Linux and Unix swapon and swapoff command
About swapon and swapoff
swapon is used to specify devices on which paging and swapping are to take place.
Calls to swapon normally occur in the system boot scripts making all swap devices available, so that the paging and swapping activity is interleaved across several devices and files.
swapoff disables swapping on the specified devices and files. When the -a flag is given, swapping is disabled on all known swap devices and files (as found in /proc/swaps or /etc/fstab).
swapoff and swapon syntax
To get swap information:
swapon -s [-h] [-V]
To enable a swap area:
swapon [-d] [-f] [-p priority] [-v] specialfile...
To disable a swap area:
swapoff [-v] specialfile...
To enable all swap areas:
swapon -a [-e] [-f] [-v]
To disable all swap areas:
swapoff -a [-v]
|-a, --all||All devices marked as "swap" in /etc/fstab are made available, except for those with the "noauto" option. Devices that are already being used as swap are silently skipped.|
|-d, --discard||Discard freed swap pages before they are reused, if the swap device supports the discard or trim operation. This may improve performance on some Solid State Devices, but often it does not. The /etc/fstab mount option discard may be also used to enable discard flag.|
|-e, --ifexists||Silently skip devices that do not exist. The /etc/fstab mount option nofail may be also used to skip non-existing device.|
|-f, --fixpgsz||Reinitialize (exec /sbin/mkswap) the swap space if its page size does not match that of the the current running kernel. mkswap initializes the whole device and does not check for bad blocks.|
|-h, --help||Display a help message, and exit.|
|-L label||Use the partition that has the specified label. (For this, access to /proc/partitions is needed.)|
|-p, --priority priority||Specify the priority of the swap device. priority is a value between 0 and 32767. Higher numbers indicate higher priority. See swapon for a full description of swap priorities. Add pri=value to the option field of /etc/fstab for use with swapon -a.|
|-s, --summary||Display swap usage summary by device. Equivalent to "cat /proc/swaps". Not available before Linux 2.1.25.|
|-U uuid||Use the partition that has the specified UUID uuid.|
|-v, --verbose||Use verbose output.|
|-V, --version||Display version information, and exit.|
You should not use swapon on a file with holes (not contiguously represented on disk). Swap over NFS may not work. swapon automatically detects and rewrites swap space signature with old software suspend data (e.g. S1SUSPEND, S2SUSPEND, ...); otherwise, you can get data corruption the next time an attempt at unsuspending is made.
swapon may not work correctly when using a swap file with some versions of btrfs (B-Tree file system). This is due to the swap file implementation in the kernel expecting to be able to write to the file directly, without the assistance of the file system. Since btrfs is a copy-on-write file system, the file location may not be static and corruption can result. btrfs actively disallows the use of files on its file systems by refusing to map the file. This can be seen in the system log as "swapon: swapfile has holes." One possible workaround is to map the file to a loopback device. This will allow the file system to determine the mapping properly but may come with a performance impact.
swapoff and swapon examples
Enable the swap partition /dev/sdc1.