NAME
swapctl,
swapon —
system swap management tool
SYNOPSIS
swapctl |
-A [-f |
-o]
[-n]
[-p
priority]
[-t
blk|noblk|auto] |
swapctl |
-U [-n]
[-t
blk|noblk|auto] |
swapctl |
-a [-p
priority] path |
swapctl |
-c -p priority
path |
swapctl |
-l | -s
[-k | -m |
-g | -h] |
DESCRIPTION
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 option.
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 command, the
-a option is treated the same as the
-A option, for backwards compatibility.
-
-
- -c
- The -c option changes the priority of the
listed swap device or file.
-
-
- -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 currently 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 problem.
-
-
- -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 byte.
-
-
- -l
- The -l option lists the current swap
devices and files, and their usage statistics.
-
-
- -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 dump
devices.
-
-
- -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 combination 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 statistics.
-
-
- -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 partitions 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 file systems.
-
-
- -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.
SWAP PRIORITY
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
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.
SWAP OPTIONS
When parsing the
/etc/fstab file for swap devices, the
following options are recognized:
- 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 filesystem. The mount
point must exist as a directory. 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
EXIT STATUS
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 found.
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 configured. If any swap partition is available or a dump
device is set, the respective query returns 0.
SEE ALSO
swapctl(2),
fstab(5),
mount_nfs(8)
HISTORY
The
swapctl program was first made available in
NetBSD 1.3. The original
swapon
program, provided for backwards compatibility, appeared in
4.0BSD.
AUTHORS
The
swapctl program was written by
Matthew R.
Green
<
mrg@eterna.com.au>.
CAVEATS
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 autoconfiguration is to use
swapctl with the
-n option.
BUGS
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
memory, causing fsck to abnormally exit and startup scripts to fail.