363 lines
8.3 KiB
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363 lines
8.3 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of SWAPON</TITLE>
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</HEAD><BODY>
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<H1>SWAPON</H1>
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Section: System Administration (8)<BR>Updated: October 2014<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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swapon, swapoff - enable/disable devices and files for paging and swapping
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>swapon</B>
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[options]
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[<I>specialfile</I>...]
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<BR>
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<B>swapoff</B>
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[<B>-va</B>]
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[<I>specialfile</I>...]
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<B>swapon</B>
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is used to specify devices on which paging and swapping are to take place.
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<P>
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The device or file used is given by the
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<I>specialfile</I>
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parameter. It may be of the form
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<B>-L</B><I> label</I>
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or
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<B>-U</B><I> uuid</I>
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to indicate a device by label or uuid.
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<P>
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Calls to
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<B>swapon</B>
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normally occur in the system boot scripts making all swap devices available, so
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that the paging and swapping activity is interleaved across several devices and
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files.
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<P>
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<B>swapoff</B>
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disables swapping on the specified devices and files.
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When the
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<B>-a</B>
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flag is given, swapping is disabled on all known swap devices and files
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(as found in
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<I>/proc/swaps</I>
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or
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<I>/etc/fstab</I>).
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<P>
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<A NAME="lbAE"> </A>
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<H2>OPTIONS</H2>
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<DL COMPACT>
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<DT id="1"><B>-a</B>,<B> --all</B>
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<DD>
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All devices marked as ``swap'' in
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<I>/etc/fstab</I>
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are made available, except for those with the ``noauto'' option.
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Devices that are already being used as swap are silently skipped.
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<DT id="2"><B>-d</B>,<B> --discard</B>[<B>=</B><I>policy</I>]
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<DD>
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Enable swap discards, if the swap backing device supports the discard or
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trim operation. This may improve performance on some Solid State Devices,
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but often it does not. The option allows one to select between two
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available swap discard policies:
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<B>--discard=once</B>
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to perform a single-time discard operation for the whole swap area at swapon;
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or
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<B>--discard=pages</B>
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to asynchronously discard freed swap pages before they are available for reuse.
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If no policy is selected, the default behavior is to enable both discard types.
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The
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<I>/etc/fstab</I>
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mount options
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<B>discard</B>,
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<B>discard=once</B>,
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or
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<B>discard=pages</B>
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may also be used to enable discard flags.
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<DT id="3"><B>-e</B>,<B> --ifexists</B>
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<DD>
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Silently skip devices that do not exist.
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The
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<I>/etc/fstab</I>
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mount option
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<B>nofail</B>
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may also be used to skip non-existing device.
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<P>
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<DT id="4"><B>-f</B>,<B> --fixpgsz</B>
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<DD>
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Reinitialize (exec mkswap) the swap space if its page size does not
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match that of the current running kernel.
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<B><A HREF="/cgi-bin/man/man2html?2+mkswap">mkswap</A></B>(2)
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initializes the whole device and does not check for bad blocks.
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<DT id="5"><B>-h</B>,<B> --help</B>
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<DD>
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Display help text and exit.
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<DT id="6"><B>-L</B><I> label</I>
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<DD>
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Use the partition that has the specified
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<I>label</I>.
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(For this, access to
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<I>/proc/partitions</I>
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is needed.)
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<DT id="7"><B>-o</B>,<B> --options </B><I>opts</I>
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<DD>
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Specify swap options by an fstab-compatible comma-separated string.
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For example:
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<DL COMPACT><DT id="8"><DD>
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<DL COMPACT><DT id="9"><DD>
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<P>
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<B>swapon -o pri=1,discard=pages,nofail /dev/sda2</B>
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<P>
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</DL>
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The <I>opts</I> string is evaluated last and overrides all other
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command line options.
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</DL>
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<DT id="10"><B>-p</B>,<B> --priority </B><I>priority</I>
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<DD>
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Specify the priority of the swap device.
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<I>priority</I>
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is a value between -1 and 32767. Higher numbers indicate
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higher priority. See
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<B><A HREF="/cgi-bin/man/man2html?2+swapon">swapon</A></B>(2)
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for a full description of swap priorities. Add
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<B>pri=</B><I>value</I>
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to the option field of
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<I>/etc/fstab</I>
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for use with
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<B>swapon -a</B>.
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When no priority is defined, it defaults to -1.
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<DT id="11"><B>-s</B>,<B> --summary</B>
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<DD>
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Display swap usage summary by device. Equivalent to "cat /proc/swaps".
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This output format is DEPRECATED in favour
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of <B>--show</B> that provides better control on output data.
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<DT id="12"><B>--show</B>[<B>=</B><I>column</I>...]
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<DD>
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Display a definable table of swap areas. See the
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<B>--help</B>
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output for a list of available columns.
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<DT id="13"><B>--output-all</B>
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<DD>
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Output all available columns.
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<DT id="14"><B>--noheadings</B>
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<DD>
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Do not print headings when displaying
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<B>--show</B>
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output.
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<DT id="15"><B>--raw</B>
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<DD>
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Display
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<B>--show</B>
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output without aligning table columns.
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<DT id="16"><B>--bytes</B>
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<DD>
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Display swap size in bytes in
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<B>--show</B>
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output instead of in user-friendly units.
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<DT id="17"><B>-U</B><I> uuid</I>
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<DD>
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Use the partition that has the specified
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<I>uuid</I>.
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<DT id="18"><B>-v</B>,<B> --verbose</B>
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<DD>
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Be verbose.
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<DT id="19"><B>-V</B>,<B> --version</B>
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<DD>
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Display version information and exit.
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</DL>
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<A NAME="lbAF"> </A>
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<H2>NOTES</H2>
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<A NAME="lbAG"> </A>
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<H3>Files with holes</H3>
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The swap file implementation in the kernel expects to be able to write to the
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file directly, without the assistance of the filesystem. This is a problem on
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files with holes or on copy-on-write files on filesystems like Btrfs.
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<P>
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Commands like
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<B><A HREF="/cgi-bin/man/man2html?1+cp">cp</A></B>(1)
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or
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<B><A HREF="/cgi-bin/man/man2html?1+truncate">truncate</A></B>(1)
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create files with holes. These files will be rejected by swapon.
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<P>
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Preallocated files created by
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<B><A HREF="/cgi-bin/man/man2html?1+fallocate">fallocate</A></B>(1)
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may be interpreted as files with holes too depending of the filesystem.
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Preallocated swap files are supported on XFS since Linux 4.18.
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<P>
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The most portable solution to create a swap file is to use
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<B><A HREF="/cgi-bin/man/man2html?1+dd">dd</A></B>(1)
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and /dev/zero.
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<A NAME="lbAH"> </A>
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<H3>Btrfs</H3>
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Swap files on Btrfs are supported since Linux 5.0 on files with nocow attribute.
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See the
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<B><A HREF="/cgi-bin/man/man2html?5+btrfs">btrfs</A></B>(5)
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manual page for more details.
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<A NAME="lbAI"> </A>
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<H3>NFS</H3>
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Swap over <B>NFS</B> may not work.
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<A NAME="lbAJ"> </A>
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<H3>Suspend</H3>
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<B>swapon</B>
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automatically detects and rewrites a swap space signature with old software
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suspend data (e.g. S1SUSPEND, S2SUSPEND, ...). The problem is that if we don't
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do it, then we get data corruption the next time an attempt at unsuspending is
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made.
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<A NAME="lbAK"> </A>
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<H2>ENVIRONMENT</H2>
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<DL COMPACT>
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<DT id="20">LIBMOUNT_DEBUG=all<DD>
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enables libmount debug output.
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<DT id="21">LIBBLKID_DEBUG=all<DD>
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enables libblkid debug output.
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<P>
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</DL>
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<A NAME="lbAL"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+swapoff">swapoff</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+swapon">swapon</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?5+fstab">fstab</A></B>(5),
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<B><A HREF="/cgi-bin/man/man2html?8+init">init</A></B>(8),
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<B><A HREF="/cgi-bin/man/man2html?1+fallocate">fallocate</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?8+mkswap">mkswap</A></B>(8),
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<B><A HREF="/cgi-bin/man/man2html?8+mount">mount</A></B>(8),
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<B><A HREF="/cgi-bin/man/man2html?8+rc">rc</A></B>(8)
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<A NAME="lbAM"> </A>
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<H2>FILES</H2>
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<BR>
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<I>/dev/sd??</I>
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standard paging devices
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<BR>
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<I>/etc/fstab</I>
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ascii filesystem description table
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<A NAME="lbAN"> </A>
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<H2>HISTORY</H2>
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The
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<B>swapon</B>
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command appeared in 4.0BSD.
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<A NAME="lbAO"> </A>
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<H2>AVAILABILITY</H2>
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The swapon command is part of the util-linux package and is available from
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<A HREF="https://www.kernel.org/pub/linux/utils/util-linux/.">https://www.kernel.org/pub/linux/utils/util-linux/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="22"><A HREF="#lbAB">NAME</A><DD>
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<DT id="23"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="24"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="25"><A HREF="#lbAE">OPTIONS</A><DD>
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<DT id="26"><A HREF="#lbAF">NOTES</A><DD>
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<DL>
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<DT id="27"><A HREF="#lbAG">Files with holes</A><DD>
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<DT id="28"><A HREF="#lbAH">Btrfs</A><DD>
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<DT id="29"><A HREF="#lbAI">NFS</A><DD>
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<DT id="30"><A HREF="#lbAJ">Suspend</A><DD>
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</DL>
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<DT id="31"><A HREF="#lbAK">ENVIRONMENT</A><DD>
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<DT id="32"><A HREF="#lbAL">SEE ALSO</A><DD>
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<DT id="33"><A HREF="#lbAM">FILES</A><DD>
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<DT id="34"><A HREF="#lbAN">HISTORY</A><DD>
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<DT id="35"><A HREF="#lbAO">AVAILABILITY</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:06:16 GMT, March 31, 2021
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</BODY>
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</HTML>
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