Linux file descriptor limit

To ensure good server performance, the total number of client connections, database files, and log files must not exceed the maximum file descriptor limit on the operating system (ulimit -n). By default, the directory server allows an unlimited number of connections but is restricted by the file descriptor limit on the operating system. Linux systems limit the number of file descriptors that any one process may open to 1024 per process. (This condition is not a problem on Solaris machines, x86, x64, or SPARC).

After the directory server has exceeded the file descriptor limit of 1024 per process, any new process and worker threads will be blocked. For example, if the directory server attempts to open a Oracle ® Berkeley JE database file when the operating system has exceeded the file descriptor limit, the directory server will no longer be able to open a connection that can lead to a corrupted database exception. Likewise, if you have a directory server that exceeds the file descriptor limit set by the operating system, the directory server can become unresponsive as the LDAP connection handler consumes all of the CPU’s processing in attempting to open a new connection.

To fix this condition, set the maximum file descriptor limit per process on Linux machines.

To Increase the File Descriptor Limit (Linux)

Display the current hard limit of your machine.

The hard limit is the maximum server limit that can be set without tuning the kernel parameters in proc file system.

Edit the /etc/security/limits.conf and add the lines:

Edit the /etc/pam.d/login by adding the line:

Use the system file limit to increase the file descriptor limit to 65535.

The system file limit is set in /proc/sys/fs/file-max .

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Linux Increase The Maximum Number Of Open Files / File Descriptors (FD)

H ow do I increase the maximum number of open files under CentOS Linux? How do I open more file descriptors under Linux?

The ulimit command provides control over the resources available to the shell and/or to processes started by it, on systems that allow such control. The maximum number of open file descriptors displayed with following command (login as the root user).

Command To List Number Of Open File Descriptors

Use the following command command to display maximum number of open file descriptors:
cat /proc/sys/fs/file-max
Output:

75000 files normal user can have open in single login session. To see the hard and soft values, issue the command as follows:
# ulimit -Hn
# ulimit -Sn
To see the hard and soft values for httpd or oracle user, issue the command as follows:
# su — username
In this example, su to oracle user, enter:
# su — oracle
$ ulimit -Hn
$ ulimit -Sn

System-wide File Descriptors (FD) Limits

The number of concurrently open file descriptors throughout the system can be changed via /etc/sysctl.conf file under Linux operating systems.

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The Number Of Maximum Files Was Reached, How Do I Fix This Problem?

Many application such as Oracle database or Apache web server needs this range quite higher. So you can increase the maximum number of open files by setting a new value in kernel variable /proc/sys/fs/file-max as follows (login as the root):
# sysctl -w fs.file-max=100000
Above command forces the limit to 100000 files. You need to edit /etc/sysctl.conf file and put following line so that after reboot the setting will remain as it is:
# vi /etc/sysctl.conf
Append a config directive as follows:
fs.file-max = 100000
Save and close the file. Users need to log out and log back in again to changes take effect or just type the following command:
# sysctl -p
Verify your settings with command:
# cat /proc/sys/fs/file-max
OR
# sysctl fs.file-max

User Level FD Limits

The above procedure sets system-wide file descriptors (FD) limits. However, you can limit httpd (or any other users) user to specific limits by editing /etc/security/limits.conf file, enter:
# vi /etc/security/limits.conf
Set httpd user soft and hard limits as follows:
httpd soft nofile 4096
httpd hard nofile 10240
Save and close the file. To see limits, enter:
# su — httpd
$ ulimit -Hn
$ ulimit -Sn

A note about RHEL/CentOS/Fedora/Scientific Linux users

Edit /etc/pam.d/login file and add/modify the following line (make sure you get pam_limts.so):

Save and close the file.

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How to set ulimit and file descriptors limit on Linux Servers

Introduction: Challenges like number of open files in any of the production environment has become common now a day. Since many applications which are Java based and Apache based, are getting installed and configured, which may lead to too many open files, file descriptors etc. If this exceeds the default limit that is set, then one may face access control problems and file opening challenges. Many production environments come to standstill kind of situations because of this.

Luckily, we have “ulimit” command in any of the Linux based server, by which one can see/set/get number of files open status/configuration details. This command is equipped with many options and with this combination one can set number of open files. Following are step-by-step commands with examples explained in detail.

To see what is the present open file limit in any Linux System

To get open file limit on any Linux server, execute the following command,

The above number shows that user can open ‘146013’ file per user login session.

This clearly indicates that individual Linux operating systems have different number of open files. This is based on dependencies and applications which are running in respective systems.

ulimit command :

As the name suggests, ulimit (user limit) is used to display and set resources limit for logged in user.When we run ulimit command with -a option then it will print all resources’ limit for the logged in user. Now let’s run “ulimit -a” on Ubuntu / Debian and CentOS systems,

Ubuntu / Debian System,

CentOS System

As we can be seen here different OS have different limits set. All these limits can be configured/changed using “ulimit” command.

To display the individual resource limit then pass the individual parameter in ulimit command, some of parameters are listed below:

ulimit -n –> It will display number of open files limit
ulimit -c –> It display the size of core file
umilit -u –> It will display the maximum user process limit for the logged in user.
ulimit -f –> It will display the maximum file size that the user can have.
umilit -m –> It will display the maximum memory size for logged in user.
ulimit -v –> It will display the maximum memory size limit

Use below commands check hard and soft limits for number of open file for the logged in user

How to fix the problem when limit on number of Maximum Files was reached ?

Let’s assume our Linux server has reached the limit of maximum number of open files and want to extend that limit system wide, for example we want to set 100000 as limit of number of open files.

Use sysctl command to pass fs.file-max parameter to kernel on the fly, execute beneath command as root user,

Above changes will be active until the next reboot, so to make these changes persistent across the reboot, edit the file /etc/sysctl.conf and add same parameter,

save and exit file,

Run the beneath command to make above changes into effect immediately without logout and reboot.

Now verify whether new changes are in effect or not.

Use below command to find out how many file descriptors are currently being utilized:

Note:- Command “sysctl -p” is used to commit the changes without reboot and logout.

Set User level resource limit via limit.conf file

“/etc/sysctl.conf” file is used to set resource limit system wide but if you want to set resource limit for specific user like Oracle, MariaDB and Apache then this can be achieved via “/etc/security/limits.conf” file.

Sample Limit.conf is shown below,

Let’s assume we want to set hard and soft limit on number of open files for linuxtechi user and for oracle user set hard and soft limit on number of open process, edit the file “/etc/security/limits.conf” and add the following lines

Save & exit the file.

Note: In case you want to put resource limit on a group instead of users, then it can also be possible via limit.conf file, in place of user name , type @ and rest of the items will be same, example is shown below,

Verify whether new changes are in effect or not,

Note: Other majorly used command is “ lsof ” which is used for finding out “how many files are opened currently”. This command is very helpful for admins.

Conclusion:

As mentioned in the introduction section “ulimit” command is very powerful and helps one to configure and make sure application installations are smoother without any bottlenecks. This command helps in fixing many of the number of file limitations in Linux based servers.

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