SWAP(8) Maintenance Commands and Procedures SWAP(8)


swap - swap administrative interface


/usr/sbin/swap -a swapname [swaplow] [swaplen]

/usr/sbin/swap -d swapname [swaplow]

/usr/sbin/swap -l [-h | -k]

/usr/sbin/swap -s [-h]


The swap utility provides a method of adding, deleting, and monitoring
the system swap areas used by the memory manager.


The following options are supported:

-a swapname [swaplow] [swaplen]

Add the specified swap area. This option can only be used by the
superuser or by one who has assumed the Primary Administrator role.
swapname is the name of the swap area or regular file. For example,
on system running a UFS root file system, specify a slice, such as
/dev/dsk/c0t0d0s1, or a regular file for a swap area. On a system
running a ZFS file system, specify a ZFS volume, such as
/dev/zvol/dsk/rpool/swap, for a swap area. Using a regular file for
swap is not supported on a ZFS file system. In addition, you cannot
use the same ZFS volume for both the swap area and a dump device when
the system is running a ZFS root file system.

swaplow is the offset in 512-byte blocks into the file where the swap
area should begin. swaplen is the desired length of the swap area in
512-byte blocks. The value of swaplen can not be less than 16. For
example, if n blocks are specified, then (n-1) blocks would be the
actual swap length. swaplen must be at least one page in length. The
size of a page of memory can be determined by using the pagesize
command. See pagesize(1). Since the first page of a swap file is
automatically skipped, and a swap file needs to be at least one page
in length, the minimum size should be a multiple of 2 pagesize bytes.
The size of a page of memory is machine-dependent.

swaplow + swaplen must be less than or equal to the size of the swap
file. If swaplen is not specified, an area will be added starting at
swaplow and extending to the end of the designated file. If neither
swaplow nor swaplen are specified, the whole file will be used except
for the first page. Swap areas are normally added automatically
during system startup by the /sbin/swapadd script. This script adds
all swap areas which have been specified in the /etc/vfstab file; for
the syntax of these specifications, see vfstab(5).

To use an NFS or local file system swapname, you should first create
a file using mkfile(8). A local file system swap file can now be
added to the running system by just running the swap -a command. For
NFS mounted swap files, the server needs to export the file. Do this
by performing the following steps:

1. Add the following line to /etc/dfs/dfstab:

share -F nfs -o \
rw=clientname,root=clientname path-to-swap-file

2. Run shareall(8).

3. Have the client add the following line to /etc/vfstab:

server:path-to-swap-file - local-path-to-swap-file nfs \
--- local-path-to-swap-file -- swap ---

4. Have the client run mount:

# mount local-path-to-swap-file

5. The client can then run swap -a to add the swap space:

# swap -a local-path-to-swap-file

-d swapname

Delete the specified swap area. This option can only be used by the
super-user. swapname is the name of the swap file: for example,
/dev/dsk/c0t0d0s1 or a regular file. swaplow is the offset in
512-byte blocks into the swap area to be deleted. If swaplow is not
specified, the area will be deleted starting at the second page. When
the command completes, swap blocks can no longer be allocated from
this area and all swap blocks previously in use in this swap area
have been moved to other swap areas.


All sizes are scaled to a human readable format. Scaling is done by
repetitively dividing by 1024.


Write the files sizes in units of 1024 bytes.


List the status of all the swap areas. The output has five columns:


The path name for the swap area.


The major/minor device number in decimal if it is a block special
device; zeroes otherwise.


The swaplow value for the area in 512-byte blocks.


The swaplen value for the area in 512-byte blocks.


The number of 512-byte blocks in this area that are not currently

The list does not include swap space in the form of physical memory
because this space is not associated with a particular swap area.

If swap -l is run while swapname is in the process of being deleted
(by swap -d), the string INDEL will appear in a sixth column of the
swap stats.


Print summary information about total swap space usage and


The total amount of swap space in bytes currently allocated for
use as backing store.


The total amount of swap space in bytes not currently allocated,
but claimed by memory mappings for possible future use.


The total amount of swap space in bytes that is either allocated
or reserved.


The total swap space in bytes that is currently available for
future reservation and allocation.

These numbers include swap space from all configured swap areas as
listed by the -l option, as well as swap space in the form of
physical memory.


A block device up to 2^63 -1 bytes can be fully utilized for swap.


See environ(7) for descriptions of the following environment variables
that affect the execution of swap: LC_CTYPE and LC_MESSAGE.


pagesize(1), getpagesize(3C), vfstab(5), attributes(7), largefile(7),
mkfile(8), shareall(8)


For information about setting up a swap area with ZFS, see the ZFS
Administration Guide.


No check is done to determine if a swap area being added overlaps with an
existing file system.

June 13, 2021 SWAP(8)