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SWAPCTL(8) System Manager's Manual SWAPCTL(8)

swapctl, swaponsystem swap management tool

swapctl -A [-p priority] [-t blk | noblk]

swapctl -a [-p priority] path

swapctl -c -p priority path

swapctl -d path

swapctl -l | -s [-k]

swapon -a | path

The swapctl program adds, removes, lists and prioritizes swap devices and files for the system. The swapon program acts the same as swapctl -a, except if swapon itself is called with -a, in which case it acts as swapctl -A.

The options are as follows:

This option causes swapctl to read the /etc/fstab file for devices and files with an “sw” type, and adds all these entries as swap devices. If no swap devices are configured, swapctl will exit with an error code.
The -a option requires that a path also be in the argument list. The path is added to the kernel's list of swap devices using the swapctl(2) system call. When using the swapon form of this command, the -a option is treated the same as the -A option, for backwards compatibility.
The -c option changes the priority of the listed swap device or file.
The -d option removes the listed path from the kernel's list of swap devices or files.
The -k option uses 1024 byte blocks instead of the default 512 byte.
The -l option lists the current swap devices and files, and their usage statistics.
The -p option sets the priority of swap devices or files to the priority argument.
The -s option displays a single line summary of current swap statistics.
blk |
This flag modifies the function of the -A option. The -t option allows the type of device to add to be specified. An argument of blk causes all block devices in /etc/fstab to be added. An argument of noblk causes all non-block devices in /etc/fstab to be added. This option is useful in early system startup, where swapping may be needed before all file systems are available, such as during disk checks of large file systems.

Lines such as the following specify swap devices in /etc/fstab:

/dev/sd1b none swap sw
d48d0e3fc1c39531.k none swap sw

The initial swap device (root disk, partition b) need not appear in /etc/fstab, though it is not an error for it to do so.

Additional flags include:

Swap devices and files may be assigned different priorities, to allow faster resources to be used first. Swap devices at the same priority are used in a round-robin fashion until there is no more space available at this priority, when the next priority level will be used. The default priority is 0, the highest. This value can be any valid integer, with higher values receiving less priority.
This option is useful for swapping to NFS files. It specifies the local mount point to mount an NFS filesystem. Typically, once this mount has succeeded, the file to be used for swapping on will be available under this point mount. For example:
server:/export/swap/client none swap sw,nfsmntpt=/swap

The swapctl utility exits 0 on success, and >0 if an error occurs.

swapctl(2), vnd(4), fstab(5), mount_nfs(8), mount_vnd(8)

The swapctl program was originally developed in NetBSD 1.3. It was ported to OpenBSD 2.6 by Tobias Weingartner. The original swapon program, provided for backwards compatibility, appeared in 4.0BSD.

The swapctl program was written by Matthew R. Green <>.

Local and remote swap files cannot be configured until the file systems they reside on are mounted read/write. The system startup scripts need to fsck(8) all local file systems before this can happen. This process requires substantial amounts of memory on some systems. If one configures no local block swap devices on a machine that has local file systems to check and rely only on swap files, the machine will have no swap space at all during system fsck(8) and may run out of real memory, causing fsck to abnormally exit and startup scripts to fail.

December 5, 2019 OpenBSD-6.8