sbin/swapctl/swapctl.8

.\"	$NetBSD: swapctl.8,v 1.39 2007/08/02 18:53:46 he Exp $
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.\" Copyright (c) 1997 Matthew R. Green
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.Dd August 2, 2007
.Dt SWAPCTL 8
.Os
.Sh NAME
.Nm swapctl ,
.Nm swapon
.Nd system swap management tool
.Sh SYNOPSIS
.Nm
.Fl A
.Op Fl f | Fl o
.Op Fl n
.Op Fl p Ar priority
.Op Fl t Ar blk|noblk|auto
.Nm
.Fl D Ar dumpdev|none
.Nm
.Fl U
.Op Fl n
.Op Fl t Ar blk|noblk|auto
.Nm
.Fl a
.Op Fl p Ar priority
.Ar path
.Nm
.Fl c
.Fl p Ar priority
.Ar path
.Nm
.Fl d
.Ar path
.Nm
.Fl l | Fl s
.Op Fl k | Fl m | Fl g | Fl h
.Nm
.Fl q
.Nm
.Fl z
.Nm swapon
.Fl a
.Op Fl t Ar blk|noblk
.Nm swapon
.Ar path
.Sh DESCRIPTION
The
.Nm
program adds, removes,
lists and prioritizes swap devices and files for the system.
The
.Nm swapon
program acts the same as the
.Nm
program, as if called with the
.Fl a
option, except if
.Nm swapon
itself is called with
.Fl a
in which case,
.Nm swapon
acts as
.Nm
with the
.Fl A
option.
.Pp
The following options are available:
.Bl -tag -width 123456
.It Fl A
This option causes
.Nm
to read the
.Pa /etc/fstab
file for devices and files with a
.Dq sw
or
.Dq dp
type, and adds all
.Dq sw
type entries as swap devices and sets the last
.Dq dp
type entry as the dump device.
If no swap devices are configured,
.Nm
will exit with an error code.
If used together with
.Fl t Ar auto
this option will not read
.Pa /etc/fstab
but query the kernel for all swap partitions on local hard disks.
.It Fl a
The
.Fl a
option requires that a
.Ar path
also be in the argument list.
The
.Ar path
is added to the kernel's list of swap devices using the
.Xr swapctl 2
system call.
When using the
.Nm swapon
form of this command, the
.Fl a
option is treated the same as the
.Fl A
option, for backwards compatibility.
.It Fl c
The
.Fl c
option changes the priority of the listed swap device or file.
.It Fl D
The
.Fl D
option requires that a
.Ar dumpdev
also be in the argument list.
The kernel dump device is set to
.Ar dumpdev .
The word
.Dq none
can be used instead of a
.Ar dumpdev
to disable the currently set dump device.
This change is made via the
.Xr 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
.Xr savecore 8
at system reboot, and analyzed to determine the problem.
.It Fl d
The
.Fl d
option removes the listed
.Ar path
from the kernel's list of swap devices or files.
.It Fl f
Used in combination with the
.Fl A
command and
.Fl t Ar auto
flag this option makes
.Nm
use the first discovered swap device to also become the dump device.
The
.Fl f
option is mutually exclusive with the
.Fl o
option.
.It Fl g
The
.Fl g
option uses (1024 * 1024 * 1024) byte blocks instead of the default 512 byte.
.It Fl h
The
.Fl h
option uses
.Xr humanize_number 3
to display the sizes.
.It Fl k
The
.Fl k
option uses 1024 byte blocks instead of the default 512 byte.
.It Fl l
The
.Fl l
option lists the current swap devices and files, and their usage statistics.
.It Fl m
The
.Fl m
option uses (1024 * 1024) byte blocks instead of the default 512 byte.
.It Fl n
Used with the
.Fl A
or
.Fl U
command, the
.Fl n
option makes
.Nm
print the action it would take, but not actually change any swap or
dump devices.
.It Fl o
Similar to the
.Fl f
flag, this
.Dq Dump Only
option makes
.Nm
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
.Fl A Fl t Ar auto
and is mutually exclusive with
.Fl f .
.It Fl p
The
.Fl p
option sets the priority of swap devices or files to the
.Ar priority
argument.
This works with the
.\" .Fl d ,
.Fl a ,
.Fl c ,
and
.Fl l
options.
.It Fl q
Query
.Pa /etc/fstab ,
checking for any defined swap or dump devices.
If any are found,
.Nm
returns with an exit status of 0, if none are found the exit status will
be 1.
.It Fl s
The
.Fl s
option displays a single line summary of current swap statistics.
.It Fl t
This flag modifies the function of the
.Fl A
and
.Fl U
options.
The
.Fl t
option allows the type of device to add to be specified.
An argument of
.Ar blk
causes all block devices in
.Pa /etc/fstab
to be added.
An argument of
.Ar noblk
causes all non-block devices in
.Pa /etc/fstab
to be added.
An argument of
.Ar 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.
.It Fl U
This option causes
.Nm
to read the
.Pa /etc/fstab
file for devices and files with a
.Dq sw
type, and remove all these entries as swap devices.
If no swap devices are unconfigured,
.Nm
will exit with an error code.
If used together with
.Fl t Ar auto
this option will not read
.Pa /etc/fstab
but unconfigure all local swap partitions.
.It Fl z
The
.Fl z
option displays the current dump device.
.El
.Sh SWAP PRIORITY
The
.Nx
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.
.Sh SWAP OPTIONS
When parsing the
.Pa /etc/fstab
file for swap devices, the following options are recognized:
.Pp
.Bl -tag -width nfsmntpt=/path -compact
.It priority=N
This option sets the priority of the specified swap device to N.
.It 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:
.Bd -literal
server:/export/swap/client none swap sw,nfsmntpt=/swap
.Ed
.El
.Sh EXIT STATUS
If the requested operation was sucessful, the
.Nm
utility exits with status 0.
If an error occurred, the exit status is 1.
.Pp
For easy scriptability, the
.Fl z
operation (query dump device) and
.Fl 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.
.Sh SEE ALSO
.Xr swapctl 2 ,
.Xr fstab 5 ,
.Xr mount_nfs 8
.Sh HISTORY
The
.Nm
program was first made available in
.Nx 1.3 .
The original
.Nm swapon
program, provided for backwards compatibility, appeared in
.Bx 4.0 .
.Sh AUTHORS
The
.Nm
program was written by
.An Matthew R. Green Aq mrg@eterna.com.au .
.Sh CAVEATS
Using the automatic swap partition detection done by the
.Fl A Fl t Ar 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
.Nm
with the
.Fl n
option.
.Sh BUGS
If no swap information is specified in
.Pa /etc/fstab ,
the system startup scripts (see
.Xr rc 8 )
will configure no swap space and your machine will behave very badly
if (more likely when) it runs out of real memory.
.Pp
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
.Xr 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
.Xr fsck 8
and may run out of real memory, causing fsck to abnormally exit and
startup scripts to fail.