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

NAME

swap - swap administrative interface

SYNOPSIS

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


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


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


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

DESCRIPTION

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

OPTIONS

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.


-h

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


-k

Write the files sizes in units of 1024 bytes.


-l

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

path

The path name for the swap area.


dev

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


swaplo

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


blocks

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


free

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

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.


-s

Print summary information about total swap space usage and
availability:

allocated

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


reserved

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


used

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


available

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.

USAGE

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

ENVIRONMENT VARIABLES

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

SEE ALSO

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

NOTES

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

WARNINGS

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

June 13, 2021 SWAP(8)