dmsetup
low level logical volume management
Synopsis
dmsetup
clear device_name
dmsetup create device_name [-u
uuid]
[--notable|--table
<table>|
table_file]
[{--addnodeoncreate|--addnodeonresume}]
[--readahead
[+]<sectors>|auto|none]
dmsetup deps
[-o options] [device_name]
dmsetup help
[-c|-C|--columns]
dmsetup info [device_name]
dmsetup info
-c|-C|--columns
[--noheadings]
[--separator separator]
[-o fields]
[-O|--sort
sort_fields] [device_name]
dmsetup load
device_name [--table
<table>|table_file]
dmsetup ls [--target
target_type] [--exec
command] [--tree] [-o
options]
dmsetup message device_name sector message
dmsetup mknodes [device_name]
dmsetup mangle [device_name]
dmsetup reload device_name
[--table
<table>|table_file]
dmsetup wipe_table device_name
dmsetup remove
[-f|--force]
[--retry] device_name
dmsetup remove_all
[-f|--force]
dmsetup rename device_name new_name
dmsetup rename device_name
--setuuid uuid
dmsetup resume device_name
[{--addnodeoncreate|--addnodeonresume}]
[--readahead
[+]<sectors>|auto|none]
dmsetup setgeometry
device_name cyl head sect start
dmsetup splitname device_name [subsystem]
dmsetup status [--target
target_type] [device_name]
dmsetup suspend [--nolockfs]
[--noflush] device_name
dmsetup table [--target
target_type] [--showkeys]
[device_name]
dmsetup targets
dmsetup udevcomplete cookie
dmsetup udevcomplete_all [age_in_minutes]
dmsetup udevcookies
dmsetup udevcreatecookie
dmsetup udevflags cookie
dmsetup udevreleasecookie [cookie]
dmsetup version
dmsetup wait device_name [event_nr]
devmap_name
major minor
devmap_name major:minor
add an example, a script, a trick and tips
examples
# A table to join two disks together
0 1028160 linear /dev/hda 0
1028160 3903762 linear /dev/hdb 0
# A table to stripe across the two disks,
# and add the spare space from
# hdb to the back of the volume
0 2056320 striped 2 32 /dev/hda 0 /dev/hdb 0
2056320 2875602 linear /dev/hdb 1028160
description
dmsetup manages
logical devices that use the device-mapper driver. Devices
are created by loading a table that specifies a target for
each sector (512 bytes) in the logical device.
The first
argument to dmsetup is a command. The second argument is the
logical device name or uuid.
Invoking the
command as devmap_name is equivalent to
dmsetup info -c --noheadings -j
major -m minor.
options
--addnodeoncreate
Ensure /dev/mapper node exists
after dmsetup create.
--addnodeonresume
Ensure /dev/mapper node exists
after dmsetup resume (default with udev).
--checks
Perform additional checks on
the operations requested and report potential problems.
Useful when debugging scripts. In some cases these checks
may slow down operations noticeably.
-c|-C|--columns
Display output in columns
rather than as Field: Value lines.
-h|--help
Outputs a summary of the
commands available, optionally including the list of report
fields (synonym with help command).
--inactive
When returning any table
information from the kernel report on the inactive table
instead of the live table. Requires kernel driver version
4.16.0 or above.
--manglename <mangling_mode>
Mangle any character not on a
whitelist using mangling_mode when processing device-mapper
device names. The names are mangled on input and unmangled
on output where the mangling_mode is one of: none (no
mangling), hex (always do the mangling) and auto (only do
the mangling if not mangled yet, do nothing if already
mangled, error on mixed; this is used by default). Character
whitelist: 0-9, A-Z, a-z, #+-.:=@_. This whitelist is also
supported by udev. Any character not on a whitelist is
replaced with its hex value (two digits) prefixed by \x.
-j|--major major
Specify the major number.
-m|--minor minor
Specify the minor number.
-n|--noheadings
Suppress the headings line when
using columnar output.
--noopencount
Tell the kernel not to supply
the open reference count for the device.
--notable
When creating a device,
don’t load any table.
--noudevrules
Do not allow udev to manage
nodes for devices in device-mapper directory.
--noudevsync
Do not synchronise with udev
when creating, renaming or removing devices.
-o|--options
Specify which fields to
display.
--readahead [+]<sectors>|auto|none
Specify read ahead size in
units of sectors. The default value is auto which
allows the kernel to choose a suitable value automatically.
The + prefix lets you specify a minimum value which
will not be used if it is smaller than the value chosen by
the kernel. The value none is equivalent to
specifying zero.
-r|--readonly
Set the table being loaded
read-only.
--table <table>
Specify a one-line table
directly on the command line.
--udevcookie
cookie
Use cookie for udev
synchronisation.
-u|--uuid
Specify the uuid.
-y|--yes
Answer yes to all prompts
automatically.
-v|--verbose [-v|--verbose]
Produce additional output.
--verifyudev
If udev synchronisation is
enabled, verify that udev operations get performed correctly
and try to fix up the device nodes afterwards if not.
--version
Display the library and kernel
driver version.
commands
clear
device_name
Destroys the table in the inactive table slot for device_name.
create
device_name [-u uuid]
[--notable|--table
<table>|table_file]
[{--addnodeoncreate|--addnodeonresume}]
[--readahead
[+]<sectors>|auto|none]
Creates a device with the given name. If table_file or
<table> is supplied, the table is loaded and made live.
Otherwise a table is read from standard input unless --notable is
used. The optional uuid can be used in place of device_name in
subsequent dmsetup commands. If successful a device will appear
as /dev/mapper/<device-name>. See below for information on
the table format.
deps
[-o options] [device_name]
Outputs a list of devices referenced by the live table for the
specified device. Device names on output can be customised by
following options: devno (major and minor pair, used by default),
blkdevname (block device name), devname (map name for
device-mapper devices, equal to blkdevname otherwise).
help
[-c|-C|--columns]
Outputs a summary of the commands available, optionally including
the list of report fields.
info
[device_name]
Outputs some brief information about the device in the form:
State: SUSPENDED|ACTIVE, READ-ONLY
Tables present: LIVE and/or INACTIVE
Open reference count
Last event sequence number (used by wait)
Major and minor device number
Number of targets in the live table
UUID
info
-c|-C|--columns [--noheadings]
[--separator separator] [-o fields]
[-O|--sort sort_fields] [device_name]
Output you can customise. Fields are comma-separated and chosen
from the following list: name, major, minor, attr, open,
segments, events, uuid. Attributes are: (L)ive, (I)nactive,
(s)uspended, (r)ead-only, read-(w)rite. Precede the list with ’+’
to append to the default selection of columns instead of
replacing it. Precede any sort_field with - for a reverse sort on
that column.
ls
[--target target_type] [--exec
command] [--tree] [-o options]
List device names. Optionally only list devices that have at
least one target of the specified type. Optionally execute a
command for each device. The device name is appended to the
supplied command. Device names on output can be customised by
following options: devno (major and minor pair, used by default),
blkdevname (block device name), devname (map name for
device-mapper devices, equal to blkdevname otherwise). --tree
displays dependencies between devices as a tree. It accepts a
comma-separate list of options. Some specify the information
displayed against each node: device/nodevice; blkdevname; active,
open, rw, uuid. Others specify how the tree is displayed: ascii,
utf, vt100; compact, inverted, notrunc.
load|reload device_name
[--table <table>|table_file]
Loads <table> or table_file into the inactive table slot
for device_name. If neither is supplied, reads a table from
standard input.
wipe_table
device_name
Wait for any I/O in-flight through the device to complete, then
replace the table with a new table that fails any new I/O sent to
the device. If successful, this should release any devices held
open by the device’s table(s).
message
device_name sector message
Send message to target. If sector not needed use 0.
mknodes
[device_name]
Ensure that the node in /dev/mapper for device_name is correct.
If no device_name is supplied, ensure that all nodes in
/dev/mapper correspond to mapped devices currently loaded by the
device-mapper kernel driver, adding, changing or removing nodes
as necessary.
mangle
[device_name]
Ensure existing device-mapper device name is in the correct
mangled form containing only whitelisted characters (supported by
udev) and do a rename if necessary. Any character not on the
whitelist will be mangled based on the --manglename settting.
remove
[-f|--force] [--retry] device_name
Removes a device. It will no longer be visible to dmsetup. Open
devices cannot be removed except with older kernels that contain
a version of device-mapper prior to 4.8.0. In this case the
device will be deleted when its open_count drops to zero. From
version 4.8.0 onwards, if a device can’t be removed because an
uninterruptible process is waiting for I/O to return from it,
adding --force will replace the table with one that fails all
I/O, which might allow the process to be killed. If an attempt to
remove a device fails, perhaps because a process run from a quick
udev rule temporarily opened the device, the --retry option will
cause the operation to be retried for a few seconds before
failing.
remove_all
[-f|--force]
Attempts to remove all device definitions i.e. reset the driver.
Use with care! From version 4.8.0 onwards, if devices can’t be
removed because uninterruptible processes are waiting for I/O to
return from them, adding --force will replace the table with one
that fails all I/O, which might allow the process to be killed.
This also runs mknodes afterwards.
rename
device_name new_name
Renames a device.
rename
device_name --setuuid uuid
Sets the uuid of a device that was created without a uuid. After
a uuid has been set it cannot be changed.
resume
device_name
[{--addnodeoncreate|--addnodeonresume}]
[--readahead
[+]<sectors>|auto|none]
Un-suspends a device. If an inactive table has been loaded, it
becomes live. Postponed I/O then gets re-queued for processing.
setgeometry device_name cyl head sect start
Sets the device geometry to C/H/S.
splitname
device_name [subsystem]
Splits given device name into subsystem constituents. Default
subsystem is LVM.
status
[--target target_type] [device_name]
Outputs status information for each of the device’s targets. With
--target, only information relating to the specified target type
is displayed.
suspend
[--nolockfs] [--noflush] device_name
Suspends a device. Any I/O that has already been mapped by the
device but has not yet completed will be flushed. Any further I/O
to that device will be postponed for as long as the device is
suspended. If there’s a filesystem on the device which supports
the operation, an attempt will be made to sync it first unless
--nolockfs is specified. Some targets such as recent (October
2006) versions of multipath may support the --noflush option.
This lets outstanding I/O that has not yet reached the device to
remain unflushed.
table
[--target target_type] [--showkeys]
[device_name]
Outputs the current table for the device in a format that can be
fed back in using the create or load commands. With --target,
only information relating to the specified target type is
displayed. Encryption keys are suppressed in the table output for
the crypt target unless the --showkeys parameter is supplied.
targets
Displays the names and versions of the currently-loaded targets.
udevcomplete
cookie
Wake any processes that are waiting for udev to complete
processing the specified cookie.
udevcomplete_all
[age_in_minutes]
Remove all cookies older than the specified number of minutes.
Any process waiting on a cookie will be resumed immediately.
udevcookies
List all existing cookies. Cookies are system-wide semaphores
with keys prefixed by two predefined bytes (0x0D4D).
udevcreatecookie
Creates a new cookie to synchronize actions with udev processing.
The output is a cookie value. Normally we don’t need to create
cookies since dmsetup creates and destroys them for each action
automatically. However, we can generate one explicitly to group
several actions together and use only one cookie instead. We can
define a cookie to use for each relevant command by using
--udevcookie option. Alternatively, we can export this value into
the environment of the dmsetup process as DM_UDEV_COOKIE variable
and it will be used automatically with all subsequent commands
until it is unset. Invoking this command will create system-wide
semaphore that needs to be cleaned up explicitly by calling
udevreleasecookie command.
udevflags
cookie
Parses given cookie value and extracts any udev control flags
encoded. The output is in environment key format that is suitable
for use in udev rules. If the flag has its symbolic name assigned
then the output is DM_UDEV_FLAG_<flag_name>=’1’,
DM_UDEV_FLAG<flag_position>=’1’ otherwise. Subsystem udev
flags don’t have symbolic names assigned and these ones are
always reported as
DM_SUBSYSTEM_UDEV_FLAG<flag_position>=’1’. There are 16
udev flags altogether.
udevreleasecookie
[cookie]
Waits for all pending udev processing bound to given cookie value
and clean up the cookie with underlying semaphore. If the cookie
is not given directly, the command will try to use a value
defined by DM_UDEV_COOKIE environment variable.
version
Outputs version information.
wait
device_name [event_nr]
Sleeps until the event counter for device_name exceeds event_nr.
Use -v to see the event number returned. To wait until the next
event is triggered, use info to find the last event
number.
environment variables
DM_DEV_DIR
The device directory name. Defaults to "/dev" and must be an
absolute path.
DM_UDEV_COOKIE
A cookie to use for all relevant commands to synchronize with
udev processing. It is an alternative to using --udevcookie
option.
table format
Each line of the table specifies a single target and is of the
form:
logical_start_sector num_sectors target_type
<target_args>
Simple target types and <target_args> include:
linear
destination_device start_sector
The traditional linear mapping.
striped
num_stripes chunk_size [destination start_sector]+
Creates a striped area.
e.g. striped 2 32 /dev/hda1 0 /dev/hdb1 0 will map the first
chunk (16k) as follows:
LV chunk 1 -> hda1, chunk 1
LV chunk 2 -> hdb1, chunk 1
LV chunk 3 -> hda1, chunk 2
LV chunk 4 -> hdb1, chunk 2
etc.
error
Errors any I/O that goes to this area. Useful for testing or for
creating devices with holes in them.
zero
Returns blocks of zeroes on reads. Any data written is discarded
silently. This is a block-device equivalent of the /dev/zero
character-device data sink described in null(4).
More complex targets include:
crypt
Transparent encryption of block devices using the kernel crypto
API.
delay
Delays reads and/or writes to different devices. Useful for
testing.
flakey
Creates a similar mapping to the linear target but exhibits
unreliable behaviour periodically. Useful for simulating failing
devices when testing.
mirror
Mirrors data across two or more devices.
multipath
Mediates access through multiple paths to the same device.
raid
Offers an interface to the kernel’s software raid driver, md.
snapshot
Supports snapshots of devices.
To find out more about the various targets and their table
formats and status lines, please read the files in the
Documentation/device-mapper directory in the kernel source tree.
(Your distribution might include a copy of this information in
the documentation directory for the device-mapper package.)
see also
Device-mapper
resource page: http://sources.redhat.com/dm/
authors
Original
version: Joe Thornber (thornber[:at:]sistina[:dot:]com)