swapon(8) - phpMan

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SWAPON(8)                             System Administration                             SWAPON(8)

       swapon, swapoff - enable/disable devices and files for paging and swapping

       Get info:
            swapon -s [-h] [-V]

            swapon [-d] [-f] [-p priority] [-v] specialfile...
            swapoff [-v] specialfile...

       Enable/disable all:
            swapon -a [-e] [-f] [-v]
            swapoff -a [-v]

       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  avail-
       able,  so  that the paging and swapping activity is interleaved across several devices and

       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

       -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 oper-
              ation.  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  dis-
              card=pages may be also used to enable discard flags.

       -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  current  running kernel.  mkswap(2) initializes the whole device and
              does not check for bad blocks.

       -h, --help
              Provide help.

       -L label
              Use the partition that has the specified label.  (For this, access to  /proc/parti-
              tions 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  avail-
              able before Linux 2.1.25.

       --show [column,column]
              Display  definable device table similar to --summary output.  See --help output for
              column list.

              Do not print headings when displaying --show output.

       --raw  Display --show output without aligning table columns.

              Display swap size in bytes in --show output instead of user friendly size and unit.
              -U uuid Use the partition that has the specified uuid.

       -v, --verbose
              Be verbose.

       -V, --version
              Display version.

       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.

              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.ker-

util-linux                                September 1995                                SWAPON(8)

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