Swap
This page provides an introduction to swap space and paging on GNU/Linux. It covers creation and activation of swap partitions and swap files.
From All about Linux swap space:
- Linux divides its physical RAM (random access memory) into chunks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.
Support for swap is provided by the Linux kernel and user-space utilities from the util-linux package.
Swap space
Swap space can take the form of a disk partition or a file. Users may create a swap space during installation or at any later time as desired. Swap space can be used for two purposes, to extend the virtual memory beyond the installed physical memory (RAM), and also for suspend-to-disk support.
Whether or not it is beneficial to extend the virtual memory with swap depends on the amount of installed physical memory. If the amount of physical memory is less than the amount of memory required to run all the desired programs, then it may be beneficial to enable swap. This avoids out of memory conditions, where the Linux kernel OOM killer mechanism will automatically attempt to free up memory by killing processes. To increase the amount of virtual memory to the required amount, add the necessary difference (or more) as swap space.
The biggest drawback of using swap when running out of memory is its lower performance, see section #Performance. Hence, enabling swap is a matter of personal preference: some prefer programs to be killed over enabling swap and others prefer enabling swap and slower system when the physical memory is exhausted.
To check swap status, use:
$ swapon --show
Or to show physical memory as well as swap usage:
$ free -h
Swap partition
A swap partition can be created with most GNU/Linux partitioning tools. Swap partitions are designated as type 82 on MBR and 0657FD6D-A4AB-43C4-84E5-0933C84B4F4F on GPT.
To set up a partition as Linux swap area, the mkswap(8) command is used. For example:
# mkswap /dev/sdxy
To enable the device for paging:
# swapon /dev/sdxy
To enable this swap partition on boot, add an entry to /etc/fstab:
UUID=device_UUID none swap defaults 0 0
where the device_UUID is the UUID of the swap space.
See fstab for the file syntax. See swapon(8) for the options syntax.
The discard option is available for SSDs and SMR drives. There are two types of discard: once (discard the whole area before use) and pages (discard asynchronously as pages are swapped out); both can be used at the same time. See Solid state drive#TRIM.
Activation by systemd
systemd activates swap partitions based on two different mechanisms. Both are executables in /usr/lib/systemd/system-generators. The generators are run on start-up and create native systemd units for mounts. The first, systemd-fstab-generator, reads the fstab to generate units, including a unit for swap. The second, systemd-gpt-auto-generator inspects the root disk to generate units. It operates on GPT disks only, and can identify swap partitions by their type GUID, see systemd#GPT partition automounting for more information.
Disabling swap
To deactivate specific swap space:
# swapoff /dev/sdxy
Alternatively use the -a switch to deactivate all swap space.
Since swap is managed by systemd, it will be activated again on the next system startup. To disable the automatic activation of detected swap space permanently, run systemctl --type swap to find the responsible .swap unit and mask it.
Swap file
As an alternative to creating an entire partition, a swap file offers the ability to vary its size on-the-fly, and is more easily removed altogether. This may be especially desirable if disk space is at a premium (e.g. a modestly-sized SSD).
| File system | Supports swap files |
|---|---|
| Bcachefs | No |
| Btrfs | Yes |
| F2FS | Yes |
| ext4 | Yes |
| JFS | Yes |
| NILFS2 | No |
| NTFS3 | Yes |
| ReiserFS | Yes |
| XFS | Yes |
| ZFS | No |
Swap file creation
Use mkswap(8) to create a swap file the size of your choosing (see Partitioning#Swap for advice). For example, creating a 4 GiB swap file:
# mkswap -U clear --size 4G --file /swapfile
Activate the swap file:
# swapon /swapfile
Finally, edit the fstab configuration to add an entry for the swap file:
/etc/fstab
/swapfile none swap defaults 0 0
For additional information, see fstab#Usage.
Swap file removal
To remove a swap file, it must be turned off first and then can be removed:
# swapoff /swapfile # rm -f /swapfile
Finally, remove the relevant entry from /etc/fstab.
Compressed block device in RAM
zswap is available by default if you use a swap file or partition, but you can avoid swap files or partitions altogether by using a compressed block device in RAM, using zram. See Improving performance#zram or zswap for more information on the differences between using zram or zswap.
Swap encryption
Performance
Swap operations are usually significantly slower than directly accessing data in RAM. However, disabling swap entirely to improve performance can sometimes lead to a degradation. If there is not enough physical memory available to hold everything, swapping out nothing leaves less memory available for file system caches, causing more frequent and costly disk usage.
Swap values can be adjusted to help performance:
Swappiness
When memory usage reaches a certain threshold, the kernel starts looking at active memory and seeing what it can free up. File data can be written out to the file system (if changed), unloaded and re-loaded later; other data must be written to swap before it can be unloaded.
The swappiness sysctl parameter represents the kernel's preference for writing to swap instead of files. It can have a value between 0 and 200 (max 100 if Linux < 5.8); the default value is 60. A low value causes the kernel to prefer freeing up open files, a high value causes the kernel to try to use swap space, and a value of 100 means IO cost is assumed to be equal.
To check the current swappiness value:
$ sysctl vm.swappiness
Alternatively, the file /proc/sys/vm/swappiness can be read in order to obtain the raw integer value.
To temporarily set the swappiness value:
# sysctl -w vm.swappiness=35
To set the swappiness value permanently, create a sysctl.d(5) configuration file. For example:
/etc/sysctl.d/99-swappiness.conf
vm.swappiness = 35
To have the boot loader set swappiness when loading the kernel, add a kernel parameter, e.g. sysctl.vm.swappiness=35.
To test and more on why this may work, take a look at this article. For a more recent counter-argument, see this article.
VFS cache pressure
Another sysctl parameter that affects swap performance is vm.vfs_cache_pressure, which controls the tendency of the kernel to reclaim the memory which is used for caching of VFS caches, versus pagecache and swap. Increasing this value increases the rate at which VFS caches are reclaimed[1]. For more information, see the Linux kernel documentation.
Priority
If you have more than one swap file or swap partition you should consider assigning a priority value (0 to 32767) for each swap area. The system will use swap areas of higher priority before using swap areas of lower priority. For example, if you have a faster disk (/dev/sda) and a slower disk (/dev/sdb), assign a higher priority to the swap area located on the fastest device. Priorities can be assigned in fstab via the pri parameter:
/dev/sda1 none swap defaults,pri=100 0 0 /dev/sdb2 none swap defaults,pri=10 0 0
Or via the --priority parameter of swapon:
# swapon --priority 100 /dev/sda1
If two or more areas have the same priority, and it is the highest priority available, pages are allocated on a round-robin basis between them.
Striping
There is no necessity to use RAID for swap performance reasons. The kernel itself can stripe swapping on several devices, if you just give them the same priority in the /etc/fstab file. Refer to The Software-RAID HOWTO for details.