Feb
12
2018
--

Webinar Thursday, February 15, 2018: Basic Internal Troubleshooting Tools for MySQL Server

Troubleshooting Tools for MySQL

Troubleshooting Tools for MySQLPlease join Percona’s Principal Support Engineer, Sveta Smirnova, as she presents “Basic Internal Troubleshooting Tools for MySQL Server” on Thursday, February 15, 2018, at 10:00 am PST (UTC-8) / 1:00 pm EST (UTC-5).

 

MySQL Server has many built-in troubleshooting tools. They are always available and can provide many insights on what is happening internally. Many graphical tools, such as Percona Monitoring and Management (PMM), use built-ins to get data for their nice graphs.

Even if you are only going to use graphical tools, it is always good to know what data they can collect. This way, you can see their limitations and won’t have incorrect expectations in the heat of battle. Built-in troubleshooting tools are accessible via SQL commands. Most of them are standard across the server, but details are component-specific.

In this webinar, I will discuss how to use them, how to troubleshoot component-specific issues and how to find additional information. I will cover SHOW commands, Information Schema, status variables and few component-specific syntaxes. I will NOT cover Performance Schema (there will be a separate webinar on that), and I will use PMM graphs to illustrate the topics whenever possible.

Register for the Basic Internal Troubleshooting Tools for MySQL Server webinar now.

Internal Troubleshooting for MySQLSveta Smirnova, Principal Technical Services Engineer

Sveta joined Percona in 2015. Her main professional interests are problem-solving, working with tricky issues, bugs, finding patterns that can solve typical issues quicker. She likes teaching others how to deal with MySQL issues, bugs and gotchas effectively. Before joining Percona Sveta worked as Support Engineer in MySQL Bugs Analysis Support Group in MySQL AB-Sun-Oracle. She is the author of book “MySQL Troubleshooting” and JSON UDF functions for MySQL.

May
20
2016
--

Introduction to Troubleshooting Performance – Troubleshooting Slow Queries webinar: Q & A

Troubleshooting Slow Queries

Troubleshooting Slow QueriesIn this blog, I will provide answers to the Q & A for the Troubleshooting Slow Queries webinar.

First, I want to thank you for attending the April 28 webinar. The recording and slides for the webinar are available here. Below is the list of your questions that I wasn’t able to answer during the webinar, with responses:

Q: I’ve heard that is a bad idea to use

select *

; what do you recommend?

A: When I used

SELECT *

 in my slides, I wanted to underline the idea that sometimes you need to select all columns from the table. There is nothing bad about it if you need them.

SELECT *

 is bad when you need only a few columns from the table. In this case, you retrieve more data than needed, which affects performance. Another issue that  

SELECT *

 can cause is if you hard-code the statement into your application, then change table definition; the application could start retrieving columns in wrong order and output (e.g., email instead of billing address). Or even worse, it will try to access a non-existent index in the result set array. The best practice is to explicitly enumerate all columns that your application needs.

Q: I heard that using 

index_field

 length will affect the indexing principle during query execution (e.g., one field is varchar and its length is not fixed, some values have short text, some values have long text. at this time). If we use this field as indexing, what happens?

A: I assume you are asking about the ability to create an index with lengths smaller than the column length? They work as follows:

Assume you have a 

TEXT

  field which contains these user questions:

  1. I’ve heard that is a bad idea to use select * what do you recommend?
  2. I heard that using 
    index_field

     length will affect the indexing principle during query execution (e.g., one field is varchar and its length is not fixed, some values have short text, some values have long text. at this time). If we use this field as indexing, what happens?

  3. ….

Since this is a 

TEXT

  field you cannot create and index on it without specifying its length, so you need to make the index as minimal as possible to uniquely identify questions. If you create an index with length 10 it will contain:

  1. I’ve heard
  2. I heard th

You will index only those parts of questions that are not very distinct from each other, and do not contain useful information about what the question is. You can create index of length 255:

  1. I’ve heard that is a bad idea to use select * what do you recommend?
  2. I heard that using index_field length will affect the indexing principle during query execution (e.g., one field is varchar and its length is not fixed, some values have short text, some values have long text. at this time). If we use this field as index

In this case, the index includes the whole first question and almost all the second question. This makes the index too large and requires us to use more disk space (which causes more IO). Also, information from the second question is probably too much.

If make index of length 75, we will have:

  1. I’ve heard that is a bad idea to use select * what do you recommend?
  2. I heard that using index_field length will affect the indexing principle du

This is more than enough for the first question and gives a good idea of what is in the second question. It also potentially will have enough unique entries to make its cardinality look more like the cardinality of real data distribution.

To conclude: choosing the correct index length is something that requires practice and analysis of your actual data. Try to make them as short as possible, but long enough so that the number of unique entries in the index will be similar to a number of unique entries in the table.

Q: Which view can we query to see stats?

A: Do you mean index statistics?

SHOW INDEX FROM table_name

 will do it.

Q: We have an InnoDB table with 47 fields (mostly text); some are ft-indexed. I tried to do an alter table, and it ran for 24 hours. What is the best way to run an alter table to add one extra field? The table has 1.9 M rows and 47 columns with many indexes.

A: Adding a column requires a table copy. Therefore, the speed of this operation depends on the table size and speed of your disk. If you are using version 5.6 and later, adding a column would not block parallel queries (and therefore is not a big deal). If you are using an older version, you can always use the pt-online-schema-change utility from Percona Toolkit. However, it will run even more slowly than the regular

ALTER TABLE

. Unfortunately, you cannot speed up the execution of

ALTER TABLE

 much. The only thing that you can do is to use a faster disk (with options, tuned to explore speed of the disk).

However, if you do not want to have this increased IO affect the production server, you can alter the table on the separate instance, then copy tablespace to production and then apply all changes to the original table from the binary logs. The steps will be something like:

  1. Ensure you use option
    innodb_file_per_table

      and the big table has individual tablespace

  2. Ensure that binary log is enabled
  3. Start a new server (you can also use an existent stand-by slave).
  4. Disable writes to the table
  5. Record the binary log position
  6. Copy the tablespace to the new server as described here.
  7. Enable writes on the production server
  8. Run
    ALTER TABLE

     on the new server you created in step 2 (it will still take 24 hours)

  9. Stop writes to the table on the production server
  10. Copy the tablespace, altered in step 7
  11. Apply all writes to this table, which are in the binary logs after position, recorded in step 4.
  12. Enable writes to the table

This scenario will take even more time overall, but will have minimal impact on the production server

Q: If there is a compound index like index1(emp_id,date), will the following query be able to use index? “select * from table1 where emp_id = 10”

A: Yes. At least it should.

Q: Are 

filesort

 and

temporary

 in extended info for explain not good?

A: Regarding

filesort

: it depends. For example, you will always have the word

filesort

” for tables which perform 

ORDER BY

 and cannot use an index for

ORDER BY

. This is not always bad. For example, in this query:

mysql> explain select emp_no, first_name from employees where emp_no <20000 order by first_nameG
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: employees
   partitions: NULL
         type: range
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 4
          ref: NULL
         rows: 18722
     filtered: 100.00
        Extra: Using where; Using filesort
1 row in set, 1 warning (0,01 sec)

the primary key used to resolve rows and

filesort

 were necessary and not avoidable. You can read about different

filesort

 algorithms here.

Regarding

Using temporary

: this means what during query execution temporary table will be created. This is can be not good, especially if the temporary table is large and cannot fit into memory. In this case, it would be written to disk and slow down operations. But, again, sometimes creating temporary tables in not avoidable, for example, if you have both

GROUP BY

 and

ORDER BY

 clauses which list columns differently as stated in the user manual.

Q: Is

key_len

 length more of a good thing for query execution?

A:

key_len

 field is not

NULL

 for all queries that use and index, and just shows the length of the key part used. It is not good or bad, it is just for information. You can use this information, for example, to identify which part of combined index is used to resolve the query.

Q: Does an alter query go for an optimizer check?

A: No. You can check it either by enabling optimizer trace, running

ALTER

 and find what trace is empty. Or by enabling the debug option and searching the resulting trace for

optimize

.

Q: A query involves four columns that are all individually covered by an index. The optimizer didn’t merge indexes because of cost, and even didn’t choose the composite index I created.

A: This depends on the table definition and query you used. I cannot provide a more detailed answer based only on this information.

Q cont.: Finally, only certain composite indexes were suitable, the column order in the complex index mattered a lot. Why couldn’t the optimizer merge the four individual single column indexes, and why did the order of the columns in the composite index matter?

A: Regarding why the optimizer could not merge four indexes, I need to see how the table is defined and which data is in these indexed columns. Regarding why the order of the columns in the composite index matters, it depends on the query. Why the optimizer can use an index, say, on

(col1, col2)

 where the conditions

col1=X AND col2=Y

 and

col2=Y AND col2=X

 for the case when you use

OR

, the order is important. For example, for the condition

col1=X OR col2=Y

, where the part

col1=X

 is always executed and the part

col2=Y

  is executed only when

col1=X

 is false. The same logic applies to queries like

SELECT col1 WHERE col2=Y ORDER BY col3

. See the user manual for details.

Q: When I try to obtain the optimizer trace on the console, the result is cut off. Even if I redirect the output to a file, how to overcome that?

A: Which version of MySQL Server do you use? The 

TRACE

 column is defined as

longtext NOT NULL

, and should not cause such issues. If it does with a newer version, report a bug at http://bugs.mysql.com/.

Q: Are there any free graphical visualizers for either EXPLAIN or the optimizer TRACE output?

A: There is graphical visualizer for

EXPLAIN

 in MySQL Workbench. But it works with online data only: you cannot run it on

EXPLAIN

 output, saved into a file. I don’t know about any visualizer for the optimizer

TRACE

 output. However, since it is

JSON

 you can simply save it to file and open in web browser. It will allow a better view than if opened in simple text editor.

Q: When do you use force index instead of

use index

 hints?

A: According to user manual “

USE INDEX (index_list)

 hint tells MySQL to use only one of the named indexes to find rows in the table” and “

FORCE INDEX

  hint acts like

USE INDEX (index_list)

, with the addition that a table scan is assumed to be very expensive . . . a table scan is used only if there is no way to use one of the named indexes to find rows in the table.” This means that when you use

USE INDEX

, you are giving a hint for the optimizer to prefer a particular index to others, but not enforcing index usage if the optimizer prefers a table scan, while

FORCE INDEX

 requires using the index. I myself use only

FORCE

 and

IGNORE

  index hints.

Q: Very informative session. I missed the beginning part. Are you going to distribute the recoded session later?

A: Yes. As usual slides and recording available here.

Dec
30
2015
--

Database Performance Webinar: Tired of MySQL Making You Wait?

Performance

database performance

Too often developers and DBAs struggle to pinpoint the root cause of MySQL database performance issues, and then spend too much time in trying to fix them. Wouldn’t it be great to bypass wasted guesswork and get right to the issue?

In our upcoming webinar Tired of MySQL Making You Wait? we’re going to help you discover how to significantly increase the performance of your applications and reduce database response time.

In this webinar, Principal Architect Alexander Rubin and Database Evangelist Janis Griffin will provide the key steps needed to identify, prioritize, and improve query performance.

They will discuss the following topics:

  • Wait time analytics using Performance / Information schemas
  • Monitoring for performance using DPA
  • Explaining plan operations focusing on temporary tables and filesort
  • Using indexes to optimize your queries
  • Using loose and tight index scans in MySQL

WHEN:

Thursday, January 7, 2016 10:00am Pacific Standard Time (UTC – 8)

PRESENTERS:

Alexander RubinPrincipal Consultant, Percona

Janis GriffinDatabase Evangelist, SolarWinds

Register now!

Percona is the only company that delivers enterprise-class software, support, consulting and managed services solutions for both MySQL and MongoDB® across traditional and cloud-based platforms that maximize application performance while streamlining database efficiencies.

Percona’s industry-recognized performance experts can maximize your database, server and application performance, lower infrastructure costs, and provide capacity and scalability planning for future growth.

Aug
24
2015
--

Advanced Query Tuning in MySQL 5.6 and MySQL 5.7 Webinar: Q&A

Thank you for attending my July 22 webinar titled “Advanced Query Tuning in MySQL 5.6 and 5.7” (my slides and a replay available here). As promised here is the list of questions and my answers (thank you for your great questions).

Q: Here is the explain example:

mysql> explain extended select id, site_id from test_index_id where site_id=1
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: test_index_id
         type: ref
possible_keys: key_site_id
          key: key_site_id
      key_len: 5
          ref: const
         rows: 1
     filtered: 100.00
        Extra: Using where; Using index

why is site_id a covered index for the query, given the fact that a) we are selecting “id”, b) key_site_id only contains site_id?

As the table is InnoDB, all secondary keys will always contain primary key (“id”); in this case the secondary index will contain all needed information to satisfy the above query and key_site_id will be “covered index”

Q: Applications change over time. Do you suggest doing a periodic analysis of indexes that are being used and drop the ones that are not? If yes, any suggestions as to tackle that?

Yes, that is a good idea. Usually it can be done easily with Percona toolkit or Performance_schema in MySQL 5.6

  1. Enable slow query log and log every query, then use Pt-index-usage tool
  2. Or use the following query (as suggested by FromDual blog post):
SELECT object_schema, object_name, index_name
  FROM performance_schema.table_io_waits_summary_by_index_usage
 WHERE index_name IS NOT NULL
   AND count_star = 0
 ORDER BY object_schema, object_name;

Q: Does the duplicate index is found on 5.6/5.7 will that causes an performance impact to the db while querying?

Duplicate keys can have negative impact on selects:

  1. MySQL can get confused and choose a wrong index
  2. Total index size can grow, which can cause MySQL to run out of RAM

Q: What is the suggested method to measure performance on queries (other than the slow query log) so as to know where to create indexes?

Slow query log is most common method. In MySQL 5.6 you can also use Performance Schema and use events_statements_summary_by_digest table.

Q: I’m not sure if this was covered in the webinar but… are there any best-practices for fulltext indexes?

That was not covered in this webinar, however, I’ve done a number of presentations regarding Full Text Indexes. For example: Creating Geo Enabled Applications with MySQL 5.6

Q: What would be the limit on index size or number of indexes you can defined per table?

There are no limits on Index size on disk, however, it will be good (performance wise) to have active indexes fit in RAM.

In InnoDB there are a number of index limitations, i.e. a table can contain a maximum of 64 secondary indexes.

Q:  If a table has two columns you would like to sum, can you have that sum indexed as a calculated index? To add to that, can that calculated index have “case when”?

Just to clarify, this is only a feature of MySQL 5.7 (not released yet).

Yes, it is documented now:

CREATE TABLE triangle (
  sidea DOUBLE,
  sideb DOUBLE,
  sidec DOUBLE AS (SQRT(sidea * sidea + sideb * sideb))
);

Q: I have noticed that you created indexes on columns like DayOfTheWeek with very low cardinality. Shouldn’t that be a bad practice normally?

Yes, you are right! Unless, you are doing queries like “select count(*) from … where DayOfTheWeek = 7” those indexes may not be very useful.

Q: I saw an article that if you don’t specify a primary key upfront mysql / innodb creates one in the background (hidden). Is it different from a primary key itself, if most of the where fields that are used not in the primary / semi primary key? And is there a way to identify the tables with the hidden primary key indexes?

The “hidden” primary key will be 6 bytes, which will also be appended (duplicated) to all secondary keys. You can create an INT primary key auto_increment, which will be smaller (if you do not plan to store more than 4 billion rows). In addition, you will not be able to use the hidden primary key in your queries.

The following query (against information_schema) can be used to find all tables without declared primary key (with “hidden” primary key):

SELECT tables.table_schema, tables.table_name, tables.table_rows
FROM information_schema.tables
LEFT JOIN (
  SELECT table_schema, table_name
  FROM information_schema.statistics
  GROUP BY table_schema, table_name, index_name
  HAVING
    SUM(
      CASE WHEN non_unique = 0 AND nullable != 'YES' THEN 1 ELSE 0 END
    ) = COUNT(*)
) puks
ON tables.table_schema = puks.table_schema AND tables.table_name = puks.table_name
WHERE puks.table_name IS NULL
AND tables.table_type = 'BASE TABLE' AND engine='InnoDB'

You may also use mysql.innodb_index_stats table to find rows with the hidden primary key:

Example:

mysql> select * from mysql.innodb_index_stats;
+---------------+------------+-----------------+---------------------+--------------+------------+-------------+-----------------------------------+
| database_name | table_name | index_name      | last_update         | stat_name    | stat_value | sample_size | stat_description                  |
+---------------+------------+-----------------+---------------------+--------------+------------+-------------+-----------------------------------+
| test          | t1         | GEN_CLUST_INDEX | 2015-08-08 20:48:23 | n_diff_pfx01 | 96         | 1           | DB_ROW_ID                         |
| test          | t1         | GEN_CLUST_INDEX | 2015-08-08 20:48:23 | n_leaf_pages | 1          | NULL        | Number of leaf pages in the index |
| test          | t1         | GEN_CLUST_INDEX | 2015-08-08 20:48:23 | size         | 1          | NULL        | Number of pages in the index      |
+---------------+------------+-----------------+---------------------+--------------+------------+-------------+-----------------------------------+

Q: You are using the alter table to create index, but how does mysql sort the data for creating the index? isn’t it uses temp table for that?

That is a very good question: the behavior of the “alter table … add index” has changed over time. As documented in Overview of Online DDL:

Historically, many DDL operations on InnoDB tables were expensive. Many ALTER TABLE operations worked by creating a new, empty table defined with the requested table options and indexes, then copying the existing rows to the new table one-by-one, updating the indexes as the rows were inserted. After all rows from the original table were copied, the old table was dropped and the copy was renamed with the name of the original table.

MySQL 5.5, and MySQL 5.1 with the InnoDB Plugin, optimized CREATE INDEX and DROP INDEX to avoid the table-copying behavior. That feature was known as Fast Index Creation

When MySQL uses “Fast Index Creation” operation it will create a set of temporary files in MySQL’s tmpdir:

To add a secondary index to an existing table, InnoDB scans the table, and sorts the rows using memory buffers and temporary files in order by the values of the secondary index key columns. The B-tree is then built in key-value order, which is more efficient than inserting rows into an index in random order.

Q: How good is InnoDB deadlocks on 5.7 comparing to 5.6 version. Is that based on parameters setup?

InnoDB deadlocks discussion is outside of the scope of this presentation. Valerii Kravchuk and Nilnandan Joshi did an excellent talk at Percona Live 2015 (slides available): Understanding Innodb Locks and Deadlocks

Q: What is the performance impact of generating a virtual column for a table having 66 Million records and generating the index. And how would you go about it? Do you have any suggestions on how to re organize indexes on the physical disk?

As MySQL 5.7 is not released yet, behavior of the virtual columns may change.  The main question here is: will it be online operations to a) add a virtual column (as this is only metadata change it should be very light operation anyway). b) add index on that virtual column. In the labs released it was not online, however this can change.

Thank you again for attending.

The post Advanced Query Tuning in MySQL 5.6 and MySQL 5.7 Webinar: Q&A appeared first on Percona Data Performance Blog.

Jun
29
2015
--

Practical MySQL Performance Optimization (July 2 webinar)

Percona MySQL webinarsApplications often become impacted by MySQL performance. Optimization is the obvious solution but where to start? Join me on July 2 at 11 a.m. Pacific and I’ll show you how to optimize MySQL in a practical way – spending time on what gives you the best return. Many of you have heard me talk on this topic before and this updated presentation will get you up to speed on the latest areas to consider when improving MySQL performance.

I’ll be pointing out the most important things you need to get right for each of the following:

  • Hardware
  • MySQL Configuration
  • Schema and Queries
  • Application Architecture

I’ll also share some tools and techniques to focus on the queries most important for your application. At the end of this webinar, aptly titled “Practical MySQL Performance Optimization,” you will know how to optimize MySQL performance in a practical way to ensure our system is “good enough” in terms of responsiveness, efficiency and scalability.

I hope to see you there! Register now to reserve your spot.

The post Practical MySQL Performance Optimization (July 2 webinar) appeared first on MySQL Performance Blog.

Apr
21
2015
--

Considering Sharding with MySQL? Join my April 22 webinar. Questions welcome!

Sharding with MySQLMySQL sharding is one of the most used and surely the most abused MySQL scaling technology. My April 2 Dzone article, “To Shard, or Not to Shard,” proved there is indeed quite an interest in this topic.

As such, I’m hosting a live webinar tomorrow (April 22) that will shed light on questions about sharding with MySQL. It’s titled: To Shard or Not to Shard That is the Question!

I’ll be answering questions such as:

  • Is sharding right for your application or should you use other scaling technologies?
  • If you’re sharding, what things do you need to consider and which questions do you need to have answered?
  • What kind of specific technologies can assist you with sharding?

I hope you can make it for this April 22 webinar. It starts at 10 a.m. Pacific time. Please register now and bring your questions, as sharing them with me and the other attendees is half of the fun of live webinars. :)

Or if you prefer, share your questions about sharding with MySQL in the comments section below, and I’ll do my best to answer them. I’ll be writing a followup post that will include all questions and my answers soon. A recording of this webinar along with my slides will also be available here afterwards.

The post Considering Sharding with MySQL? Join my April 22 webinar. Questions welcome! appeared first on MySQL Performance Blog.

Apr
21
2015
--

Considering Sharding with MySQL? Join my April 22 webinar. Questions welcome!

Sharding with MySQLMySQL sharding is one of the most used and surely the most abused MySQL scaling technology. My April 2 Dzone article, “To Shard, or Not to Shard,” proved there is indeed quite an interest in this topic.

As such, I’m hosting a live webinar tomorrow (April 22) that will shed light on questions about sharding with MySQL. It’s titled: To Shard or Not to Shard That is the Question!

I’ll be answering questions such as:

  • Is sharding right for your application or should you use other scaling technologies?
  • If you’re sharding, what things do you need to consider and which questions do you need to have answered?
  • What kind of specific technologies can assist you with sharding?

I hope you can make it for this April 22 webinar. It starts at 10 a.m. Pacific time. Please register now and bring your questions, as sharing them with me and the other attendees is half of the fun of live webinars. :)

Or if you prefer, share your questions about sharding with MySQL in the comments section below, and I’ll do my best to answer them. I’ll be writing a followup post that will include all questions and my answers soon. A recording of this webinar along with my slides will also be available here afterwards.

The post Considering Sharding with MySQL? Join my April 22 webinar. Questions welcome! appeared first on MySQL Performance Blog.

Mar
13
2015
--

Q&A: Multi-threaded Replication in MySQL 5.6 and MySQL 5.7

My webinar “Multi-threaded Replication in MySQL 5.6 and 5.7″ on February 25 generated several excellent questions following the presentation (available here for playback along with the slides). I didn’t have time to answer many of the questions during the session and so in this post I answer all of them. Thanks to everyone who attended!

Q&A: Multi-threaded Replication in MySQL 5.6 and MySQL 5.7Q: What do you expect from MTS with logical clock? Do you think performance would be good as with per database?
A: MTS with 5.6 is not usable if you have a single database. I do not have numbers, but this is quite frequent. With 5.7 everyone should be able to benefit from multi-threaded replication.

Q: When MySQL 5.6 was released, performance of MTS was lower, than in 5.5, for example. Is this addressed now?
A: I am not sure which specific issue or bug you are referring, but if your data is spread across several databases

Q: How does Percona XtraBackup work with MTS? What are the changes in mysqldump?
A: As long as you are using GTIDs, you can safely take a backup from a slave using multi-threaded replication: with XtraBackup, add the --slave-info option as usual when taking a backup from a slave and with mysqldump, use --master-data instead of --dump-slave.

Q: For checkpoint position, what if MTS thread apply Insert before creating table where it inserting data. How MTR checkpoint will keep track of these transactions applying by different thread on slave?
A: The worker threads track all execution gaps to make sure that out-of-order execution is safe and to be able to replay all events without forgetting any of them. So it is not possible that a worker thread will insert data in a table that has not been created yet.

Q: Can you use MTS with all binlog_format options?
A: Yes

Q: Is there any way to have the threads work so that no database contention happens?
A: The short answer is no: the goal of the worker threads is to execute the incoming transactions as fast as possible. If that results in database contention, you should probably decrease the number of worker threads.

Q: Why doesn’t multi-threaded replication perform well on a single DB?
A: With 5.6, parallelization is based on isolating the transactions going to each database. If you only have a single DB, no parallelization is possible. You should look at 5.7 and the logical clock algorithm.

Q: Are there any implications with regards to GTIDs and Multi-Threaded replication when running a Master-to-Master setup?
A: I cannot think of any, however I am not sure master-master replication is still very relevant when using GTIDs.

Q: Is there any inconvenience with memory or cache when using more workers than the number of databases?
A: If the number of workers is just a bit higher than the number of databases (like 5 workers for 3 databases), there should not be any issue. However with ridiculously high numbers (500 workers for 2 databases), there might be performance degradation. I have not tested such cases, so I cannot give a good answer there. However the idea is that the number of workers should be close to the number of databases and should exceed the number of cores on the server.

Q: Is there multi-threaded replication in MySQL 5.7?
A: Yes, multi-threaded replication is available in MySQL 5.7 and offers improvements compared to MySQL 5.6 (mainly the parallelization with logical clock).

Q: Have you used DIM_STAT to created load and measure SLAVE Lag? Any interesting take-a-ways from that effort?
A: I used sysbench to generate load and Seconds_Behind_Master from SHOW SLAVE STATUS to measure slave lag. That mainly shows that if your workload is a good fit for MTS (multiple databases and load shared evenly across ), performance benefits can be significant.

Q: Does multi-threaded replication also work with Percona XtraDB Cluster/Percona Server?
A: Percona Server 5.6 is based on MySQL 5.6, so you can use multi-threaded replication exactly as you would use it on MySQL 5.6.

On Percona XtraDB Cluster, it is a bit different: replication inside the cluster uses Galera replication, which has nothing to do with MySQL replication. Note that Galera has offered parallel replication from the beginning (parallel applying of the replicated writesets to be accurate). However if you are using asynchronous replicas, these replicas can benefit from multi-threaded replication if they are running on MySQL/Percona Server 5.6.

Q: What happens to cross db transactions? Do they not replicate?
A: These transactions will replicate, but they will have to wait until all preceding transactions have been executed. Stated differently, cross db transactions introduce serialization, so you should avoid them as much as possible if you want to benefit from parallel applying.

To be accurate, if you have db1, db2 and db3 and if you execute a transaction involving db1 and db2, transactions on db3 can still be applied in parallel. So if you have many databases, cross db transactions may not be that bad.

Q: When using MTS without GTIDs, is “Seconds_Behind_Master” from SHOW SLAVE STATUS valid?
A: Seconds_Behind_Master is based on Exec_Master_Log_Pos. And with MTS, Exec_Master_Log_Pos is not reliable as it indicates the position of the latest checkpoint and not the position of the latest executed transaction. However in practice, checkpoints will happen at least every 300ms by default, so Seconds_Behind_Master is still a good indication of the replication lag. Of course you should keep in mind the usual limitations, such as with multi-tiered replication (if the setup is A->B->C, C will report its lag against B, not against A) or when there is a replication error (then Seconds_Behind_Master is NULL).

Q: How can all the servers be realistically restarted at the same time? There could be a few sec intervals if you have multiple servers [That was when I explained how to enable GTID replication].
A: With MySQL 5.6, the requirements are pretty strict when it comes to enabling GTIDs: all servers must be restarted at the same point in time in the replication stream. As you mention, it is difficult if you have several servers, so the only viable solution is: stop the writes on the master, wait until replication has caught up on all slaves, stop all servers, change the configuration, restart all servers.

What it means is that there is a time range when all servers are down. This is a showstopper for many people, and that’s why Percona Server 5.6 now includes a patch from Facebook that allows an online migration to GTIDs and that’s why MySQL 5.7.6 also offers this option.

* * *

Thanks for all of the great questions – and I hope to see you next month at OpenStack Live and the Percona Live MySQL Conference and Expo 2015 (April 13-16) – both at the Santa Clara conference center in sunny Silicon Valley. Get more info here.

The post Q&A: Multi-threaded Replication in MySQL 5.6 and MySQL 5.7 appeared first on MySQL Performance Blog.

Jan
08
2015
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Managing data using open source technologies? Learn what’s hot in 2015!

Whether you’re looking at the overall MySQL ecosystem or the all-data management landscape, the choice of technologies has never been larger than it is in 2015.

Having so many options is great but it also can be very hard to make a selection. I’m going to help narrow the list next week during a Webinar titled, “Open Source Technologies you should evaluate in 2015,” January 14 at 10 a.m. PST.

During the hour I’ll share which technologies I think worthy of consideration in 2015 – open source and proprietary technologies that allow you to manage your data easier, increase development pace, scale better and improve availability and security. I’ll also discuss recent developments in MySQL, NoSQL and NewSQL, Cloud and general advances in hardware.

Open source technologies you should evaluate in 2015Specifically, some of the areas I’ll address will include:

  • Cloud-based Database as a Service (DBaaS) such as Amazon RDS for MySQL, Amazon RDS for Aurora, Google Cloud, and OpenStack Trove
  • MySQL 5.7
  • Hybrid database environments with MySQL plus MongoDB or other NoSQL solutions
  • Advanced Monitoring capabilities such as Percona Cloud Tools
  • Other performance enhancements such as solid state devices (SSD) and the TokuDB storage engine

I hope to see you next week! (Register now to reserve your spot!)

The post Managing data using open source technologies? Learn what’s hot in 2015! appeared first on MySQL Performance Blog.

Sep
03
2014
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Migrating to Percona XtraDB Cluster 2014 edition: Sept. 10 MySQL webinar

Join Jay Janssen Sept. 10 at 10 a.m. PDT and learn how to migrate to Percona XtraDB Cluster 5.6Join me online next week (September 10 at 10 a.m. PDT) for my live webinar on Migrating to Percona XtraDB Cluster.  This was a popular webinar that I gave a few years ago, so I’m doing it again with updates for Percona XtraDB Cluster 5.6 (PXC) and all the latest in the Galera world.

This webinar will be really good for people interested in getting an overview of what PXC/Galera is, what it would take to adopt it for your application, and some of the differences and challenges it brings compared with a conventional MySQL Master/slave setup.  I’d highly suggest attending if you are considering Galera in your environment and want to get a better understanding of its uses and antipatterns.

Additionally, I’ll cover such questions as:

  • What are the requirements for running Percona XtraDB Cluster?
  • Will I have to reload all my tables?
  • How does configuration for the cluster differ from configuring a stand-alone InnoDB server?
  • How should my application interact with the Cluster?
  • Can I use Percona XtraDB Cluster if I only have two MySQL servers currently?
  • How can I move to the Cluster and keep downtime to a minimum?
  • How can I migrate to Percona XtraDB Cluster gradually?

I hope to see you next Wednesday. And please feel free to ask questions in advance in the comments section below. Next week’s live event, like all of our MySQL webinars, is free. Register here!

The post Migrating to Percona XtraDB Cluster 2014 edition: Sept. 10 MySQL webinar appeared first on MySQL Performance Blog.

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