Swap

Related articles
fstab
Hibernation
zswap
zram
Swap on video ram
ZFS#Swap volume
dm-crypt/Swap encryption
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 by the partition type GUID
0657FD6D-A4AB-43C4-84E5-0933C84B4F4F
on GPT (
8200
type for
gdisk
,
swap
type for
fdisk
) and type ID
82
on MBR.
To set up a partition as Linux swap area, the
mkswap(8)
command is used. For example:
# mkswap /dev/sd
xy
Warning
All data on the specified partition will be lost.
To enable the device for paging:
# swapon /dev/sd
xy
See
swapon(8)
for the options syntax.
Enabling at boot
To enable the swap partition at boot time, either:
use
systemd#GPT partition automounting
or add an entry to
/etc/fstab
. E.g.:
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, and
systemd#systemd.mount - mounting
.
Disabling swap
To deactivate specific swap space:
# swapoff /dev/sd
xy
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
XFS
Yes
ZFS
No
Swap file creation
Note
For
Btrfs
, follow the procedure described in
Btrfs#Swap file
instead of the steps below.
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
As an alternative to fstab, a swap unit can be created (see
systemd.swap(5)
):
/etc/systemd/system/swapfile.swap
[Swap]
What=/swapfile

[Install]
WantedBy=swap.target
Perform a
daemon-reload
and
enable
swapfile.swap
.
For additional information, see
fstab#Usage
.
Note
The swap file must be specified by its location on the file system, not by its UUID or LABEL.
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
.
Swap encryption
See
dm-crypt/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, and 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.
Note
There is a common misconception that swappiness affects the memory threshold or prevents using swap space, but it only affects the preference for freeing up file pages over swap. See:
this article
for a more detailed explanation or the
kernel source code
where it is used.
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
.
Reasons for choosing a different swappiness can include:
Lowering the swappiness
has been suggested
for desktop responsiveness. The argument is that because a lot of perceived performance (responsiveness) is determined by how fast a program responds to user input, anonymous pages (program memory) should be kept in RAM and freeing up open files should be prioritized even at some expense of
actual
performance.
The
minimal reasonable swappiness
is 1 because a swappiness of 0 causes an extreme bias against anonymous page eviction, preventing them from being scanned for reclaiming or swapping except in the most extreme cases of memory contention. It is generally not desired to not reclaim truly unused anonymous pages.
A higher swappiness of 100 or beyond is desired (by definition) when the IO cost of swapping is equal or lower than the cost of reading from a file. This can happen when
the swap is not backed by the disk
, especially in the case of
zram
. This can also happen when swap IO is being intercepted/cached by a lower-cost mechanism such as
zswap
.
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 on what it does, see the
Linux kernel documentation
.
The default value is 100, which states that filesystem cache is about as important as the other caches, so they should be reclaimed at about an equal weight. On desktops it has been argued that
filesystem cache is more important than the other caches
because filesystem browsing times affects operation latency (perceived responsiveness) more than the other caches, resulting a suggested value of 50. On the other hand, a higher value
has been suggested
when the VFS cache holds metadata on many small files that are not touched again. For more information on tuning this parameter, see
OpenSUSE tuning guide
(which recommends experimenting and checking the types of caches via
slaptop
).
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 and a slower disk, assign a higher priority to the swap area located on the fastest device. Priorities can be assigned in
fstab
via the
pri
parameter:
UUID=f9fe0b69-a280-415d-a03a-a32752370dee none swap defaults,pri=100 0 0
UUID=d7eb6062-01c8-4367-ac1d-3bf1167de8bb 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.
Discard (a.k.a. trim)
See
Solid state drive#swap
.
Compressed block device in RAM
See
Improving performance#Swap on zram or zswap
.
Using the swap space only for hibernation
If you only need swap as a hibernation image storage space, then you can use
zswap
and disable its writeback so that there are no disk writes from regular swap usage. See
Power management/Suspend and hibernate#Disable zswap writeback to use the swap space only for hibernation
.
See also
In defence of swap: common misconceptions