Mar
17
2016
--

MySQL replication primer with pt-table-checksum / pt-table-sync, part 2

MySQL replication

MySQL replicationThis is the second and last tutorial blog post on how to use pt-table-checksum / pt-table-sync tools for MySQL replication.

In the first post, I showed you how to use the

pt-table-checksum

  /

pt-table-sync

  tools in a typical MySQL replication setup. In this post, we’ll discuss MySQL replication for more advanced topologies. I will show you how to use these tools in a chained master/slave replication setup in general, and in Percona XtraDB Cluster specifically. (Here is another good post about how to use the

pt-table-checksum

  tool in Percona XtraDB Cluster.)

Let me first show you an example where I tried to run

pt-table-checksum

 /

pt-table-sync

 on a chained master/slave setup. In this example, the setup is Master -> Slave1 -> Slave2, where the master has a binary log format set to STATEMENT, while the chained Slave1 has a binary log format set to ROW.

Master:
mysql> SHOW GLOBAL VARIABLES LIKE 'binlog_format';
+---------------+-----------+
| Variable_name | Value     |
+---------------+-----------+
| binlog_format | STATEMENT |
+---------------+-----------+
Slave1:
mysql> SHOW VARIABLES LIKE 'binlog_format';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| binlog_format | ROW   |
+---------------+-------+
[root@master ~]# pt-table-checksum --replicate=percona.checksums --ignore-databases mysql h=localhost,u=checksum_user,p=checksum_password
Replica slave1 has binlog_format ROW which could cause pt-table-checksum to break replication.  Please read "Replicas using row-based replication" in the LIMITATIONS section of the tool's documentation.  If you understand the risks, specify --no-check-binlog-format to disable this check.

As soon as I tried to execute

pt-table-checksum

 on this replication topology, it failed because the binary logs use different formats. The

pt-table-checksum

 documentation mentions this issue in the Limitations section.

The problem here is that Slave1 receives changes from its Master, which will be STATEMENT (

pt-table-checksum

  will force this into STATEMENT binlog_format in its session). Slave1 executes the checksum queries, and all seems good until that point. But then Slave1 will also write the changes to its binary log to replicate to Slave2.

pt-table-checksum

  only works properly when STATEMENT-based formatting is used to generate the checksum. If it’s ROW-based, the row changes to the checksums table are just replicated from the chain master,  and no checksumming happens. In this case, this is what happens with Slave2, which is why 

pt-table-checksum

  doesn’t perform its checksum.

To remedy this problem we need to ignore this checking by using –no-check-binlog-format, or we need to change the binary format to STATEMENT on chain master (Slave1).

In our second example, I’m going to run

pt-table-checksum

  on a three-node Percona XtraDB Cluster. Data inconsistencies can occur in Galera Cluster because of human errors or bugs. For this purpose, I inserted a few rows in Node1, and the Node1/Node2 test.dummy table data look like the following:

Node1 & Node2:
mysql> SELECT * FROM test.dummy;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | c    |
|  4 | d    |
|  5 | e    |
+----+------+
5 rows in set (0.00 sec)

Whereas Node3 is missing last two records, as shown below (I intentionally deleted the last couple rows with the setting wsrep_on off/on in between):

Node3:
mysql> SELECT * FROM dummy;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | c    |
+----+------+
3 rows in set (0.00 sec)

I have filled the DSN table accordingly with checksum user credentials:

mysql> INSERT INTO percona.dsns (dsn) VALUES ('h=10.0.3.10,u=checksum_user,p=checksum_password,P=3306');
mysql> INSERT INTO percona.dsns (dsn) VALUES ('h=10.0.3.14,u=checksum_user,p=checksum_password,P=3306');
mysql> INSERT INTO percona.dsns (dsn) VALUES ('h=10.0.3.169,u=checksum_user,p=checksum_password,P=3306');

pt-table-checksum

  on Percona XtraDB Cluster works with a single cluster, where all nodes are cluster nodes and not regular replicas. The example above is a good one – Node1, Node2 and Node3 all belong to a single cluster. Cluster-to-cluster setups aren’t supported. (Refer to documentation for more details.)

Let’s try to run

pt-table-checksum

  from authoritative node Node1:

root@node1:# pt-table-checksum --replicate percona.checksums --ignore-databases=mysql h=localhost,u=checksum_user,p=checksum_password --recursion-method=dsn=D=percona,t=dsns
Not checking replica lag on node2 because it is a cluster node.
Not checking replica lag on node3 because it is a cluster node.
            TS ERRORS  DIFFS     ROWS  CHUNKS SKIPPED    TIME TABLE
08-21T08:27:12      0      1        5       1       0   0.045 test.dummy
root@node1:~# pt-table-checksum --replicate=percona.checksums --replicate-check-only --ignore-databases=mysql h=localhost,u=checksum_user,p=checksum_password --recursion-method=dsn=D=percona,t=dsns
Not checking replica lag on node2 because it is a cluster node.
Not checking replica lag on node3 because it is a cluster node.
Differences on node3
TABLE CHUNK CNT_DIFF CRC_DIFF CHUNK_INDEX LOWER_BOUNDARY UPPER_BOUNDARY
test.dummy 1 -2 1

pt-table-checksum

  prints messages during the checksum process saying it can’t check the replica lag. It’s obvious why: SHOW SLAVE STATUS is not supported on cluster nodes as it would be in a traditional master -> slave(s) replication setup. Further,

pt-table-checksum

  found the difference on the test.dummy table. Now,

pt-table-checksum

  also supports “recursion-method=cluster,” which will auto-discover cluster nodes by querying the variable wsrep_incoming_addresses status. This new recursion-method “cluster” works equally well in Percona XtraDB Cluster. You may use “recursion-method=cluster” in place of “recursion-method=dsn” Let’s find the differences again using recursion-method “cluster”:

root@node1:~# pt-table-checksum --empty-replicate-table --replicate percona.checksums --ignore-databases=mysql h=localhost,u=checksum_user,p=checksum_password --recursion-method=cluster
Not checking replica lag on node2 because it is a cluster node.
Not checking replica lag on node3 because it is a cluster node.
            TS ERRORS  DIFFS     ROWS  CHUNKS SKIPPED    TIME TABLE
02-24T02:19:48      0      0        3       1       0   0.017 percona.dsns
02-24T02:19:48      0      1        5       1       0   0.017 test.dummy
root@node1:~# pt-table-checksum --replicate=percona.checksums --replicate-check-only --ignore-databases=mysql h=localhost,u=checksum_user,p=checksum_password --recursion-method=cluster
Not checking replica lag on node2 because it is a cluster node.
Not checking replica lag on node3 because it is a cluster node.
Differences on node3
TABLE CHUNK CNT_DIFF CRC_DIFF CHUNK_INDEX LOWER_BOUNDARY UPPER_BOUNDARY
test.dummy 1 -2 1

The differences are found on Node3. Now the next big challenge is to synchronize Node3 with the other nodes in the cluster.

pt-table-sync

  is the tool to synchronize MySQL table data differences, but unfortunately, the

pt-table-sync

  tool is not supported on Galera Cluster. I tried few hacks with pt-table-sync in Percona XtraDB Cluster, but no luck (as shown below):

Sync test.dummy on node1 to node3:
root@node1:~# pt-table-sync --dry-run h=localhost,u=checksum_user,p=checksum_password,D=test,t=dummy h=10.0.3.10
# NOTE: --dry-run does not show if data needs to be synced because it
#       does not access, compare or sync data.  --dry-run only shows
#       the work that would be done.
# Syncing D=test,h=10.0.3.10,p=...,t=dummy,u=checksum_user in dry-run mode, without accessing or comparing data
# DELETE REPLACE INSERT UPDATE ALGORITHM START    END      EXIT DATABASE.TABLE
#      0       0      0      0 Chunk     09:11:31 09:11:31 0    test.dummy
root@node1:~# pt-table-sync --print h=localhost,u=checksum_user,p=checksum_password,D=test,t=dummy h=10.0.3.10
INSERT INTO `test`.`dummy`(`id`, `name`) VALUES ('4', 'd') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:D=test,h=localhost,p=...,t=dummy,u=checksum_user dst_db:test dst_tbl:dummy dst_dsn:D=test,h=10.0.3.10,p=...,t=dummy,u=checksum_user lock:0 transaction:1 changing_src:0 replicate:0 bidirectional:0 pid:48372 user:root host:node1*/;
INSERT INTO `test`.`dummy`(`id`, `name`) VALUES ('5', 'e') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:D=test,h=localhost,p=...,t=dummy,u=checksum_user dst_db:test dst_tbl:dummy dst_dsn:D=test,h=10.0.3.10,p=...,t=dummy,u=checksum_user lock:0 transaction:1 changing_src:0 replicate:0 bidirectional:0 pid:48372 user:root host:node1*/;

I tried to run

pt-table-sync

  from Node1 to sync the differences on Node3. I first used –dry-run to make sure everything was good. Later, I passed the –print option to review queries before actually syncing. In XtraDB Cluster,

pt-table-sync

  prints INSERT/UPDATE/DELETE queries instead of REPLACE queries. Executing

pt-table-sync

  with the –execute option will break the cluster, as those changes will be replicated to all the cluster nodes. Since Node1 and Node2 already have those records in their table, the cluster will break with a “Duplicate Key” error. To workaround this problem, use the

pt-table-sync

  –replace option (as shown below):

root@node1:/var/lib/mysql# pt-table-sync --print --replace h=localhost,u=checksum_user,p=checksum_password,D=test,t=dummy h=10.0.3.10
REPLACE INTO `test`.`dummy`(`id`, `name`) VALUES ('4', 'd') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:D=test,h=localhost,p=...,t=dummy,u=checksum_user dst_db:test dst_tbl:dummy dst_dsn:D=test,h=10.0.3.10,p=...,t=dummy,u=checksum_user lock:0 transaction:1 changing_src:0 replicate:0 bidirectional:0 pid:942 user:root host:node1*/;
REPLACE INTO `test`.`dummy`(`id`, `name`) VALUES ('5', 'e') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:D=test,h=localhost,p=...,t=dummy,u=checksum_user dst_db:test dst_tbl:dummy dst_dsn:D=test,h=10.0.3.10,p=...,t=dummy,u=checksum_user lock:0 transaction:1 changing_src:0 replicate:0 bidirectional:0 pid:942 user:root host:node1*/;

When I run

pt-table-sync

  with “–execute” and “–replace”, it breaks with deadlock errors (see here: https://bugs.launchpad.net/percona-toolkit/+bug/1250450). Let’s try to run those REPLACE statements from one of the authoritative nodes (i.e., Node1):

mysql> SHOW BINLOG EVENTS IN 'node4-bin.000002';
+------------------+-----+-------------+-----------+-------------+-----------------------------------------------+
| Log_name         | Pos | Event_type  | Server_id | End_log_pos | Info                                          |
+------------------+-----+-------------+-----------+-------------+-----------------------------------------------+
| node4-bin.000002 |   4 | Format_desc |         1 |         120 | Server ver: 5.6.24-72.2-56-log, Binlog ver: 4 |
+------------------+-----+-------------+-----------+-------------+-----------------------------------------------+
1 row in set (0.03 sec)
mysql> source replace.sql;
Query OK, 1 row affected (0.02 sec)
Query OK, 1 row affected (0.00 sec)
mysql> SHOW BINLOG EVENTS IN 'node4-bin.000002';
+------------------+-----+-------------+-----------+-------------+-----------------------------------------------+
| Log_name         | Pos | Event_type  | Server_id | End_log_pos | Info                                          |
+------------------+-----+-------------+-----------+-------------+-----------------------------------------------+
| node4-bin.000002 |   4 | Format_desc |         1 |         120 | Server ver: 5.6.24-72.2-56-log, Binlog ver: 4 |
+------------------+-----+-------------+-----------+-------------+-----------------------------------------------+
1 row in set (0.00 sec)

As you can see,

pt-table-sync

  tool prints REPLACE INTO statements, which seems correct as those will be no-op changes for nodes that already have that data, and replication will sync unsynchronized nodes. But here is the problem: Galera Cluster works through a ROW binary log format, and REPLACE statements won’t get to an unsynchronized node since there won’t be a binary log event recorded for REPLACE statements into the binary log if executed from a node that is already synchronized (Node1 and Node2 in this case) since a ROW image change didn’t occur.

So how do we synchronized those changes in Galera Cluster? I’ve found a couple of workarounds to synchronize nodes in Galera Cluster!

Approach # 1:
Generate a .sql script via

pt-table-sync

  tool with –print option, and review the changes that need to occur on the unsynchronized node. Once you are satisfied, you can execute this script directly on the problematic node with binary logging disabled for that particular execution (you can push SET SQL_LOG_BIN=0 at first and enable SQL_LOG_BIN=1 again at last line of script). This ensures those changes don’t synchronize to other nodes of the cluster.

NOTE: This could be risky if you forgot to disable binary logging (i.e., SET SQL_LOG_BIN=0 for the synchronization process) because as previously explained changes will replicate to all nodes, and all the remaining nodes in the cluster will go down (as those rows already exist on the other nodes). So, be careful with this method!

Also, it’s important to mention that when manually syncing with an SQL file, it may not be an atomic operation. Between the time it takes to generate the SQL file and execute it, there is a chance that the rows in question could change. To deal with this, you should engage LOCK TABLE tablename WRITE followed by UNLOCK TABLES to release the lock after the syncing process. In fact, the most secure method is to stop writing to the table that needs to be synchronized on all the nodes before running

pt-table-sync

. You can resume writes on the table after the sync operation for the target table completes. One other important thing is if there are a vast number of table rows to sync via this method, it makes sense to enable wsrep_desync (set global

wsrep_desync=on

 ) until the node gets synchronized, and then disable

wsrep_desync

  again.

As during this synchronization process, this node may lag behind and flow control will no longer take care of the desynced node.

mysql> SHOW STATUS LIKE 'wsrep_cluster_size%';
+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| wsrep_cluster_size | 3     |
+--------------------+-------+
mysql> SET SQL_LOG_BIN=0;
Query OK, 0 rows affected (0.00 sec)
mysql> LOCK TABLE dummy WRITE;
Query OK, 0 rows affected (0.00 sec)
mysql> INSERT INTO `test`.`dummy`(`id`, `name`) VALUES ('4', 'd') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:D=test,h=localhost,p=...,t=dummy,u=checksum_user dst_db:test dst_tbl:dummy dst_dsn:D=test,h=10.0.3.14,p=...,t=dummy,u=checksum_user lock:0 transaction:1 changing_src:0 replicate:0 bidirectional:0 pid:3800 user:root host:node3*/;
Query OK, 1 row affected (0.00 sec)
mysql> INSERT INTO `test`.`dummy`(`id`, `name`) VALUES ('5', 'e') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:D=test,h=localhost,p=...,t=dummy,u=checksum_user dst_db:test dst_tbl:dummy dst_dsn:D=test,h=10.0.3.14,p=...,t=dummy,u=checksum_user lock:0 transaction:1 changing_src:0 replicate:0 bidirectional:0 pid:3800 user:root host:node3*/;
Query OK, 1 row affected (0.00 sec)
mysql> select * from test.dummy;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | c    |
|  4 | d    |
|  5 | e    |
+----+------+
5 rows in set (0.00 sec)
mysql> SHOW STATUS LIKE 'wsrep_cluster_size%';
+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| wsrep_cluster_size | 3     |
+--------------------+-------+
1 row in set (0.01 sec)
mysql> UNLOCK TABLES;
Query OK, 0 rows affected (0.00 sec)
mysql> SET SQL_LOG_BIN=1;
Query OK, 0 rows affected (0.00 sec)

Approach # 2:
Another approach is to rebuild database table in question. I got this idea from Jervin’s impressive post. You need to use the pt-online-schema-change tool and a NOOP ALTER. You need to run it through one of the authoritative synchronization nodes of the cluster, as below:

root@node2:~# pt-online-schema-change --execute --alter 'ENGINE=INNODB' D=test,t=dummy
No slaves found.  See --recursion-method if host node4 has slaves.
Not checking slave lag because no slaves were found and --check-slave-lag was not specified.
Operation, tries, wait:
  copy_rows, 10, 0.25
  create_triggers, 10, 1
  drop_triggers, 10, 1
  swap_tables, 10, 1
  update_foreign_keys, 10, 1
Altering `test`.`dummy`...
Creating new table...
Created new table test._dummy_new OK.
Altering new table...
Altered `test`.`_dummy_new` OK.
2016-02-11T01:23:22 Creating triggers...
2016-02-11T01:23:22 Created triggers OK.
2016-02-11T01:23:22 Copying approximately 5 rows...
2016-02-11T01:23:22 Copied rows OK.
2016-02-11T01:23:22 Swapping tables...
2016-02-11T01:23:22 Swapped original and new tables OK.
2016-02-11T01:23:22 Dropping old table...
2016-02-11T01:23:22 Dropped old table `test`.`_dummy_old` OK.
2016-02-11T01:23:22 Dropping triggers...
2016-02-11T01:23:22 Dropped triggers OK.
Successfully altered `test`.`dummy`.

Once the table is rebuild, ROW images will be copied to the node needing synchronization. It might be a good idea to use the –dry-run option before doing the actual changes with –execute.

You’ll need to decide which of these approaches to choose: while this second approach seems a much safer option, it may take some time to complete. If the database table is in gigabytes and terabytes, and the differences in rows are few compared to the actual table size, then the first approach could be faster (but riskier).

In next example, I have added an async node as a replica of Node1, where Node1 acts as the master server for the async slave. The setup looks like:

node1 <-> node2 <-> node3
 |
 |
 +-> replica (async)

pt-table-checksum

  can only detect differences on a replica from the master node (i.e., Node1). In this case, the tool needs to be executed from the master node (Node1). If you run

pt-table-checksum

  from Node2 or Node3, it won’t detect diffs on the replica because its master node is Node1. You can find details about this in the documentation.

Let’s try to see all this in below examples. In this test, I’m going to run the

pt-table-checksum

  tool from Node1 (master) of the async node (slave) to see if it can find the differences on the async replica node.

root@node1:~# pt-table-checksum --replicate percona.checksums --ignore-databases=mysql h=localhost,u=checksum_user,p=checksum_password
            TS ERRORS  DIFFS     ROWS  CHUNKS SKIPPED    TIME TABLE
09-03T03:02:48      0      0        3       1       0   0.022 percona.dsns
09-03T03:02:48      0      1        5       1       0   0.041 test.dummy
root@node1:~# pt-table-checksum --replicate=percona.checksums --replicate-check-only --ignore-databases=mysql h=localhost,u=checksum_user,p=checksum_password
Differences on async1
TABLE CHUNK CNT_DIFF CRC_DIFF CHUNK_INDEX LOWER_BOUNDARY UPPER_BOUNDARY
test.dummy 1 -2 1

The

pt-table-checksum

  tool can find the differences on async node test.dummy table. Let’s try to sync those diffs with the help of

pt-table-sync tool

:

root@async1:~# pt-table-sync --dry-run --sync-to-master h=localhost,u=checksum_user,p=checksum_password,D=test,t=dummy
# NOTE: --dry-run does not show if data needs to be synced because it
#       does not access, compare or sync data.  --dry-run only shows
#       the work that would be done.
# Syncing D=test,h=localhost,p=...,t=dummy,u=checksum_user in dry-run mode, without accessing or comparing data
# DELETE REPLACE INSERT UPDATE ALGORITHM START    END      EXIT DATABASE.TABLE
#      0       0      0      0 Chunk     08:38:55 08:38:55 0    test.dummy
root@async1:~# pt-table-sync --print --sync-to-master h=localhost,u=checksum_user,p=checksum_password,D=test,t=dummy
REPLACE INTO `test`.`dummy`(`id`, `name`) VALUES ('4', 'd') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:D=test,P=3306,h=10.0.3.14,p=...,t=dummy,u=checksum_user dst_db:test dst_tbl:dummy dst_dsn:D=test,h=localhost,p=...,t=dummy,u=checksum_user lock:1 transaction:1 changing_src:1 replicate:0 bidirectional:0 pid:49118 user:root host:async1*/;
REPLACE INTO `test`.`dummy`(`id`, `name`) VALUES ('5', 'e') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:D=test,P=3306,h=10.0.3.14,p=...,t=dummy,u=checksum_user dst_db:test dst_tbl:dummy dst_dsn:D=test,h=localhost,p=...,t=dummy,u=checksum_user lock:1 transaction:1 changing_src:1 replicate:0 bidirectional:0 pid:49118 user:root host:async1*/;
root@async1:~# pt-table-sync --execute --sync-to-master h=localhost,u=checksum_user,p=checksum_password,D=test,t=dummy
Deadlock found when trying to get lock; try restarting transaction at line 6115 while doing test.dummy on localhost

As you can see, I executed

pt-table-sync

 tool from the async node to find the exact differences, and it printed the two rows that were missing on the dummy table on the async replica. Further, when executed with the –execute option, the tool fails with a “Deadlock” error. In reality, there is no deadlock found from the InnoDB status. In order to sync the slave with its master, you can take one of the approaches described above: either use the .sql script to execute into slave server directly, or rebuild the table on the master Galera node and it will rebuild across the slaves.

Let me show you one final example where I tried to run

pt-table-checksum

  from Node3 to find the differences on the async slave with Node1 as its master host:

root@node3:~# pt-table-checksum --replicate percona.checksums --ignore-databases=mysql h=localhost,u=checksum_user,p=checksum_password
Diffs cannot be detected because no slaves were found.  Please read the --recursion-method documentation for information.
node3 is a cluster node but no other nodes or regular replicas were found.  Use --recursion-method=dsn to specify the other nodes in the cluster.

Here, Node3 refused to find differences because there is no direct replica attached to it. The documentation mentions this behavior, as cluster environment, differences can only be detected on the master node directly connected to async slave (as is the case here with Node1 to async replica). In order to run a checksum from Node3 to check for differences between cluster nodes, use the –recursion-method mentioned in previous examples.

Conclusion:
Galera Cluster doesn’t support

pt-table-sync

 in general. But here we’ve found a couple of workarounds to solve this problem:

  • You can sync those changes manually by redirecting the
    pt-table-sync --print

      output to file to generate a .SQL template, review changes carefully and execute it on nodes with the missing data. Again, this approach is fast but possibly dangerous.

  • Another option is the noop alter method using
    pt-online-schema-change

      to rebuild the table. It’s the recommended method, but it could be slower.

Finally, after synchronizing the data changes it is a good idea to re-run the checksum tool to verify any discrepancies.

Aug
11
2015
--

MySQL replication primer with pt-table-checksum and pt-table-sync

MySQL replication is a process that allows you to easily maintain multiple copies of MySQL data by having them copied automatically from a master to a slave database.

It’s essential to make sure the slave servers have the same set of data as the master to ensure data is consistent within the replication stream. MySQL slave server data can drift from the master for many reasons – e.g. replication errors, accidental direct updates on slave, etc.

Here at Percona Support we highly recommend that our customers periodically run the pt-table-checksum tool to verify data consistency within replication streams. Specifically, after fixing replication errors on slave servers to ensure that the slave has identical data as its master. As you don’t want to put yourself in a situation where you need to failover to a slave server for some reason and you find different data on that slave server.

In this post, I will examine the pt-table-checksum and pt-table-sync tools usage from Percona Toolkit on different replication topologies. We often receive queries from customers about how to run these tools and I hope this post will help.

Percona Toolkit is a free collection of advanced command-line tools to perform a variety of MySQL server and system tasks that are too difficult or complex to perform manually.

One of those tools is pt-table-checksum, which works by dividing table rows into chunks of rows. The size of a chunk changes dynamically during the operation to avoid overloading the server. pt-table-checksum has many safeguards including variation into chunk size to make sure queries run in a desired amount of time.

pt-table-checksum verifies chunk size by running EXPLAIN query on each chunk. It also monitors slave server’s continuously in order to make sure replicas not falls too far behind and in this case tool pauses itself to allow slave to catch up. Along with that there are many other safeguards builtin and you can find all the details in this documentation

In my first example case, I am going to run pt-table-checksum against pair of replication servers – i.e. master having only one slave in replication topology. We will run pt-table-checksum tool on master server to verify data integrity on slave and in case If differences found by pt-table-checksum tool we will sync those changes on slave server via pt-table-sync tool.

I have created a dummy table under test database and inserted 10 records on master server as below:

mysql-master> create table dummy (id int(11) not null auto_increment primary key, name char(5)) engine=innodb;
Query OK, 0 rows affected (0.08 sec)
mysql-master> insert into dummy VALUES (1,'a'), (2,'b'), (3,'c'), (4,'d'), (5,'e'), (6,'f'), (7,'g'), (8,'h'), (9,'i'), (10,'j');
Query OK, 10 rows affected (0.00 sec)
Records: 10  Duplicates: 0  Warnings: 0
mysql-master> select * from dummy;
+------+------+
| id   | name |
+------+------+
|    1 | a    |
|    2 | b    |
|    3 | c    |
|    4 | d    |
|    5 | e    |
|    6 | f    |
|    7 | g    |
|    8 | h    |
|    9 | i    |
|   10 | j    |
+------+------+
10 rows in set (0.00 sec)

Then I intentionally deleted a few records from the slave server to make it inconsistent with the master for the purpose of this post.

mysql-slave> delete from dummy where id>5;
Query OK, 5 rows affected (0.03 sec)
mysql-slave> select * from dummy;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | c    |
|  4 | d    |
|  5 | e    |
+----+------+
5 rows in set (0.00 sec)

Now, in this case the master server has 10 records on the dummy table while the slave server has only 5 records missing records from id>5 – we will run pt-table-checksum at this point on the master server to see if the pt-table-checksum tool catches those differences.

[root@master]# pt-table-checksum --replicate=percona.checksums --ignore-databases mysql h=localhost,u=checksum_user,p=checksum_password
            TS ERRORS  DIFFS     ROWS  CHUNKS SKIPPED    TIME TABLE
07-11T18:30:13      0      1       10       1       0   1.044 test.dummy

This needs to be executed on the master. The user and password you specify will be used to not only connect to the master but the slaves as well. You need the following privileges for the pt-table-checksum mysql user:

mysql-master> GRANT REPLICATION SLAVE,PROCESS,SUPER, SELECT ON *.* TO `checksum_user`@'%' IDENTIFIED BY 'checksum_password';
mysql-master> GRANT ALL PRIVILEGES ON percona.* TO `checksum_user`@'%';

Earlier, in pt-table-checksum command, I used –replicate option which writes replication queries to mentioned table percona.checksums. Next I passed  –ignore-databases option which accepts comma separated list of databases to ignore. Moreover, –create-replicate-table and —empty-replicate-table options are “Yes” by default and you can specify both options explicitly if you want to create database table different then percona.checksums.

pt-table-checksum reported 1 DIFF which is number of chunks which are different from master on one or more slaves. You can find details about tabular columns e.g. TS, ERRORS and so on on documentation of pt-table-checksum. After that, I ran next command to identify which table has difference on slave.

[root@master]# pt-table-checksum --replicate=percona.checksums --replicate-check-only --ignore-databases mysql h=localhost,u=checksum_user,p=checksum_password
Differences on slave
TABLE CHUNK CNT_DIFF CRC_DIFF CHUNK_INDEX LOWER_BOUNDARY UPPER_BOUNDARY
test.dummy 1 -5 1

In this command I used –replicate-check-only option which only reports the tables with having differences vice versa only checksum differences on detected replicas are printed. It doesn’t checksum any tables. It checks replicas for differences found by previous checksumming, and then exits.

You may also login to the slave and also execute below query to find out which tables have inconsistencies.

mysql-slave> SELECT db, tbl, SUM(this_cnt) AS total_rows, COUNT(*) AS chunks
FROM percona.checksums
WHERE (
master_cnt <> this_cnt
OR master_crc <> this_crc
OR ISNULL(master_crc) <> ISNULL(this_crc))
GROUP BY db, tbl;

pt-table-checksum identified test.dummy table is different on slave now we are going to use pt-table-sync tool to synchronize table data between MySQL servers.

[root@slave]# pt-table-sync --print --replicate=percona.checksums --sync-to-master h=localhost,u=checksum_user,p=checksum_password
REPLACE INTO `test`.`dummy`(`id`, `name`) VALUES ('6', 'f') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:P=3306,h=192.168.0.130,p=...,u=checksum_user dst_db:test dst_tbl:dummy dst_dsn:h=localhost,p=...,u=checksum_user lock:1 transaction:1 changing_src:percona.checksums replicate:percona.checksums bidirectional:0 pid:24683 user:root host:slave*/;
REPLACE INTO `test`.`dummy`(`id`, `name`) VALUES ('7', 'g') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:P=3306,h=192.168.0.130,p=...,u=checksum_user dst_db:test dst_tbl:dummy dst_dsn:h=localhost,p=...,u=checksum_user lock:1 transaction:1 changing_src:percona.checksums replicate:percona.checksums bidirectional:0 pid:24683 user:root host:slave*/;
REPLACE INTO `test`.`dummy`(`id`, `name`) VALUES ('8', 'h') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:P=3306,h=192.168.0.130,p=...,u=checksum_user dst_db:test dst_tbl:dummy dst_dsn:h=localhost,p=...,u=checksum_user lock:1 transaction:1 changing_src:percona.checksums replicate:percona.checksums bidirectional:0 pid:24683 user:root host:slave*/;
REPLACE INTO `test`.`dummy`(`id`, `name`) VALUES ('9', 'i') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:P=3306,h=192.168.0.130,p=...,u=checksum_user dst_db:test dst_tbl:dummy dst_dsn:h=localhost,p=...,u=checksum_user lock:1 transaction:1 changing_src:percona.checksums replicate:percona.checksums bidirectional:0 pid:24683 user:root host:slave*/;
REPLACE INTO `test`.`dummy`(`id`, `name`) VALUES ('10', 'j') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:P=3306,h=192.168.0.130,p=...,u=checksum_user dst_db:test dst_tbl:dummy dst_dsn:h=localhost,p=...,u=checksum_user lock:1 transaction:1 changing_src:percona.checksums replicate:percona.checksums bidirectional:0 pid:24683 user:root host:slave*/;

I ran the pt-table-sync tool from an opposite host this time i.e. from the slave as I used the –sync-to-master option which treats DSN as slave and syncs to master. Again, pt-table-sync will use the mysql username and password you specify to connect to the slave as well as to its master. –replicate option here examines the specified table to find out the data differences and –print just prints the SQL  (REPLACE queries) not actually executes it.

You may audit the queries before executing to sync data between master/slave.  You may see it printed only missing records on the slave. Once you are happy with the results, you can substitute –print with –execute to do actual synchronization.

As a reminder, these queries always executed on the master as this is the only safe way to do the changes on slave. However, on the master it’s no-op changes as these records already exists on master but then falls to slave via replication stream to sync it with master.

If you find lots of differences on your slave server it may lag during synchronization of those changes. As I mentioned earlier, you can use –print option to go through your queries which are going to be executed to sync slave with master server. I found this post useful if you see a huge difference in the table between master/slave(s).

Note, you may use the –dry-run option initially which only analyzes print information about the sync algorithm and then exits. It shows verbose output; it doesn’t do any changes though. –dry-run parameter will basically instruct pt-table-sync to not actually do the sync, but just perform some checks.

Let me present another replication topology, where the master has two slaves where slave2 is running on non-default port 3307 while master and slave1 running on port 3306. Further, slave2 is out of sync with master and I will show you how to sync slave2 which running on port 3307 with master.

mysql-master> SELECT * FROM dummy;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | c    |
|  4 | d    |
|  5 | e    |
+----+------+
5 rows in set (0.00 sec)
mysql-slave1> SELECT * FROM test.dummy;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | c    |
|  4 | d    |
|  5 | e    |
+----+------+
5 rows in set (0.00 sec)
mysql-slave2> SELECT * FROM test.dummy;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | c    |
+----+------+

Let’s run pt-table-checksum tool on master database server.

[root@master]# pt-table-checksum --replicate percona.checksums --ignore-databases=mysql h=192.168.0.130,u=checksum_user,p=checksum_password --recursion-method=dsn=D=percona,t=dsns
            TS ERRORS  DIFFS     ROWS  CHUNKS SKIPPED    TIME TABLE
07-23T13:57:39      0      0        2       1       0   0.310 percona.dsns
07-23T13:57:39      0      1        5       1       0   0.036 test.dummy

I used –recursion-method parameter this time which is method to use find slaves in replication stream and it’s pretty useful when your servers run on non-standard port i.e. other than 3306. I created dsns table under percona database with following entries. You may find dsns table structure in documentation.

mysql> SELECT * FROM dsns;
+----+-----------+------------------------------------------------------------+
| id | parent_id | dsn                                                        |
+----+-----------+------------------------------------------------------------+
|  1 |         1 | h=192.168.0.134,u=checksum_user,p=checksum_password,P=3306 |
|  2 |         2 | h=192.168.0.132,u=checksum_user,p=checksum_password,P=3307 |
+----+-----------+------------------------------------------------------------+

Next I ran below pt-table-checksum command to identify which slave server has differences on test.dummy table.

[root@master]# pt-table-checksum --replicate=percona.checksums --replicate-check-only --ignore-databases=mysql h=192.168.0.130,u=checksum_user,p=checksum_password --recursion-method=dsn=D=percona,t=dsns
Differences on slave2
TABLE CHUNK CNT_DIFF CRC_DIFF CHUNK_INDEX LOWER_BOUNDARY UPPER_BOUNDARY
test.dummy 1 -2 1

This shows that slave2 has different data on test.dummy table as compared to the master. Now let’s run pt-table-sync tool to sync those differences and make slave2 identical as the master.

[root@slave2] ./pt-table-sync --print --replicate=percona.checksums --sync-to-master h=192.168.0.132,u=checksum_user,p=checksum_password
REPLACE INTO `test`.`dummy`(`id`, `name`) VALUES ('4', 'd') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:P=3306,h=192.168.0.130,p=...,u=checksum dst_db:test dst_tbl:dummy dst_dsn:h=192.168.0.132,p=...,u=checksum lock:1 transaction:1 changing_src:percona.checksums replicate:percona.checksums bidirectional:0 pid:1514 user:root host:slave2*/;
REPLACE INTO `test`.`dummy`(`id`, `name`) VALUES ('5', 'e') /*percona-toolkit src_db:test src_tbl:dummy src_dsn:P=3306,h=192.168.0.130,p=...,u=checksum dst_db:test dst_tbl:dummy dst_dsn:h=192.168.0.132,p=...,u=checksum lock:1 transaction:1 changing_src:percona.checksums replicate:percona.checksums bidirectional:0 pid:1514 user:root host:slave2*/;

It shows 2 rows are different on slave2. Substituting –print with –execute synchronized the differences on slave2 and re-running pt-table-checksum tool shows no more differences.

Conclusion:
pt-table-checksum and pt-table-sync are the finest tools from Percona Toolkit to validate data between master/slave(s). With the help of these tools you can easily identify data drifts and fix them. I mentioned a couple of replication topologies above about how to check replication consistency and how to fix it in case of data drift. You may script pt-table-checksum / pt-table-sync steps and cron checksum script to periodically check the data consistency within replication stream.

This procedure is only safe for a single level master-slave(s) hierarchy. I will discuss the procedure for other topologies in future posts – i.e. I will describe more complex scenarios on how to use these tools in chain replication i.e. master -> slave1 -> slave2 pair and in Percona XtraDB Cluster setup.

The post MySQL replication primer with pt-table-checksum and pt-table-sync appeared first on MySQL Performance Blog.

Apr
15
2015
--

Checking table definition consistency with mysqldiff

Data inconsistencies in replication environments are a pretty common. There are lots of posts that explain how to fix those using pt-table-checksum and pt-table-sync. Usually we only care about the data but from time to time we receive this question in support:

How can I check the table definition consistency between servers?

Replication also allow us to have different table definition between master and slaves. For example, there are some cases that you need some indexes on slaves for querying purposes but are not really needed on the master. There are some other cases where those differences are just a mistake that needs to be fixed.

mysqldiff, included in Oracle’s MySQL Utilities, can help us to find those differences and get the information we need to fix those them. In this post I’m going to show you how to use it with an example.

Find table definition inconsistencies

mysqldiff allows us to find those inconsistencies checking the differences between the tables on the same server (different databases) or on different servers (also possible on different databases). In this example I’m going to search for differences in table definitions between two different servers, server1 and server2.

The command line is pretty simple. This is used to compare the tables on “test” database:

mysqldiff --server1=user@host1 --server2=user@host2 test:test

If the database name is different:

mysqldiff --server1=user@host1 --server2=user@host2 testdb:anotherdb

If the table name is different:

mysqldiff --server1=user@host1 --server2=user@host2 testdb.table1:anotherdb.anothertable

Now I want to check the table definition consistency between two servers. The database’s name is “employees”:

# mysqldiff --force --server1=root:msandbox@127.0.0.1:21489 --server2=root:msandbox@127.0.0.1:21490 employees:employees
# WARNING: Using a password on the command line interface can be insecure.
# server1 on 127.0.0.1: ... connected.
# server2 on 127.0.0.1: ... connected.
# Comparing `employees` to `employees`                             [PASS]
# Comparing `employees`.`departments` to `employees`.`departments`   [FAIL]
# Object definitions differ. (--changes-for=server1)
#
--- `employees`.`departments`
+++ `employees`.`departments`
@@ -1,6 +1,6 @@
 CREATE TABLE `departments` (
   `dept_no` char(4) NOT NULL,
-  `dept_name` varchar(40) NOT NULL,
+  `dept_name` varchar(256) DEFAULT NULL,
   PRIMARY KEY (`dept_no`),
   UNIQUE KEY `dept_name` (`dept_name`)
 ) ENGINE=InnoDB DEFAULT CHARSET=latin1
# Comparing `employees`.`dept_emp` to `employees`.`dept_emp`       [PASS]
# Comparing `employees`.`dept_manager` to `employees`.`dept_manager`   [PASS]
# Comparing `employees`.`employees` to `employees`.`employees`     [FAIL]
# Object definitions differ. (--changes-for=server1)
#
--- `employees`.`employees`
+++ `employees`.`employees`
@@ -5,5 +5,6 @@
   `last_name` varchar(16) NOT NULL,
   `gender` enum('M','F') NOT NULL,
   `hire_date` date NOT NULL,
-  PRIMARY KEY (`emp_no`)
+  PRIMARY KEY (`emp_no`),
+  KEY `last_name` (`last_name`,`first_name`)
 ) ENGINE=InnoDB DEFAULT CHARSET=latin1
# Comparing `employees`.`salaries` to `employees`.`salaries`       [PASS]
# Comparing `employees`.`titles` to `employees`.`titles`           [PASS]
Compare failed. One or more differences found.

There are at least two differences. One in departments table and another one in employees table. The output is similar to diff. By default the tool stops after finding the first difference. That’s why we use –force, to tell the tool to continue checking all the tables.

It shows us that on departments the dept_name is varchar(40) on server1 and varchar(256) on server2. For “employees” table, it has a KEY (last_name, first_name) on the server2 that is not present on server1. Why is it taking server2 as a reference? Because of this line:

# Object definitions differ. (--changes-for=server1)

So, the changes shown on the diff are for server1. If you want server2 to be the one to be changed and server1 used as reference, then –changes-for=server2 would be needed.

In some cases the diff output is not really useful. We actually need a SQL query to do the changes on the server. We just need to add –difftype=sql to the command line:

# mysqldiff --force --difftype=sql --server1=root:msandbox@127.0.0.1:21489 --server2=root:msandbox@127.0.0.1:21490 employees:employees
[...]
# Comparing `employees`.`departments` to `employees`.`departments`   [FAIL]
# Transformation for --changes-for=server1:
ALTER TABLE `employees`.`departments`
  DROP INDEX dept_name,
  ADD UNIQUE INDEX dept_name (dept_name),
  CHANGE COLUMN dept_name dept_name varchar(256) NULL;
[...]
# Comparing `employees`.`employees` to `employees`.`employees`     [FAIL]
# Transformation for --changes-for=server1:
#
ALTER TABLE `employees`.`employees`
  DROP PRIMARY KEY,
  ADD PRIMARY KEY(`emp_no`),
  ADD INDEX last_name (last_name,first_name);

As we can see, the tool is not perfect. There are two problems here:

1- On “departments table” it drops a UNIQUE key that is present in both servers only to add it again. Waste of time and resources.

2- On “employees” table it drops and recreate the PRIMARY KEY, again something that is not needed a all.

I have created a bug report but this also teaches us a good lesson. Don’t just copy and paste commands without first double checking it.

What mysqldiff runs under the hood?

Mostly queries on INFORMATION_SCHEMA. These are the ones used to check inconsistencies on departments:

SHOW CREATE TABLE `departments`;
SELECT TABLE_SCHEMA, TABLE_NAME, ENGINE, AUTO_INCREMENT, AVG_ROW_LENGTH, CHECKSUM, TABLE_COLLATION, TABLE_COMMENT, ROW_FORMAT, CREATE_OPTIONS
  FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments';
SELECT ORDINAL_POSITION, COLUMN_NAME, COLUMN_TYPE, IS_NULLABLE,
         COLUMN_DEFAULT, EXTRA, COLUMN_COMMENT, COLUMN_KEY
  FROM INFORMATION_SCHEMA.COLUMNS
  WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments';
SELECT PARTITION_NAME, SUBPARTITION_NAME, PARTITION_ORDINAL_POSITION,
         SUBPARTITION_ORDINAL_POSITION, PARTITION_METHOD, SUBPARTITION_METHOD,
         PARTITION_EXPRESSION, SUBPARTITION_EXPRESSION, PARTITION_DESCRIPTION
  FROM INFORMATION_SCHEMA.PARTITIONS
  WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments';
SELECT CONSTRAINT_NAME, COLUMN_NAME, REFERENCED_TABLE_SCHEMA,
         REFERENCED_TABLE_NAME, REFERENCED_COLUMN_NAME
  FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
  WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments' AND
        REFERENCED_TABLE_SCHEMA IS NOT NULL;
SELECT TABLE_SCHEMA, TABLE_NAME, ENGINE, AUTO_INCREMENT, AVG_ROW_LENGTH, CHECKSUM, TABLE_COLLATION, TABLE_COMMENT, ROW_FORMAT, CREATE_OPTIONS
  FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments';
SELECT ORDINAL_POSITION, COLUMN_NAME, COLUMN_TYPE, IS_NULLABLE,
         COLUMN_DEFAULT, EXTRA, COLUMN_COMMENT, COLUMN_KEY
  FROM INFORMATION_SCHEMA.COLUMNS
  WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments';
SELECT PARTITION_NAME, SUBPARTITION_NAME, PARTITION_ORDINAL_POSITION,
         SUBPARTITION_ORDINAL_POSITION, PARTITION_METHOD, SUBPARTITION_METHOD,
         PARTITION_EXPRESSION, SUBPARTITION_EXPRESSION, PARTITION_DESCRIPTION
  FROM INFORMATION_SCHEMA.PARTITIONS
  WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments';

As a summary, it checks partitions, row format, collation, constraints and so on.

Conclusion

There are different tools for different purposes. We can check the data consistency with pt-table-checkum/pt-table-sync but also the table definitions with mysqldiff.

The post Checking table definition consistency with mysqldiff appeared first on MySQL Performance Blog.

Apr
15
2015
--

Checking table definition consistency with mysqldiff

Data inconsistencies in replication environments are a pretty common. There are lots of posts that explain how to fix those using pt-table-checksum and pt-table-sync. Usually we only care about the data but from time to time we receive this question in support:

How can I check the table definition consistency between servers?

Replication also allow us to have different table definition between master and slaves. For example, there are some cases that you need some indexes on slaves for querying purposes but are not really needed on the master. There are some other cases where those differences are just a mistake that needs to be fixed.

mysqldiff, included in Oracle’s MySQL Utilities, can help us to find those differences and get the information we need to fix those them. In this post I’m going to show you how to use it with an example.

Find table definition inconsistencies

mysqldiff allows us to find those inconsistencies checking the differences between the tables on the same server (different databases) or on different servers (also possible on different databases). In this example I’m going to search for differences in table definitions between two different servers, server1 and server2.

The command line is pretty simple. This is used to compare the tables on “test” database:

mysqldiff --server1=user@host1 --server2=user@host2 test:test

If the database name is different:

mysqldiff --server1=user@host1 --server2=user@host2 testdb:anotherdb

If the table name is different:

mysqldiff --server1=user@host1 --server2=user@host2 testdb.table1:anotherdb.anothertable

Now I want to check the table definition consistency between two servers. The database’s name is “employees”:

# mysqldiff --force --server1=root:msandbox@127.0.0.1:21489 --server2=root:msandbox@127.0.0.1:21490 employees:employees
# WARNING: Using a password on the command line interface can be insecure.
# server1 on 127.0.0.1: ... connected.
# server2 on 127.0.0.1: ... connected.
# Comparing `employees` to `employees`                             [PASS]
# Comparing `employees`.`departments` to `employees`.`departments`   [FAIL]
# Object definitions differ. (--changes-for=server1)
#
--- `employees`.`departments`
+++ `employees`.`departments`
@@ -1,6 +1,6 @@
 CREATE TABLE `departments` (
   `dept_no` char(4) NOT NULL,
-  `dept_name` varchar(40) NOT NULL,
+  `dept_name` varchar(256) DEFAULT NULL,
   PRIMARY KEY (`dept_no`),
   UNIQUE KEY `dept_name` (`dept_name`)
 ) ENGINE=InnoDB DEFAULT CHARSET=latin1
# Comparing `employees`.`dept_emp` to `employees`.`dept_emp`       [PASS]
# Comparing `employees`.`dept_manager` to `employees`.`dept_manager`   [PASS]
# Comparing `employees`.`employees` to `employees`.`employees`     [FAIL]
# Object definitions differ. (--changes-for=server1)
#
--- `employees`.`employees`
+++ `employees`.`employees`
@@ -5,5 +5,6 @@
   `last_name` varchar(16) NOT NULL,
   `gender` enum('M','F') NOT NULL,
   `hire_date` date NOT NULL,
-  PRIMARY KEY (`emp_no`)
+  PRIMARY KEY (`emp_no`),
+  KEY `last_name` (`last_name`,`first_name`)
 ) ENGINE=InnoDB DEFAULT CHARSET=latin1
# Comparing `employees`.`salaries` to `employees`.`salaries`       [PASS]
# Comparing `employees`.`titles` to `employees`.`titles`           [PASS]
Compare failed. One or more differences found.

There are at least two differences. One in departments table and another one in employees table. The output is similar to diff. By default the tool stops after finding the first difference. That’s why we use –force, to tell the tool to continue checking all the tables.

It shows us that on departments the dept_name is varchar(40) on server1 and varchar(256) on server2. For “employees” table, it has a KEY (last_name, first_name) on the server2 that is not present on server1. Why is it taking server2 as a reference? Because of this line:

# Object definitions differ. (--changes-for=server1)

So, the changes shown on the diff are for server1. If you want server2 to be the one to be changed and server1 used as reference, then –changes-for=server2 would be needed.

In some cases the diff output is not really useful. We actually need a SQL query to do the changes on the server. We just need to add –difftype=sql to the command line:

# mysqldiff --force --difftype=sql --server1=root:msandbox@127.0.0.1:21489 --server2=root:msandbox@127.0.0.1:21490 employees:employees
[...]
# Comparing `employees`.`departments` to `employees`.`departments`   [FAIL]
# Transformation for --changes-for=server1:
ALTER TABLE `employees`.`departments`
  DROP INDEX dept_name,
  ADD UNIQUE INDEX dept_name (dept_name),
  CHANGE COLUMN dept_name dept_name varchar(256) NULL;
[...]
# Comparing `employees`.`employees` to `employees`.`employees`     [FAIL]
# Transformation for --changes-for=server1:
#
ALTER TABLE `employees`.`employees`
  DROP PRIMARY KEY,
  ADD PRIMARY KEY(`emp_no`),
  ADD INDEX last_name (last_name,first_name);

As we can see, the tool is not perfect. There are two problems here:

1- On “departments table” it drops a UNIQUE key that is present in both servers only to add it again. Waste of time and resources.

2- On “employees” table it drops and recreate the PRIMARY KEY, again something that is not needed a all.

I have created a bug report but this also teaches us a good lesson. Don’t just copy and paste commands without first double checking it.

What mysqldiff runs under the hood?

Mostly queries on INFORMATION_SCHEMA. These are the ones used to check inconsistencies on departments:

SHOW CREATE TABLE `departments`;
SELECT TABLE_SCHEMA, TABLE_NAME, ENGINE, AUTO_INCREMENT, AVG_ROW_LENGTH, CHECKSUM, TABLE_COLLATION, TABLE_COMMENT, ROW_FORMAT, CREATE_OPTIONS
  FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments';
SELECT ORDINAL_POSITION, COLUMN_NAME, COLUMN_TYPE, IS_NULLABLE,
         COLUMN_DEFAULT, EXTRA, COLUMN_COMMENT, COLUMN_KEY
  FROM INFORMATION_SCHEMA.COLUMNS
  WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments';
SELECT PARTITION_NAME, SUBPARTITION_NAME, PARTITION_ORDINAL_POSITION,
         SUBPARTITION_ORDINAL_POSITION, PARTITION_METHOD, SUBPARTITION_METHOD,
         PARTITION_EXPRESSION, SUBPARTITION_EXPRESSION, PARTITION_DESCRIPTION
  FROM INFORMATION_SCHEMA.PARTITIONS
  WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments';
SELECT CONSTRAINT_NAME, COLUMN_NAME, REFERENCED_TABLE_SCHEMA,
         REFERENCED_TABLE_NAME, REFERENCED_COLUMN_NAME
  FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
  WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments' AND
        REFERENCED_TABLE_SCHEMA IS NOT NULL;
SELECT TABLE_SCHEMA, TABLE_NAME, ENGINE, AUTO_INCREMENT, AVG_ROW_LENGTH, CHECKSUM, TABLE_COLLATION, TABLE_COMMENT, ROW_FORMAT, CREATE_OPTIONS
  FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments';
SELECT ORDINAL_POSITION, COLUMN_NAME, COLUMN_TYPE, IS_NULLABLE,
         COLUMN_DEFAULT, EXTRA, COLUMN_COMMENT, COLUMN_KEY
  FROM INFORMATION_SCHEMA.COLUMNS
  WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments';
SELECT PARTITION_NAME, SUBPARTITION_NAME, PARTITION_ORDINAL_POSITION,
         SUBPARTITION_ORDINAL_POSITION, PARTITION_METHOD, SUBPARTITION_METHOD,
         PARTITION_EXPRESSION, SUBPARTITION_EXPRESSION, PARTITION_DESCRIPTION
  FROM INFORMATION_SCHEMA.PARTITIONS
  WHERE TABLE_SCHEMA = 'employees' AND TABLE_NAME = 'departments';

As a summary, it checks partitions, row format, collation, constraints and so on.

Conclusion

There are different tools for different purposes. We can check the data consistency with pt-table-checkum/pt-table-sync but also the table definitions with mysqldiff.

The post Checking table definition consistency with mysqldiff appeared first on MySQL Performance Blog.

Mar
09
2015
--

5 free handy tools for monitoring and managing MySQL replication

MySQL Replication is very simple to set up. In this post I’ll discuss its importance and five handy tools for monitoring and managing MySQL replication.

What is MySQL Replication? It’s the process of copying the (real-time events) data from one master instance to another slave instance and maintaining the redundant consistent data in a different machine. This enables a distributed database system that shares the same level of information.

In MySQL the replication works based on the three threads as shown below.

1) I/O thread on the slave server:  To start on receiving replication events, an I/O thread starts on the slave server and connects to the master server.

2) Master connection handler thread:  As a connection handier, master starts a thread whenever a replication slave connects to a master. The master server sends the events from its binary log file to the slave I/O thread, notifying slave about newly written events to its binary log. The slave I/O thread which records them to in the slave’s relay log file.

3) Slave SQL thread:  When it starts, immediately reads the events from the relay log and applies on the Slave DB. Once it finishes the processing of every relay log and if the I/O thread is writing the events to a new relay log file then it deletes the processed one. Suppose if the  I/O thread is writing  the events on a relay log and which is the same file SQL thread is reading then the SQL thread pauses until more events are available in the relay log.

MySQL replication (slave) redundant instances is an excellent method of improving data performance and stability. It ensures the availability of another copy of a database whenever there arises any issues with the master server. One of the other advantages is the report query (select) offloading to a slave server, which is a common practice to reduce the workload of the master DB server as there are multiple servers that are able to respond to the queries. The third advantage is to schedule the backup from the slave server, etc.

All the benefits discussed above are smart and effective only if replication is up and running and the data is in sync with the master.

Let us see the set of very useful tools from Percona Toolkit which help you in monitoring and managing the MySQL replication (Slaves).

5 free handy tools for monitoring and managing MySQL replication1) pt-heartbeat: Tool measures/monitor replication lag on a MySQL in real time. It is important to have a replication monitoring system to confirm that replication is up and running and lag is current.

In typical way of monitoring, we use “SHOW SLAVE STATUS” to find out the information like Slave_IO_Running: Yes, Slave_SQL_Running: Yes and Seconds_Behind_Master: 0 etc, but is not reliable as  Seconds_Behind_Master shows difference between last timestamp read in the binlogs and current time. Many reasons like slow network, long running queries, blocking operations or a 2nd level slaves(Master > slave1> slave2) etc. can produce an irrelevant value for the variable.

So I recommend to use pt-heartbeat, which depends only on the heartbeat record being replicated to slave instead of the above said unreliable method of finding the lag. pt-heartbeat will insert/update a row in the master and the time delay is calculated depending on when the data was inserted and when it became available to read in the slave. It works at any depth in the replication hierarchy. For example, it will reliably report how far a slave lags its original master (master’s master).

Example :

On Master:
[root@Tst1Master ~]#pt-heartbeat --daemonize -D test --update -h<IP address> --create-table
On Slave:
[root@Tst1Slave ~]#pt-heartbeat -D test --monitor --master-server-id 1
0.00s [  0.00s,  0.00s,  0.00s ]
0.00s [  0.00s,  0.00s,  0.00s ]
0.00s [  0.00s,  0.00s,  0.00s ]
0.00s [  0.00s,  0.00s,  0.00s ]

We used to schedule the backup from the slave to avoid the additional load with the master server. In this case it is important to confirm the slave is current with the master to ascertain the backup is having the recent data. Here is a simple script you can use to verify the replication status on a periodical basis(cron) and to know the status just before the backup scheduled.

#!/bin/bash
#     <300 - [Good]
#     300> <600 - [Warning]
#     > 600 - [Critical]
MAIL_FROM="root@`hostname`"
MAIL_TO="mailid@mail.com"
Warningthreshold=300
Criticalthreshold=600
backup=$1
CMD=$(/root/bin/pt-heartbeat -D test --master-server-id 1 --check | cut -d. -f1)
# Pass the parameter "test.sh backup" to denote the call is from the backup script.
if [ $CMD -lt $Warningthreshold ]
then
MESSAGE=`date +'%m:%d:%Y %H:%M:%S'`" [Good] current delay: "$CMD;
elif [ $CMD -gt $Warningthreshold ] && [ $CMD -lt $Criticalthreshold ]
then
MESSAGE=`date +'%m:%d:%Y %H:%M:%S'`" [Warning] current delay: "$CMD;
elif [ $CMD -gt $Criticalthreshold ]
then
MESSAGE=`date +'%m:%d:%Y %H:%M:%S'`" [Critical] current delay: $CMD Check the replication"
else
MESSAGE=`date +'%m:%d:%Y %H:%M:%S'`" [Error] Replication status check failed need to investigate."
fi
#No arguments supplied"
if [ -z "$1" ] && [ $CMD -gt $Warningthreshold ]
then
(echo "Subject: Replication status on `hostname`";
echo "Replication status : "
echo $MESSAGE
)  | /usr/sbin/sendmail -O NoRecipientAction=add-to -f${MAIL_FROM} ${MAIL_TO}
elif [ $# -eq 1 ]
then
(echo "Subject: Replication status check prior to backup on `hostname`";
echo "Replication status prior to backup:"
echo $MESSAGE
)  | /usr/sbin/sendmail -O NoRecipientAction=add-to -f${MAIL_FROM} ${MAIL_TO}
fi

2) pt-slave-find: Finds and prints replication hierarchy of the slaves – shows you the topology and replication hierarchy of your MySQL replication instances.

Example :

[root@Tst1Master ~]# ./pt-slave-find --host=192.168.56.10
192.168.56.10
Version         5.6.22-72.0-log
Server ID       1
Uptime          42:09 (started 2015-03-03T01:40:42)
Replication     Is not a slave, has 1 slaves connected, is not read_only
Filters
Binary logging  STATEMENT
Slave status
Slave mode      STRICT
Auto-increment  increment 1, offset 1
InnoDB version  5.6.22-72.0
+- 192.168.56.11
   Version         5.6.22-72.0
   Server ID       2
   Uptime          41:48 (started 2015-03-03T01:41:03)
   Replication     Is a slave, has 0 slaves connected, is not read_only
   Filters
   Binary logging  STATEMENT
   Slave status    0 seconds behind, running, no errors
   Slave mode      STRICT
   Auto-increment  increment 1, offset 1
   InnoDB version  5.6.22-72.0

[root@Tst1Master ~]# ./pt-slave-find --host=192.168.56.10 --report-format=hostname
192.168.56.10
+- 192.168.56.11

3) pt-slave-restart: Watches the MySQL replication slaves for any error and tries to restart the replication.

The tool is very useful for skipping statements that cause errors and continuing replication. If you use this carelessly, the slave will be having the inconsistent data. However  when you use the tool, I recommended you to confirm the consistency of data between master and slave with help of pt-table-checksum.

Example : Restart the slave for error-numbers=1062 (Duplicate entry ‘1’ for key ‘PRIMARY’)

#pt-slave-restart --socket=/var/lib/mysql/custom-feeds/mysql.sock --ask-pass --error-numbers=1062

4) pt-table-checksum: Performs an online replication consistency check by executing checksum queries on the master, which produces different results on replicas that are inconsistent with the master.

Example :

[root@Tst1Master ~]# ./pt-table-checksum -dD
            TS ERRORS  DIFFS     ROWS  CHUNKS SKIPPED    TIME TABLE
03-03T02:34:44      0      1        2       1       0   0.011 d.t

Note: It  is important to run the pt-table-checksum tool regardless of whether or not you’ve ever skipped an event with pt-slave-restart to make sure we are having the identical data on the slave side.

5) pt-table-sync: Sync the slave with their master (synchronizes data efficiently between MySQL tables.)

Example :

[root@Tst1Slave ~]# ./pt-table-sync -dD --print  --sync-to-master 192.168.56.11
REPLACE INTO `d`.`t`(`id`, `data`) VALUES ('1', 'Test1') /*percona-toolkit src_db:d src_tbl:t src_dsn:P=3306,h=192.168.56.10 dst_db:d dst_tbl:t dst_dsn:h=192.168.56.11 lock:1 transaction:1 changing_src:1 replicate:0 bidirectional:0 pid:6435 user:root host:Tst1Slave.mysql*/;
REPLACE INTO `d`.`t`(`id`, `data`) VALUES ('2', 'Test2') /*percona-toolkit src_db:d src_tbl:t src_dsn:P=3306,h=192.168.56.10 dst_db:d dst_tbl:t dst_dsn:h=192.168.56.11 lock:1 transaction:1 changing_src:1 replicate:0 bidirectional:0 pid:6435 user:root host:Tst1Slave.mysql*/;
[root@Tst1Slave ~]#

[root@Tst1Slave ~]# ./pt-table-sync -dD  --verbose  --execute  --sync-to-master 192.168.56.11
# Syncing h=192.168.56.11
# DELETE REPLACE INSERT UPDATE ALGORITHM START    END      EXIT DATABASE.TABLE
#      0       2      0      0 Chunk     03:38:09 03:38:09 2    d.t

We have successfully sync the tables so let us try the checksum again and confirm the table is in sync.

[root@Tst1Master ~]# ./pt-table-checksum -dD
            TS ERRORS  DIFFS     ROWS  CHUNKS SKIPPED    TIME TABLE
03-03T03:03:40      0      0        2       1       0   0.111 d.t

The aforesaid are the simple examples and based on your needs, you can choose the tools, options or modify the scripts. I also recommend that you to go through the documentations for more details on each tools.

The post 5 free handy tools for monitoring and managing MySQL replication appeared first on MySQL Performance Blog.

Nov
04
2014
--

Data inconsistencies on MySQL replicas: Beyond pt-table-checksum

Percona Toolkit’s pt-table-checksum is a great tool to find data inconsistencies between a MySQL master and its replicas. However it is sometimes not enough to know that there are inconsistencies and let pt-table-sync fix the issue: you may want to know which exact rows are different to identify the statements that created the inconsistency. This post shows one way to achieve that goal.

The issue

Let’s assume you have 2 servers running MySQL 5.5: db1 the master and db2 the replica. You want to upgrade to MySQL 5.6 using an in-place upgrade and to play safe, you will upgrade db2 (the slave) first. If all goes well you will promote it and upgrade db1.

A good thing to do after upgrading db2 is to check for potential data inconsistencies with pt-table-checksum. Once checksumming is done, you can run the following query on db2 to see if there is any data drift:

mysql> SELECT db, tbl,
       SUM(this_cnt) AS total_rows,
       COUNT(*) AS chunks
FROM percona.checksums
WHERE (master_cnt <> this_cnt
       OR master_crc <> this_crc
       OR ISNULL(master_crc) <> ISNULL(this_crc))
GROUP BY db, tbl;
+------+-------+------------+--------+
| db   | tbl   | total_rows | chunks |
+------+-------+------------+--------+
| mydb | items |    3745563 |     17 |
+------+-------+------------+--------+

This indicates that inconsistencies can be found in mydb.items in 17 chunks. Now the question is: which rows are different on db1 and db2?

The solution

The previous query shows that we will find inconsistencies in 17 of the chunks pt-table-checksum used. But what is a chunk?

mysql> SELECT *
       FROM percona.checksums
       WHERE this_crc != master_crc
       AND tbl='items'G
***************** 1. row *****************
            db: mydb
           tbl: items
         chunk: 28
    chunk_time: 0.123122
   chunk_index: PRIMARY
lower_boundary: 7487511
upper_boundary: 7563474
      this_crc: 2c11da8d
      this_cnt: 75964
    master_crc: 66a1c22c
    master_cnt: 75964
            ts: 2014-10-22 01:21:26
[...]

So the first chunk with inconsistencies is chunk #28, which is the set of rows where the primary key is >= 7487511 and <= 7563474.

Let’s export all these rows on db1 and db2 instance ::

# db1
mysql> SELECT * INTO outfile '/tmp/items_db1.txt'
       FROM mydb.items
       WHERE id BETWEEN 7487511 AND 7563474;
# db2
mysql> SELECT * INTO outfile '/tmp/items_db2.txt'
       FROM mydb.items
       WHERE id BETWEEN 7487511 AND 7563474;

Then let’s use diff to isolate non-matching rows

# Using diff to compare rows
# diff items_db1.txt items_db2.txt
75872,75874c75872,75874
< 7563382 2127002 3 0 2014-10-22 02:51:33
< 7563383 2127002 4 0 2014-10-22 02:51:33
< 7563384 2127002 5 0 2014-10-22 02:51:33
---
> 7563382 2127002 3 0 2014-10-22 02:51:34
> 7563383 2127002 4 0 2014-10-22 02:51:34
> 7563384 2127002 5 0 2014-10-22 02:51:34
[...]

We can see that some datetime fields are off by 1 second on the 5.6 instance.

In this case, the binlogs showed queries like:

INSERT INTO items ([...],posted_at) VALUES ([...],'2014-10-22 02:51:33.835249');

MySQL 5.5 rounds '2014-10-22 02:51:33.835249' to '2014-10-22 02:51:33' (ignoring the fractional part), while MySQL 5.6 rounds it to '2014-10-22 02:51:34'.

Now it’s easy to fix the application so that it works both with MySQL 5.5 and 5.6 and then continue testing MySQL 5.6.

Conclusion

The method shown above is an easy way to find the exact records that are inconsistent between the MySQL master and a replica. It is not useful if you only want to resync the slave (in this case, just run pt-table-sync) but it can be a first step in understanding how inconsistencies are created.

The post Data inconsistencies on MySQL replicas: Beyond pt-table-checksum appeared first on MySQL Performance Blog.

Oct
13
2014
--

How to avoid hash collisions when using MySQL’s CRC32 function

Percona Toolkit’s  pt-table-checksum performs an online replication consistency check by executing checksum queries on the master, which produces different results on replicas that are inconsistent with the master – and the tool pt-table-sync synchronizes data efficiently between MySQL tables.

The tools by default use the CRC32. Other good choices include MD5 and SHA1. If you have installed the FNV_64 user-defined function, pt-table-sync will detect it and prefer to use it, because it is much faster than the built-ins. You can also use MURMUR_HASH if you’ve installed that user-defined function. Both of these are distributed with Maatkit. For details please see the tool’s documentation.

Below are test cases similar to what you might have encountered. By using the table checksum we can confirm that the two tables are identical and useful to verify a slave server is in sync with its master. The following test cases with pt-table-checksum and pt-table-sync will help you use the tools more accurately.

For example, in a master-slave setup we have a table with a primary key on column “a” and a unique key on column “b”. Here the master and slave tables are not in sync and the tables are having two identical values and two distinct values. The pt-table-checksum tool should be able to identify the difference between master and slave and the pt-table-sync in this case should sync the tables with two REPLACE queries.

+-----+-----+    +-----+-----+
|  a  |  b  |    |  a  |  b  |
+-----+-----+    +-----+-----+
|  2  |  1  |    |  2  |  1  |
|  1  |  2  |    |  1  |  2  |
|  4  |  3  |    |  3  |  3  |
|  3  |  4  |    |  4  |  4  |
+-----+-----+    +-----+-----+

Case 1:  Non-cryptographic Hash function (CRC32) and the Hash collision.

The tables in the source and target have two different columns and in general way of thinking the tools should identify the difference. But the below scenarios explain how the tools can be wrongly used and how to avoid them – and make things more consistent and reliable when using the tools in your production.

The tools by default use the CRC32 checksums and it is prone to hash collisions. In the below case the non-cryptographic function (CRC32) is not able to identify the two distinct values as the function generates the same value even we are having the distinct values in the tables.

CREATE TABLE `t1` (
  `a` int(11) NOT NULL,
  `b` int(11) NOT NULL,
  PRIMARY KEY (`a`),
  UNIQUE KEY `b` (`b`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Master           Slave
+-----+-----+    +-----+-----+
|  a  |  b  |    |  a  |  b  |
+-----+-----+    +-----+-----+
|  2  |  1  |    |  2  |  1  |
|  1  |  2  |    |  1  |  2  |
|  4  |  3  |    |  3  |  3  |
|  3  |  4  |    |  4  |  4  |
+-----+-----+    +-----+-----+

Master:
[root@localhost mysql]#  pt-table-checksum --replicate=percona.checksum --create-replicate-table --databases=db1 --tables=t1
localhost --user=root --password=*** --no-check-binlog-format
            TS ERRORS  DIFFS     ROWS  CHUNKS SKIPPED    TIME TABLE
09-17T00:59:45      0      0        4       1       0   1.081 db1.t1
Slave:
[root@localhost bin]# ./pt-table-sync --print --execute --replicate=percona.checksum --tables db1.t1 --user=root --password=***
 --verbose  --sync-to-master  192.**.**.**
# Syncing via replication h=192.**.**.**,p=...,u=root
# DELETE REPLACE INSERT UPDATE ALGORITHM START    END      EXIT DATABASE.TABLE

Narrowed down to BIT_XOR:

Master:
mysql> SELECT BIT_XOR(CAST(CRC32(CONCAT_WS('#', `a`, `b`)) AS UNSIGNED)) FROM `db1`.`t1`;
+------------------------------------------------------------+
| BIT_XOR(CAST(CRC32(CONCAT_WS('#', `a`, `b`)) AS UNSIGNED)) |
+------------------------------------------------------------+
|                                                    6581445 |
+------------------------------------------------------------+
1 row in set (0.00 sec)
Slave:
mysql> SELECT BIT_XOR(CAST(CRC32(CONCAT_WS('#', `a`, `b`)) AS UNSIGNED)) FROM `db1`.`t1`;
+------------------------------------------------------------+
| BIT_XOR(CAST(CRC32(CONCAT_WS('#', `a`, `b`)) AS UNSIGNED)) |
+------------------------------------------------------------+
|                                                    6581445 |
+------------------------------------------------------------+
1 row in set (0.16 sec)

Case 2: As the tools are not able to identify the difference, let us add a new row to the slave and check if the tools are able to identify the distinct values. So I am adding a new row (5,5) to the slave.

mysql> insert into db1.t1 values(5,5);
Query OK, 1 row affected (0.05 sec)
Master           Slave
+-----+-----+    +-----+-----+
|  a  |  b  |    |  a  |  b  |
+-----+-----+    +-----+-----+
|  2  |  1  |    |  2  |  1  |
|  1  |  2  |    |  1  |  2  |
|  4  |  3  |    |  3  |  3  |
|  3  |  4  |    |  4  |  4  |
+-----+-----+    |  5  |  5  |
                 +-----+-----+

[root@localhost mysql]#  pt-table-checksum --replicate=percona.checksum --create-replicate-table --databases=db1 --tables=t1
 localhost --user=root --password=*** --no-check-binlog-format
            TS ERRORS  DIFFS     ROWS  CHUNKS SKIPPED    TIME TABLE
09-17T01:01:13      0      1        4       1       0   1.054 db1.t1
[root@localhost bin]# ./pt-table-sync --print --execute --replicate=percona.checksum --tables db1.t1 --user=root --password=***
 --verbose  --sync-to-master  192.**.**.**
# Syncing via replication h=192.**.**.**,p=...,u=root
# DELETE REPLACE INSERT UPDATE ALGORITHM START    END      EXIT DATABASE.TABLE
DELETE FROM `db1`.`t1` WHERE `a`='5' LIMIT 1 /*percona-toolkit src_db:db1 src_tbl:t1 src_dsn:P=3306,h=192.**.**.**.
10,p=...,u=root dst_db:db1 dst_tbl:t1 dst_dsn:h=192.**.**.**,p=...,u=root lock:1 transaction:1 changing_src:percona.checksum
replicate:percona.checksum bidirectional:0 pid:5205 user:root host:localhost.localdomain*/;
REPLACE INTO `db1`.`t1`(`a`, `b`) VALUES ('3', '4') /*percona-toolkit src_db:db1 src_tbl:t1 src_dsn:P=3306,h=192.**.**.**,
p=...,u=root dst_db:db1 dst_tbl:t1 dst_dsn:h=192.**.**.**,p=...,u=root lock:1 transaction:1 changing_src:percona.checksum
 replicate:percona.checksum bidirectional:0 pid:5205 user:root host:localhost.localdomain*/;
REPLACE INTO `db1`.`t1`(`a`, `b`) VALUES ('4', '3') /*percona-toolkit src_db:db1 src_tbl:t1 src_dsn:P=3306,h=192.**.**.**,
p=...,u=root dst_db:db1 dst_tbl:t1 dst_dsn:h=192.**.**.**,p=...,u=root lock:1 transaction:1 changing_src:percona.checksum
replicate:percona.checksum bidirectional:0 pid:5205 user:root host:localhost.localdomain*/;
#      1       2      0      0 Chunk     01:01:43 01:01:43 2    db1.t1

Well, apparently the tools are now able to identify the newly added row in the slave and the two other rows having the difference.

Case 3: Advantage of Cryptographic Hash functions (Ex: Secure MD5)

As such let us make the tables as in the case1 and ask the tools to use the cryptographic (secure MD5) hash functions instead the usual non-cryptographic function. The default CRC32 function provides no security due to their simple mathematical structure and too prone to hash collisions but the MD5 provides better level of integrity. So let us try with the –function=md5 and see the result.

Master           Slave
+-----+-----+    +-----+-----+
|  a  |  b  |    |  a  |  b  |
+-----+-----+    +-----+-----+
|  2  |  1  |    |  2  |  1  |
|  1  |  2  |    |  1  |  2  |
|  4  |  3  |    |  3  |  3  |
|  3  |  4  |    |  4  |  4  |
+-----+-----+    +-----+-----+

Narrowed down to BIT_XOR:

Master:
mysql> SELECT 'test', 't2', '1', NULL, NULL, NULL, COUNT(*) AS cnt, COALESCE(LOWER(CONCAT(LPAD(CONV(BIT_XOR(CAST(CONV(SUBSTRING
(@crc, 1, 16), 16, 10) AS UNSIGNED)), 10, 16), 16, '0'), LPAD(CONV(BIT_XOR(CAST(CONV(SUBSTRING(@crc := md5(CONCAT_WS('#', `a`, `b`))
, 17, 16), 16, 10) AS UNSIGNED)), 10, 16), 16, '0'))), 0) AS crc FROM `db1`.`t1`;
+------+----+---+------+------+------+-----+----------------------------------+
| test | t2 | 1 | NULL | NULL | NULL | cnt | crc                              |
+------+----+---+------+------+------+-----+----------------------------------+
| test | t2 | 1 | NULL | NULL | NULL |   4 | 000000000000000063f65b71e539df48 |
+------+----+---+------+------+------+-----+----------------------------------+
1 row in set (0.00 sec)
Slave:
mysql> SELECT 'test', 't2', '1', NULL, NULL, NULL, COUNT(*) AS cnt, COALESCE(LOWER(CONCAT(LPAD(CONV(BIT_XOR(CAST(CONV(SUBSTRING
(@crc, 1, 16), 16, 10) AS UNSIGNED)), 10, 16), 16, '0'), LPAD(CONV(BIT_XOR(CAST(CONV(SUBSTRING(@crc := md5(CONCAT_WS('#', `a`, `b`))
, 17, 16), 16, 10) AS UNSIGNED)), 10, 16), 16, '0'))), 0) AS crc FROM `db1`.`t1`;
+------+----+---+------+------+------+-----+----------------------------------+
| test | t2 | 1 | NULL | NULL | NULL | cnt | crc                              |
+------+----+---+------+------+------+-----+----------------------------------+
| test | t2 | 1 | NULL | NULL | NULL |   4 | 0000000000000000df024e1a4a32c31f |
+------+----+---+------+------+------+-----+----------------------------------+
1 row in set (0.00 sec)

[root@localhost mysql]# pt-table-checksum --replicate=percona.checksum --create-replicate-table --function=md5 --databases=db1
 --tables=t1 localhost --user=root --password=*** --no-check-binlog-format
            TS ERRORS  DIFFS     ROWS  CHUNKS SKIPPED    TIME TABLE
09-23T23:57:52      0      1       12       1       0   0.292 db1.t1
[root@localhost bin]# ./pt-table-sync --print --execute --replicate=percona.checksum --tables db1.t1 --user=root --password=amma
 --verbose --function=md5 --sync-to-master  192.***.***.***
# Syncing via replication h=192.168.56.102,p=...,u=root
# DELETE REPLACE INSERT UPDATE ALGORITHM START    END      EXIT DATABASE.TABLE
REPLACE INTO `db1`.`t1`(`a`, `b`) VALUES ('3', '4') /*percona-toolkit src_db:db1 src_tbl:t1 src_dsn:P=3306,h=192.168.56.101,p=...,
u=root dst_db:db1 dst_tbl:t1 dst_dsn:h=192.***.***.***,p=...,u=root lock:1 transaction:1 changing_src:percona.checksum
replicate:percona.checksum bidirectional:0 pid:5608 user:root host:localhost.localdomain*/;
REPLACE INTO `db1`.`t1`(`a`, `b`) VALUES ('4', '3') /*percona-toolkit src_db:db1 src_tbl:t1 src_dsn:P=3306,h=192.168.56.101,p=...,
u=root dst_db:db1 dst_tbl:t1 dst_dsn:h=192.***.**.***,p=...,u=root lock:1 transaction:1 changing_src:percona.checksum
replicate:percona.checksum bidirectional:0 pid:5608 user:root host:localhost.localdomain*/;
#      0       2      0      0 Chunk     04:46:04 04:46:04 2    db1.t1

Master           Slave
+-----+-----+    +-----+-----+
|  a  |  b  |    |  a  |  b  |
+-----+-----+    +-----+-----+
|  2  |  1  |    |  2  |  1  |
|  1  |  2  |    |  1  |  2  |
|  4  |  3  |    |  4  |  3  |
|  3  |  4  |    |  3  |  4  |
+-----+-----+    +-----+-----+

The MD5 did the trick and solved the problem. See the BIT_XOR result for the MD5 given above and the function is able to identify the distinct values in the tables and resulted with the different crc values. The MD5 (Message-Digest algorithm 5) is a well-known cryptographic hash function with a 128-bit resulting hash value. MD5 is widely used in security-related applications, and is also frequently used to check the integrity but MD5() and SHA1() are very CPU-intensive with slower checksumming if chunk-time is included.

 

The post How to avoid hash collisions when using MySQL’s CRC32 function appeared first on MySQL Performance Blog.

Sep
17
2014
--

Syncing MySQL slave table with pt-online-schema-change

I recently encountered a situation in which after running Percona Toolkit’s pt-table-checksum on a customer system, 95% of the table on the MySQL master was different on the MySQL slave. Although this table was not a critical part of the infrastructure, from time to time, writes to the table from the master would break replication. Additionally, this table has about 6 million rows, and running pt-table-sync would take sometime. Rebuilding the slave from backup of the master would not be an easy option as well since the slave acts as an archive where it has a lot more data than the master.

So how did we solve it? With pt-online-schema-change and a NOOP ALTER.

pt-online-schema-change --alter 'ENGINE=INNODB' D=dbname,t=tblname

How is it possible? pt-online-schema-change works by creating a shadow copy of the original table and start copying the rows from the latter to the former. An additional set of TRIGGERs also ensures that any additional changes to existing rows after they have been copied to the shadow version will propagate.

There is little one caveat though, the 

binlog_format

  on the master would have to be ROW so the actual ROW images from the master would be copied to the slave. If your binlog_format is set to STATEMENT, you’d still end up with the same inconsistency. Since change statements will be logged as STATEMENT, the NOOP ALTER will operate on the slave table instead of copying from the master. You can configure the binlog_format for the alter process via the –set-vars option i.e. 

--set-vars 'binlog_format=ROW'

  to make it more easier!

The post Syncing MySQL slave table with pt-online-schema-change appeared first on MySQL Performance Blog.

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