.BR \-f , " \-\-fixpgsz"
Reinitialize (exec mkswap) the swap space if its page size does not
match that of the current running kernel.
-.BR mkswap (2)
+.BR mkswap (8)
initializes the whole device and does not check for bad blocks.
.TP
.BR \-h , " \-\-help"
.RS
.RS
.sp
-.B "swapon -o pri=1,discard=pages,nofail /dev/sda2"
+.B "swapon \-o pri=1,discard=pages,nofail /dev/sda2"
.sp
.RE
The \fIopts\fP string is evaluated last and overrides all other
to the option field of
.I /etc/fstab
for use with
-.BR "swapon -a" .
+.BR "swapon \-a" .
When no priority is defined, it defaults to \-1.
.TP
.BR \-s , " \-\-summary"
.TP
.BR \-V , " \-\-version"
Display version information and exit.
-.SH NOTES
-You should not use \fBswapon\fR on a file with holes.
-This can be seen in the system log as
-.RS
-.sp
-.B "swapon: swapfile has holes."
-.sp
-.RE
-The swap file implementation in the kernel expects to be able to write to the
-file directly, without the assistance of the filesystem. This is a problem on
-preallocated files (e.g.
-.BR fallocate (1))
-on filesystems like \fBXFS\fR or \fBext4\fR, and on copy-on-write
-filesystems like \fBbtrfs\fR.
-.PP
-It is recommended to use
-.BR dd (1)
-and
-.I /dev/zero
-to avoid holes on XFS and ext4.
-.PP
-.B swapon
-may not work correctly when using a swap file with some versions of
-\fBbtrfs\fR. This is due to btrfs being a copy-on-write filesystem: the
-file location may not be static and corruption can result. Btrfs actively
-disallows the use of swap files on its filesystems by refusing to map the file.
-.PP
-One possible workaround is to map the swap
-file to a loopback device. This will allow the filesystem to determine the
-mapping properly but may come with a performance impact.
-.PP
-Swap over \fBNFS\fR may not work.
-.PP
-.B swapon
-automatically detects and rewrites a 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.
-
.SH ENVIRONMENT
.IP LIBMOUNT_DEBUG=all
enables libmount debug output.
.IP LIBBLKID_DEBUG=all
enables libblkid debug output.
-.SH SEE ALSO
-.BR swapoff (2),
-.BR swapon (2),
-.BR fstab (5),
-.BR init (8),
-.BR mkswap (8),
-.BR mount (8),
-.BR rc (8)
.SH FILES
.br
.I /dev/sd??
.br
.I /etc/fstab
ascii filesystem description table
+.SH NOTES
+.SS Files with holes
+The swap file implementation in the kernel expects to be able to write to the
+file directly, without the assistance of the filesystem. This is a problem on
+files with holes or on copy-on-write files on filesystems like Btrfs.
+.sp
+Commands like
+.BR cp (1)
+or
+.BR truncate (1)
+create files with holes. These files will be rejected by swapon.
+.sp
+Preallocated files created by
+.BR fallocate (1)
+may be interpreted as files with holes too depending of the filesystem.
+Preallocated swap files are supported on XFS since Linux 4.18.
+.sp
+The most portable solution to create a swap file is to use
+.BR dd (1)
+and /dev/zero.
+.SS Btrfs
+Swap files on Btrfs are supported since Linux 5.0 on files with nocow attribute.
+See the
+.BR btrfs (5)
+manual page for more details.
+.SS NFS
+Swap over \fBNFS\fR may not work.
+.SS Suspend
+.B swapon
+automatically detects and rewrites a 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.
.SH HISTORY
The
.B swapon
command appeared in 4.0BSD.
+.SH SEE ALSO
+.BR swapoff (2),
+.BR swapon (2),
+.BR fstab (5),
+.BR init (8),
+.BR fallocate (1),
+.BR mkswap (8),
+.BR mount (8),
+.BR rc (8)
.SH AVAILABILITY
The swapon command is part of the util-linux package and is available from
https://www.kernel.org/pub/linux/utils/util-linux/.
projects / thirdparty / util-linux.git / blobdiff