This is only relevant if you are running Snow Owl with an embedded Elasticsearch and not connecting it to an existing cluster.
Snow Owl (with embedded Elasticsearch) uses a lot of file descriptors or file handles. Running out of file descriptors can be disastrous and will most probably lead to data loss. Make sure to increase the limit on the number of open files descriptors for the user running Snow Owl to 65,536 or higher.
For the .zip and .tar.gz packages, set ulimit -n 65536 as root before starting Snow Owl, or set nofile to 65536 in /etc/security/limits.conf.
RPM and Debian packages already default the maximum number of file descriptors to 65536 and do not require further configuration.
Most operating systems try to use as much memory as possible for file system caches and eagerly swap out unused application memory. This can result in parts of the JVM heap or even its executable pages being swapped out to disk.
Swapping is very bad for performance, and should be avoided at all costs. It can cause garbage collections to last for minutes instead of milliseconds and can cause services to respond slowly or even time out.
There are two approaches to disabling swapping. The preferred option is to completely disable swap, but if this is not an option, you can minimize swappiness.
Usually Snow Owl is the only service running on a box, and its memory usage is controlled by the JVM options. There should be no need to have swap enabled.
On Linux systems, you can disable swap temporarily by running:
To disable it permanently, you will need to edit the /etc/fstab file and comment out any lines that contain the word swap.
Another option available on Linux systems is to ensure that the sysctl value vm.swappiness is set to 1. This reduces the kernel’s tendency to swap and should not lead to swapping under normal circumstances, while still allowing the whole system to swap in emergency conditions.
Snow Owl uses a number of thread pools for different types of operations. It is important that it is able to create new threads whenever needed. Make sure that the number of threads that the Snow Owl user can create is at least 4096.
This can be done by setting ulimit -u 4096 as root before starting Snow Owl, or by setting nproc to 4096 in /etc/security/limits.conf.
The package distributions when run as services under systemd will configure the number of threads for the Snow Owl process automatically. No additional configuration is required.
Snow Owl uses a mmapfs directory by default to store its data. The default operating system limits on mmap counts is likely to be too low, which may result in out of memory exceptions.
On Linux, you can increase the limits by running the following command as root:
To set this value permanently, update the vm.max_map_count setting in /etc/sysctl.conf. To verify after rebooting, run sysctl vm.max_map_count.
The RPM and Debian packages will configure this setting automatically. No further configuration is required.
Ideally, Snow Owl should run alone on a server and use all of the resources available to it. In order to do so, you need to configure your operating system to allow the user running Snow Owl to access more resources than allowed by default.
The following settings must be considered before going to production:
Where to configure systems settings depends on which package you have used to install Snow Owl, and which operating system you are using.
When using the .zip or .tar.gz packages, system settings can be configured:
temporarily with ulimit, or
permanently in /etc/security/limits.conf.
When using the RPM or Debian packages, most system settings are set in the system configuration file. However, systems which use systemd require that system limits are specified in a systemd configuration file.
On Linux systems, ulimit can be used to change resource limits on a temporary basis. Limits usually need to be set as root before switching to the user that will run Snow Owl. For example, to set the number of open file handles (ulimit -n) to 65,536, you can do the following:
The new limit is only applied during the current session.
You can consult all currently applied limits with ulimit -a.
On Linux systems, persistent limits can be set for a particular user by editing the /etc/security/limits.conf file. To set the maximum number of open files for the snowowl user to 65,536, add the following line to the limits.conf file:
This change will only take effect the next time the snowowl user opens a new session.
When using the RPM or Debian packages, system settings and environment variables can be specified in the system configuration file, which is located in:
However, for systems which uses systemd, system limits need to be specified via systemd.
When using the RPM or Debian packages on systems that use systemd, system limits must be specified via systemd.
The systemd service file (/usr/lib/systemd/system/snowowl.service) contains the limits that are applied by default.
To override them, add a file called /etc/systemd/system/snowowl.service.d/override.conf (alternatively, you may run sudo systemctl edit snowowl which opens the file automatically inside your default editor). Set any changes in this file, such as:
Once finished, run the following command to reload units:
Package
Location
RPM
/etc/sysconfig/snowowl
Debian
/etc/default/snowowl