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SWAPCTL(8) NetBSD System Manager's Manual SWAPCTL(8)
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swapctl, swapon -- system swap management tool
swapctl -A [-f | -o] [-n] [-p priority] [-t blk|noblk|auto]
swapctl -D dumpdev|none
swapctl -U [-n] [-t blk|noblk|auto]
swapctl -a [-p priority] path
swapctl -c -p priority path
swapctl -d path
swapctl -l | -s [-k | -m | -g | -h]
swapon -a [-t blk|noblk]
The swapctl program adds, removes, lists, and prioritizes swap devices
and files for the system. The swapon program acts the same as the
swapctl program as if called with the -a option, except if swapon itself
is called with -a in which case swapon acts as swapctl with the -A
The following options are available:
-A This option causes swapctl to read the /etc/fstab file for
devices and files with a ``sw'' or ``dp'' type, and adds all
``sw'' type entries as swap devices and sets the last ``dp'' type
entry as the dump device. If no swap devices are configured,
swapctl will exit with an error code. If used together with -t
auto this option will not read /etc/fstab but query the kernel
for all swap partitions on local hard disks.
-a 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 com-
mand, the -a option is treated the same as the -A option, for
-c The -c option changes the priority of the listed swap device or
-D The -D option requires that a dumpdev also be in the argument
list. The kernel dump device is set to dumpdev. The word
``none'' can be used instead of a dumpdev to disable the cur-
rently set dump device. This change is made via the swapctl(2)
system call. The dump device is used when the system crashes to
write a current snapshot of real memory, to be saved later with
savecore(8) at system reboot, and analyzed to determine the prob-
-d The -d option removes the listed path from the kernel's list of
swap devices or files.
-f Used in combination with the -A command and -t auto flag this
option makes swapctl use the first discovered swap device to also
become the dump device. The -f option is mutually exclusive with
the -o option.
-g The -g option uses (1024 * 1024 * 1024) byte blocks instead of
the default 512 byte.
-h The -h option uses humanize_number(3) to display the sizes.
-k The -k option uses 1024 byte blocks instead of the default 512
-l The -l option lists the current swap devices and files, and their
-m The -m option uses (1024 * 1024) byte blocks instead of the
default 512 byte.
-n Used with the -A or -U command, the -n option makes swapctl print
the action it would take, but not actually change any swap or
-o Similar to the -f flag, this ``Dump Only'' option makes swapctl
find the first swap device and configure it as dump device. No
swap device is changed. This option needs to be used in combina-
tion with -A -t auto and is mutually exclusive with -f.
-p The -p option sets the priority of swap devices or files to the
priority argument. This works with the -a, -c, and -l options.
-q Query /etc/fstab, checking for any defined swap or dump devices.
If any are found, swapctl returns with an exit status of 0, if
none are found the exit status will be 1.
-s The -s option displays a single line summary of current swap sta-
-t This flag modifies the function of the -A and -U options. 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. An argument of auto causes all swap par-
titions on local hard disks to be used. 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
-U This option causes swapctl to read the /etc/fstab file for
devices and files with a ``sw'' type, and remove all these
entries as swap devices. If no swap devices are unconfigured,
swapctl will exit with an error code. If used together with -t
auto this option will not read /etc/fstab but unconfigure all
local swap partitions.
-z The -z option displays the current dump device.
The NetBSD swap system allows different swap devices and files to be
assigned different priorities, to allow the faster resources to be used
first. Swap devices at the same priority are used in a round-robin fash-
ion until there is no more space available at this priority, when the
next priority level will be used. The default priority is 0, the high-
est. This value can be any valid integer, with higher values receiving
When parsing the /etc/fstab file for swap devices, the following options
priority=N This option sets the priority of the specified swap
device to N.
nfsmntpt=/path This option is useful for swapping to NFS files. It
specifies the local mount point to mount an NFS filesys-
tem. The mount point must exist as a directory. Typi-
cally, once this mount has succeeded, the file to be used
for swapping on will be available under this point mount.
server:/export/swap/client none swap sw,nfsmntpt=/swap
If the requested operation was successful, the swapctl utility exits with
status 0. If an error occurred, the exit status is 1.
The -A and -U operations (add or remove swap devices listed in fstab(5))
return an exit status of 2 to report that no suitable swap devices were
The -z operation (query dump device) and -l (list swap partitions) return
an exit status of 1 if no dump device or swap partition has been config-
ured. If any swap partition is available or a dump device is set, the
respective query returns 0.
swapctl(2), fstab(5), mount_nfs(8)
The swapctl program was first made available in NetBSD 1.3. The original
swapon program, provided for backwards compatibility, appeared in 4.0BSD.
The swapctl program was written by Matthew R. Green <firstname.lastname@example.org>.
Using the automatic swap partition detection done by the -A -t auto
option may be dangereous. Depending on the on-disk partitioning scheme
used, the type of a partition may not be accurately recognizable as a
swap partition. The autodetection might recognize and use partitions on
removable media like USB sticks. An easy way to test the autoconfigura-
tion is to use swapctl with the -n option.
If no swap information is specified in /etc/fstab, the system startup
scripts (see rc(8)) will configure no swap space and your machine will
behave very badly if (more likely when) it runs out of real memory.
Local and remote swap files cannot be configured until after 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
you configure 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 mem-
ory, causing fsck to abnormally exit and startup scripts to fail.
NetBSD 10.99 June 1, 2011 NetBSD 10.99