SWAPON(8) manual page
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swapon, swapoff - enable/disable
devices and files for paging and swapping
swapon [options] [specialfile...]
swapoff [-va] [specialfile...]
swapon is used to specify devices
on which paging and swapping are to take place.
The device or file used
is given by the specialfile parameter. It may be of the form -L label or
-U uuid to indicate a device by label or uuid.
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).
- -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[=policy]
- Enable swap discards, if the swap backing device
supports the discard or trim operation. This may improve performance on
some Solid State Devices, but often it does not. The option allows one
to select between two available swap discard policies: --discard=once to
perform a single-time discard operation for the whole swap area at swapon;
or --discard=pages to discard freed swap pages before they are reused, while
swapping. If no policy is selected, the default behavior is to enable both
discard types. The /etc/fstab mount options discard, discard=once, or discard=pages
may also be used to enable discard flags.
- -e, --ifexists
- Silently skip devices
that do not exist. The /etc/fstab mount option nofail may also be 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 current running
kernel. mkswap(2)
initializes the whole device and does not check for bad
blocks.
- -h, --help
- Display help text 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 -1 and 32767. Higher numbers indicate higher priority. See
swapon(2)
for a full description of swap priorities. Add pri=value to the
option field of /etc/fstab for use with swapon -a. When priority is not defined
it defaults to -1.
- -s, --summary
- Display swap usage summary by device. Equivalent
to "cat /proc/swaps". Not available before Linux 2.1.25. This output format
is DEPRECATED in favour of --show that provides better control on output
data.
- --show [column, ...]
- Display definable device table similar to --summary
output. See --help output for a list of available columns.
- --noheadings
- Do not
print headings when displaying --show output.
- --raw
- Display --show output without
aligning table columns.
- --bytes
- Display swap size in bytes in --show output
instead of in user-friendly units.
- -U uuid
- Use the partition that has the
specified uuid.
- -v, --verbose
- Be verbose.
- -V, --version
- Display version information
and exit.
You should not use swapon on a file with holes. Swap over
NFS may not work.
swapon automatically detects and rewrites swap space signature
with old software suspend data (e.g S1SUSPEND, S2SUSPEND, ...). The problem
is that if we don’t do it, then we 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. 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.
- LIBMOUNT_DEBUG=0xffff
- enables debug output.
swapon(2)
, swapoff(2)
, fstab(5)
, init(8)
, mkswap(8)
, rc(8)
, mount(8)
/dev/sd?? standard paging devices
/etc/fstab ascii filesystem description table
The swapon command
appeared in 4.0BSD.
The swapon command is part of the util-linux
package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
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