# Swap

Related articles

- Swap on video ram
- fstab

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 will usually be a disk partition but can also be a file. Users may create a swap space during installation of Arch Linux or at any later time should it become necessary. Swap space is generally recommended for users with less than 1 GB of RAM, but becomes more a matter of personal preference on systems with gratuitous amounts of physical RAM (though it is required for suspend-to-disk support).

To check swap status, use:

    $ swapon -s

Or:

    $ free -m

Note: There is no performance advantage to either a contiguous swap file or a partition, both are treated the same way.

## Swap partition

A swap partition can be created with most GNU/Linux partitioning tools (e.g. `fdisk`, `cfdisk`). Swap partitions are typically designated as type 82, however it is possible to use any partition type as swap.

To set up a Linux swap area, the `mkswap` command is used. For example:

    # mkswap /dev/sda2

Warning: All data on the specified partition will be lost.

The mkswap utility generates an UUID for the partition by default, use the `-U` flag in case you want to specify custom UUID:

    # mkswap -U custom_UUID /dev/sda2

To enable the device for paging:

    # swapon /dev/sda2

To enable this swap partition on boot, add an entry to fstab:

    /etc/fstab

    /dev/sda2 none swap defaults 0 0

Note: Adding an entry to fstab is optional in most cases with systemd. See the next subsection.
Note: If using an SSD with TRIM support, consider using `defaults,discard` in the swap line in fstab. If activating swap manually with swapon, using the `-d` or `--discard` parameter achieves the same. See `man 8 swapon` 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 code 82.

Note: If both generators create a systemd unit for the same swap partition, they are conflicting, resulting in an error message.

This can be solved by one of the following options:

- removing the swap entry from `/etc/fstab`
- changing the swap partition's type code from 82 to an arbitrary type code
- deleting the systemd-gpt-auto-generator, and adding its path to the NoExtract rule for pacman

## 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).

Note: The Btrfs file system does not currently support swap files. Failure to heed this warning may result in file system corruption. Though it should be noted that one may use a swap file on Btrfs if mounted through a loop device. This method will result in severely degraded swap performance.

### Swap file creation

As root use `fallocate` to create a swap file the size of your choosing (M = Megabytes, G = Gigabytes) (`dd` can also be used but will take longer). For example, creating a 512 MB swap file:

    # fallocate -l 512M /swapfile
    Or
    # 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

Edit `/etc/fstab` and add an entry for the swap file:

    /swapfile none swap defaults 0 0

### Remove swap file

Warning: swap is managed by systemd and will be reenabled by it after some time.

To remove a swap file, the current swap file must be turned off.

As root:

    # swapoff -a

Remove swap file:

    # rm -f /swapfile

## Swap with USB device

Thanks to modularity offered by Linux, we can have multiple swap partitions spread over different devices. If you have a very full hard disk, USB device can be used as partition temporally. But this method has some severe disadvantage：

- USB device is slower than hard disk.
- flash memories have limited write cycles. Using it as swap partition will kill it quickly.
- when another device is attached to the computer, no swap can be used.

To add a a USB device to SWAP, first take a USB flash and partition it with a swap partition.You can use graphical tools such as Gparted or console tools like fdisk. Make sure to label the partition as SWAP before writing the partition table.

Warning: Make sure you are writing the partition to the correct disk!

Next open `/etc/fstab`.

Now add a new entry, just under the current swap entry, which take the current swap partition over the new USB one

    UUID=... none swap defaults,pri=10 0 0

where UUID is taken from the output of the command

    ls -l /dev/disk/by-uuid/ | grep /dev/sdc1

Just replace sdc1 with your new USB swap partition. `sdb1`

Tip: We use UUID because when you attach other devices to the computer it could modify the device order

Last, add

    pri=0

in the original swap entry for teaching fstab to use HD swap only when USB is full

This guide will work for other memory such as SD cards, etc.

## Performance Tuning

Swap values can be adjusted to help performance.

### Swappiness

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. Setting this parameter to a low value will reduce swapping from RAM, and is known to improve responsiveness on many systems.

To check the current swappiness value:

    $ cat /proc/sys/vm/swappiness

To temporarily set the swappiness value:

    # sysctl vm.swappiness=10

To set the swappiness value permanently, edit a sysctl configuration file

    /etc/sysctl.d/99-sysctl.conf

    vm.swappiness=10

To test and more on why this may work, take a look at this article.

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 faster 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 `-p` (or `--priority`) parameter of swapon:

    # swapon -p 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.

Category:

- File systems