This page provides an introduction to swap space and paging on GNU/Linux. It covers creation and activation of swap partitions and swap files.
- 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 thepackage.
- 1 Swap space
- 2 Swap partition
- 3 Swap file
- 4 Swap with USB device
- 5 Swap encryption
- 6 Performance
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), a.k.a "enable swap", and also for suspend-to-disk support.
If it is beneficial to enable swap depends on the amount of installed physical memory, and the amount of memory required to run all the desired programs. If the amount of physical memory is less than the required amount, then it is 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 as swap space. For example, if your programs require 7.5 GB of memory to run, and there are 4 GB of physical memory installed, add the difference of 3.5 GB in swap space. Add more swap space to account for future requirements. It is a matter of personal preference if you prefer programs to be killed over enabling swap. The biggest drawback to enabling swap is its lower performance, see section #Performance.
To check swap status, use:
$ swapon --show
$ free -h
free also indicates if memory is running short, which can be remedied by enabling swap or increasing swap.
A swap partition can be created with most GNU/Linux partitioning tools. Swap partitions are typically designated as type
82. Even though it is possible to use any partition type as swap, it is recommended to use type
82 in most cases since systemd will automatically detect it and mount it (see below).
To set up a partition as Linux swap area, the
mkswap 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 fstab:
UUID=<UUID> none swap defaults 0 0
where the <UUID> is taken from the command:
lsblk -no UUID /dev/sdxy
- The fstab-entry is optional if the swap partition is located on a device using GPT. See the next subsection.
- If using an SSD with TRIM support, consider using
defaults,discardin the swap line in fstab. If activating swap manually with swapon, using the
--discardparameter achieves the same. See for details.
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.
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.
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).
Swap file creation
# fallocate -l 512M /swapfile
# dd if=/dev/zero of=/swapfile bs=1M count=512
Set the right permissions (a world-readable swap file is a huge local vulnerability)
# chmod 600 /swapfile
After creating the correctly sized file, format it to swap:
# mkswap /swapfile
Activate the swap file:
# swapon /swapfile
Finally, edit fstab to add an entry for the swap file:
/swapfile none swap defaults 0 0
Remove swap file
To remove a swap file, it must be turned off first and then can be removed:
# swapoff -a # rm -f /swapfile
Finally remove the relevant entry from
Install the package. Set
swapfc_enabled=1 in the Swap File Chunked section of
/etc/systemd/swap.conf. Start/enable the
systemd-swap service. Visit the authors GitHub page for more information and setting up the recommended configuration.
systemd-swap[..]: WARN: swapFC: ENOSPCand no swap file is being created, you need to set
Swap with USB device
Thanks to the modularity offered by Linux, we can have multiple swap partitions spread over different devices. If you have a very full hard disk, a USB device can be used as a swap partition temporarily. However, this method has some severe disadvantages:
- A USB device is slower than a hard disk
- Flash memory has a limited number of write cycles. Using it as a swap partition can kill it quickly
To add a USB device to swap, first take a USB flash drive and partition it for swap as described in #Swap partition.
/etc/fstab and add
to the mount options of the original swap entry so that the USB swap partition will take priority over the old swap partition.
This guide will work for other memory such as SD cards, etc.
Swap operations are usually significantly slower than directly accessing data in RAM. Disabling swap entirely to improve performance can sometimes lead to a degradation, since it decreases the memory available for VFS caches, causing more frequent and costly disk I/O.
Swap values can be adjusted to help performance:
The swappiness sysctl parameter represents the kernel's preference (or avoidance) of swap space. Swappiness can have a value between 0 and 100, the default value is 60. A low value causes the kernel to avoid swapping, a higher value causes the kernel to try to use swap space. Using a low value on sufficient memory is known to improve responsiveness on many systems.
To check the current swappiness value:
$ cat /sys/fs/cgroup/memory/memory.swappiness
$ cat /proc/sys/vm/swappiness
/procis a lot less organized and is kept only for compatibility purposes, you are encouraged to use
To temporarily set the swappiness value:
# sysctl vm.swappiness=10
To set the swappiness value permanently, edit a sysctl configuration file
To test and more on why this may work, take a look at 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. For more information, see the Linux kernel documentation.
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
/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.
Using zswap or zram
Zswap is a Linux kernel feature providing a compressed write-back cache for swapped pages. This increases the performance and decreases the IO-Operations. ZRAM creates a virtual compressed Swap-file in memory as alternative to a swapfile on disc.
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.