In this blog, we’ll look at a mystery around setting the
open_files_limit
variable in MySQL and Percona Server for MySQL.
MySQL Server needs file descriptors to run. It uses them to open new connections, store tables in the cache, create temporary tables to resolve complicated queries and access persistent ones. If
mysqld
is not able to open new files when needed, it can stop functioning correctly. A common symptom of this issue is error 24:
"Too many open files"
.
The number of file descriptors
mysqld
can open simultaneously is defined by the configuration
open_files_limit
option. You would expect it to work like any other MySQL Server option: set in the configuration file, restart
mysqld
and use more or fewer descriptors. All other configuration variables work this way. But
open_files_limit
also depends on the operating system (OS) limits. This makes setting the variable more complicated.
mysqld
As a user, when you start any application it cannot have limits set to be greater than the limits defined by the operating system for the user in question. Therefore, you would intuitively expect
mysqld
to set
open_files_limit
to any value that is less than the OS limit. This is not the case, however. No matter what value you set for the
open_files_limit
variable, the OS limit is used unless it is set to “infinity”.
sveta@Thinkie:~$ ulimit -n 32000 sveta@Thinkie:$ cat /etc/my.cnf [mysqld] open-files-limit=16000 ... sveta@Thinkie:$ ./bin/mysqld & sveta@Thinkie:$ mysql -uroot Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 3 Server version: 5.7.19-17-debug-log Source distribution Copyright (c) 2009-2017 Percona LLC and/or its affiliates Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type 'help;' or 'h' for help. Type 'c' to clear the current input statement. mysql> select @@open_files_limit; +--------------------+ | @@open_files_limit | +--------------------+ | 32000 | +--------------------+ 1 row in set (0.00 sec)
The reason for this can be found in the code contained in the
mysys/my_file.c
file:
static uint set_max_open_files(uint max_file_limit)
{
struct rlimit rlimit;
uint old_cur;
DBUG_ENTER("set_max_open_files");
DBUG_PRINT("enter",("files: %u", max_file_limit));
if (!getrlimit(RLIMIT_NOFILE,&rlimit))
{
old_cur= (uint) rlimit.rlim_cur;
DBUG_PRINT("info", ("rlim_cur: %u rlim_max: %u",
(uint) rlimit.rlim_cur,
(uint) rlimit.rlim_max));
if (rlimit.rlim_cur == RLIM_INFINITY)
rlimit.rlim_cur = max_file_limit;
if (rlimit.rlim_cur >= max_file_limit)
DBUG_RETURN(rlimit.rlim_cur); /* purecov: inspected */
rlimit.rlim_cur= rlimit.rlim_max= max_file_limit;
if (setrlimit(RLIMIT_NOFILE, &rlimit))
max_file_limit= old_cur; /* Use original value */
else
{
rlimit.rlim_cur= 0; /* Safety if next call fails */
(void) getrlimit(RLIMIT_NOFILE,&rlimit);
DBUG_PRINT("info", ("rlim_cur: %u", (uint) rlimit.rlim_cur));
if (rlimit.rlim_cur) /* If call didn't fail */
max_file_limit= (uint) rlimit.rlim_cur;
}
}
DBUG_PRINT("exit",("max_file_limit: %u", max_file_limit));
DBUG_RETURN(max_file_limit);
}
Particularly these lines:
if (rlimit.rlim_cur >= max_file_limit) DBUG_RETURN(rlimit.rlim_cur); /* purecov: inspected */
This code tells
mysqld
to take the maximum value of what is specified in either the variable
open_files_limit
, or the soft system user limit.
I reported this behavior as documentation bug #87681.
mysqld_safe
mysqld_safe
has its own
open_files_limit
option. This option allows you to overwrite the system soft limit any way you want. However, on:
- Red Hat Enterprise Linux 7
- Oracle Linux 7
- CentOS 7
- SUSE Linux Enterprise Server 12
- Fedora 25 and 26
- Debian 8 or higher
- Ubuntu 16.04 LTS or higher
This option as specified under the
[mysqld_safe]
header in the configuration file is not used when you start
mysqld
as a service. To explain the reason for this behavior, we need to step back into history.
init.d
For a long time, many Linux Operating Systems used init.d to start certain commands together with the OS. The Init daemon executes scripts (usually located in the directory
/etc/init.d
) at system startup, depending on the runlevel.
The different implementations of
init.d
vary, but they have known drawbacks. For example,
init.d
starts everything sequentially. This means a new process has to wait if another has already started. This makes the startup process on multi-core machine slow. Another drawback related to our topic is that daemons started by
init.d
inherit OS limits from the root user. If a program needs to be run by another user, the switch needs to happen in the startup script itself. But the order of option files that such users read can be different, depending if they are logged in via the
ssh
,
su
or
sudo
commands.
MySQL Server
MySQL Server’s startup sequence for the service is as follow:
- <Perform another job>
- Start
mysqld_safe
as
mysql
user:
su - mysql -s /bin/bash -c "mysqld_safe > /dev/null &"
This behavior has existed at least since version 5.5.
Percona Server for MySQL
Before version 5.7, Percona Server for MySQL had a different startup sequence:
- <Perform another job>
- Start
mysqld_safe
as root and pass option
--user=mysql
to it:
"${PERCONA_PREFIX}"/bin/mysqld_safe > /dev/null 2>&1 &
With this sequence, you only need to set a hard limit for a mysql user in the file
/etc/security/limits.conf
, and
mysqld_safe
will do the rest.
In version 5.7, Percona Server for MySQL backported the startup sequence from MySQL Server. Since then, setting a hard limit on the number of open files for mysql users in
/etc/security/limits.conf
is not enough. You also need to have a row
session required pam_limits.so
in the file
/etc/pam.d/common-session
. This is needed because the startup sequence for
mysql
users changed due to the design of
init.d
.
SystemD
Linux developers performed several trials to find a better startup solution than
init.d
. Speaking for MySQL and Percona Server for MySQL startup, the most important innovation is SystemD. SystemD is becoming more and more popular. Therefore MySQL and Percona Server for MySQL do not use
init.d
on Red Hat Enterprise Linux 7, Oracle Linux 7, CentOS 7, SUSE Linux Enterprise Server 12, Fedora 25 and 26, Debian 8 or higher and Ubuntu 16.04 LTS or higher. Instead, they use SystemD.
What does this mean for MySQL users?
Scripts started by SystemD start as required by the system user from the start. Therefore they do not inherit limits from the root user and use their own limits specified in
/etc/security/limits.conf
. If you need to have your
mysqld
process limits differ from the defaults for user
mysql
, you need to set the option
LimitNOFILE
under the
[Service]
section in the service configuration file. Again, you cannot then lower this limit using
open_files_limit
option, unless you set it to
Infinity
.
Both packages
To make things more complex, Percona Server for MySQL packages for Ubuntu contain both the
mysql.server
script (used by
init.d
) and the service description for SystemD. In fact, SystemD is used after install — but you might be confused when looking at only the package files.
Conclusion
You should set the
open_files_limit
variable together with the operating system limits. You should study how
init.d
or SystemD works if you see values that you don’t expect.
How to change
open_files_limit
variable?
| Operating System | Startup daemon | Where to put configuration |
|---|---|---|
| Red Hat Enterprise Linux 7, Oracle Linux 7, CentOS 7 SUSE Linux Enterprise Server 12 Fedora 25 and 26 Debian 8+ Ubuntu 16.04 LTS+ |
SystemD |
/etc/security/limits.conf and /etc/pam.d/common-session Service configuration: sudo systemctl edit mysql [mysqld] section of the configuration file |
| Others | init.d |
/etc/security/limits.conf and /etc/pam.d/common-session [mysqld_safe] section of the configuration file [mysqld] section of the configuration file |
Which values of
open_files_limit
variable make sense?
| Soft User Limit |
open_files_limit range |
|---|---|
| Infinity | Any |
| Positive | Greater/equal than soft user limit and smaller than hard user limit |
Percona announces the release of 
