Why to avoid swapping

Obviously, accessing swap memory from disk is far slower than accessing RAM directly. This is particularly bad on a database server because:

  • MariaDB's internal algorithms assume that memory is not swap, and are highly inefficient if it is. Some algorithms are intended to avoid or delay disk IO, and use memory where possible - performing this with swap can be worse than just doing it on disk in the first place.
  • Swap increases IO over just using disk in the first place as pages are actively swapped in and out of swap. Even something like removing a dirty page that is no longer going to be stored in memory, while designed to improve efficiency, will under a swap situation cost more IO.
  • Database locks are particularly inefficient in swap. They are designed to be obtained and released often and quickly, and pausing to perform disk IO will have a serious impact on their usability.

The main way to avoid swapping is to make sure you have enough RAM for all processes that need to run on the machine. Setting the system variables too high can mean that under load the server run's short of memory, and needs to use swap. So understanding what settings to use and how these impact your server's memory usage is critical.

Setting swappiness on Linux

Linux has a swappiness setting which determines the balance between swapping out pages (chunks of memory) from RAM to a preconfigured swap space on the hard drive.

The setting is from 0 to 100, with lower values meaning a lower likelihood of swapping. The default is usually 60 - you can check this by running:

sysctl vm.swappiness

The default setting encourages the server to use swap. Since there probably won't be much else on the database server besides MariaDB processes to put into swap, you'll probably want to reduce this to zero to avoid swapping as much as possible. You can change the default by adding a line to the sysctl.conf file (usually found in /etc/sysctl.conf).

To set the swappiness to zero, add the line:

vm.swappiness = 0

This normally takes effect after a reboot, but you can change the value without rebooting as follows:

sysctl -w vm.swappiness=0

Since RHEL 6.4, setting swappiness=0 more aggressively avoids swapping out, which increases the risk of OOM killing under strong memory and I/O pressure.

A low swappiness setting is recommended for database workloads. For MariaDB databases, it is recommended to set swappiness to a value of 1.

vm.swappiness = 1

Disabling swap altogether

While some disable swap altogether, and you certainly want to avoid any database processes from using it, it can be prudent to leave some swap space to at least allow the kernel to fall over gracefully should a spike occur. Having emergency swap available at least allows you some scope to kill any runaway processes.

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