path: root/man/dmsetup.8_main

.TH DMSETUP 8 "Apr 06 2006" "Linux" "MAINTENANCE COMMANDS"
.
.SH NAME
.
dmsetup \(em low level logical volume management
.
.SH SYNOPSIS
.
.\".nh
.ad l
.PD 0
.HP 9
.B dmsetup
.de CMD_CLEAR
.  BR clear
.  IR device_name
..
.CMD_CLEAR
.
.HP
.B dmsetup
.de CMD_CREATE
.  ad l
.  nh
.  BR create
.  IR device_name
.  RB [ -n | --notable |\: --table
.  IR table |\: table_file ]
.  RB [ --readahead
.  RB [ + ] \fIsectors |\: auto | none ]
.  RB [ -u | --uuid
.  IR uuid ]
.  RB [ --addnodeoncreate |\: --addnodeonresume ]
.  hy
.  ad b
..
.CMD_CREATE
.
.HP
.B dmsetup
.de CMD_CREATE_CONCISE
.  ad l
.  BR create
.  BR --concise
.  RI [ concise_device_specification ]
.  ad b
..
.CMD_CREATE_CONCISE
.
.HP
.B dmsetup
.de CMD_DEPS
.  ad l
.  BR deps
.  RB [ -o
.  IR options ]
.  RI [ device_name ...]
.  ad b
..
.CMD_DEPS
.
.HP
.B dmsetup
.de CMD_HELP
.  BR help
.  RB [ -c | -C | --columns ]
..
.CMD_HELP
.
.HP
.B dmsetup
.de CMD_INFO
.  BR info
.  RI [ device_name ...]
..
.CMD_INFO
.
.HP
.B dmsetup
.de CMD_INFOLONG
.  ad l
.  nh
.  BR info
.  BR -c | -C | --columns
.  RB [ --count
.  IR count ]
.  RB [ --interval
.  IR seconds ]
.  RB [ --noheadings ]
.  RB [ -o
.  IR fields ]
.  RB [ -O | --sort
.  IR sort_fields ]
.  RB [ --nameprefixes ]
.  RB [ --separator
.  IR separator ]
.  RI [ device_name ]
.  hy
.  ad b
..
.CMD_INFOLONG
.
.HP
.B dmsetup
.de CMD_LOAD
.  ad l
.  BR load
.  IR device_name
.  RB [ --table
.  IR table | table_file ]
.  ad b
..
.CMD_LOAD
.
.HP
.B dmsetup
.de CMD_LS
.  ad l
.  nh
.  BR ls
.  RB [ --target
.  IR target_type ]
.  RB [ -o
.  IR options ]
.  RB [ --exec
.  IR command ]
.  RB [ --tree ]
.  hy
.  ad b
..
.CMD_LS
.
.HP
.B dmsetup
.de CMD_MANGLE
.  BR mangle
.  RI [ device_name ...]
..
.CMD_MANGLE
.
.HP
.B dmsetup
.de CMD_MEASURE
.  BR measure
.  RI [ device_name ...]
..
.CMD_MEASURE
.
.HP
.B dmsetup
.de CMD_MESSAGE
.  BR message
.  IR device_name
.  IR sector
.  IR message
..
.CMD_MESSAGE
.
.HP
.B dmsetup
.de CMD_MKNODES
.  BR mknodes
.  RI [ device_name ...]
..
.CMD_MKNODES
.
.HP
.B dmsetup
.de CMD_RELOAD
.  ad l
.  BR reload
.  IR device_name
.  RB [ --table
.  IR table | table_file ]
.  ad b
..
.CMD_RELOAD
.
.HP
.B dmsetup
.de CMD_REMOVE
.  ad l
.  BR remove
.  RB [ -f | --force ]
.  RB [ --retry ]
.  RB [ --deferred ]
.  IR device_name ...
.  ad b
..
.CMD_REMOVE
.
.HP
.B dmsetup
.de CMD_REMOVE_ALL
.  BR remove_all
.  RB [ -f | --force ]
.  RB [ --deferred ]
..
.CMD_REMOVE_ALL
.
.HP
.B dmsetup
.de CMD_RENAME
.  BR rename
.  IR device_name
.  IR new_name
..
.CMD_RENAME
.
.HP
.B dmsetup
.de CMD_RENAME_UUID
.  BR rename
.  IR device_name
.  BR --setuuid
.  IR uuid
..
.CMD_RENAME_UUID
.
.HP
.B dmsetup
.de CMD_RESUME
.  ad l
.  BR resume
.  IR device_name ...
.  RB [ --addnodeoncreate | --addnodeonresume ]
.  RB [ --noflush ]
.  RB [ --nolockfs ]
.  RB \%[ --readahead
.  RB \%[ + ] \fIsectors | auto | none ]
.  ad b
..
.CMD_RESUME
.
.HP
.B dmsetup
.de CMD_SETGEOMETRY
.  ad l
.  nh
.  BR setgeometry
.  IR device_name
.  IR cyl
.  IR head
.  IR sect
.  IR start
.  hy
.  ad b
..
.CMD_SETGEOMETRY
.
.HP
.B dmsetup
.de CMD_SPLITNAME
.  BR splitname
.  IR device_name
.  RI [ subsystem ]
..
.CMD_SPLITNAME
.
.HP
.B dmsetup
.de CMD_STATS
.  BR stats
.  IR command
.  RI [ options ]
..
.CMD_STATS
.
.HP
.B dmsetup
.de CMD_STATUS
.  ad l
.  BR status
.  RB [ --target
.  IR target_type ]
.  RB [ --noflush ]
.  RI [ device_name ...]
.  ad b
..
.CMD_STATUS
.
.HP
.B dmsetup
.de CMD_SUSPEND
.  ad l
.  nh
.  BR suspend
.  RB [ --nolockfs ]
.  RB [ --noflush ]
.  IR device_name ...
.  hy
.  ad b
..
.CMD_SUSPEND
.
.HP
.B dmsetup
.de CMD_TABLE
.  ad l
.  nh
.  BR table
.  RB [ --concise ]
.  RB [ --target
.  IR target_type ]
.  RB [ --showkeys ]
.  RI [ device_name ...]
.  hy
.  ad b
..
.CMD_TABLE
.
.HP
.B dmsetup
.de CMD_TARGETS
.  BR targets
..
.CMD_TARGETS
.
.HP
.B dmsetup
.de CMD_UDEVCOMPLETE
.  BR udevcomplete
.  IR cookie
..
.CMD_UDEVCOMPLETE
.
.HP
.B dmsetup
.de CMD_UDEVCOMPLETE_ALL
.  BR udevcomplete_all
.  RI [ age_in_minutes ]
..
.CMD_UDEVCOMPLETE_ALL
.
.HP
.B dmsetup
.de CMD_UDEVCOOKIES
.  BR udevcookie
..
.CMD_UDEVCOOKIES
.
.HP
.B dmsetup
.de CMD_UDEVCREATECOOKIE
.  BR udevcreatecookie
..
.CMD_UDEVCREATECOOKIE
.
.HP
.B dmsetup
.de CMD_UDEVFLAGS
.  BR udevflags
.  IR cookie
..
.CMD_UDEVFLAGS
.
.HP
.B dmsetup
.de CMD_UDEVRELEASECOOKIE
.  BR udevreleasecookie
.  RI [ cookie ]
..
.CMD_UDEVRELEASECOOKIE
.
.HP
.B dmsetup
.de CMD_VERSION
.  BR version
..
.CMD_VERSION
.
.HP
.B dmsetup
.de CMD_WAIT
.  ad l
.  nh
.  BR wait
.  RB [ --noflush ]
.  IR device_name
.  RI [ event_nr ]
.  hy
.  ad b
..
.CMD_WAIT
.
.HP
.B dmsetup
.de CMD_WIPE_TABLE
.  ad l
.  nh
.  BR wipe_table
.  IR device_name ...
.  RB [ -f | --force ]
.  RB [ --noflush ]
.  RB [ --nolockfs ]
.  hy
.  ad b
..
.CMD_WIPE_TABLE
.
.TP
\ \&
.
.TP
.B devmap_name \fImajor minor
.TP
.B devmap_name \fImajor:minor
.PD
.ad b
.
.SH 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.
.P
The first argument to dmsetup is a command.
The second argument is the logical device name or uuid.
.P
Invoking the dmsetup tool as \fBdevmap_name\fP
(which is not normally distributed and is supported
only for historical reasons) is equivalent to
.BI \%dmsetup\ info\ -c\ --noheadings\ -j \ major\  -m \ minor \c
\fR.
.\" dot above here fixes -Thtml rendering for next HP option
.
.SH OPTIONS
.
.TP
.B --addnodeoncreate
Ensure \fI/dev/mapper\fP node exists after \fBdmsetup create\fP.
.
.TP
.B --addnodeonresume
Ensure \fI/dev/mapper\fP node exists after \fBdmsetup\ resume\fP (default with udev).
.
.TP
.B --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.
.
.TP
.BR -c | -C | --columns
Display output in columns rather than as Field: Value lines.
.
.TP
.B --count \fIcount
Specify the number of times to repeat a report. Set this to zero
continue until interrupted.  The default interval is one second.
.
.TP
.BR -f | --force
Try harder to complete operation.
.
.TP
.BR -h | --help
Outputs a summary of the commands available, optionally including
the list of report fields (synonym with \fBhelp\fP command).
.
.TP
.B --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.
.
.TP
.B --interval \fIseconds
Specify the interval in seconds between successive iterations for
repeating reports. If \fB--interval\fP is specified but \fB--count\fP
is not, reports will continue to repeat until interrupted.
The default interval is one second.
.
.TP
.BR --manglename " " auto | hex | none
Mangle any character not on a whitelist using mangling_mode when
processing device-mapper device names and UUIDs. The names and UUIDs
are mangled on input and unmangled on output where the mangling mode
is one of:
\fBauto\fP (only do the mangling if not mangled yet, do nothing
if already mangled, error on mixed),
\fBhex\fP (always do the mangling) and
\fBnone\fP (no mangling).
Default mode is \fB#DEFAULT_MANGLING#\fP.
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.
Mangling mode could be also set through
\fBDM_DEFAULT_NAME_MANGLING_MODE\fP
environment variable.
.
.TP
.BR -j | --major " " \fImajor
Specify the major number.
.
.TP
.BR -m | --minor " " \fIminor
Specify the minor number.
.
.TP
.BR -n | --notable
When creating a device, don't load any table.
.
.TP
.B --nameprefixes
Add a "DM_" prefix plus the field name to the output.  Useful with
\fB--noheadings\fP to produce a list of
field=value pairs that can be used to set environment variables
(for example, in
.BR udev (7)
rules).
.
.TP
.B --noheadings
Suppress the headings line when using columnar output.
.
.TP
.B --noflush
Do not flush outstanding I/O when suspending a device, or do not
commit thin-pool metadata when obtaining thin-pool status.
.
.TP
.B --nolockfs
Do not attempt to synchronize filesystem eg, when suspending a device.
.
.TP
.B --noopencount
Tell the kernel not to supply the open reference count for the device.
.
.TP
.B --noudevrules
Do not allow udev to manage nodes for devices in device-mapper directory.
.
.TP
.B --noudevsync
Do not synchronise with udev when creating, renaming or removing devices.
.
.TP
.BR -o | --options " " \fIoptions
Specify which fields to display.
.
.TP
.BR --readahead \ [ + ] \fIsectors | auto | none
Specify read ahead size in units of sectors.
The default value is \fBauto\fP which allows the kernel to choose
a suitable value automatically.  The \fB+\fP 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 \fBnone\fP is equivalent to specifying zero.
.
.TP
.BR -r | --readonly
Set the table being loaded read-only.
.
.TP
.BR -S | --select " " \fIselection
Process only items that match \fIselection\fP criteria.  If the command is
producing report output, adding the "selected" column (\fB-o
selected\fP) displays all rows and shows 1 if the row matches the
\fIselection\fP and 0 otherwise. The selection criteria are defined by
specifying column names and their valid values while making use of supported
comparison operators. As a quick help and to see full list of column names that
can be used in selection and the set of supported selection operators, check
the output of \fBdmsetup\ info\ -c\ -S\ help\fP command.
.
.TP
.B --table \fItable
Specify a one-line table directly on the command line.
See below for more information on the table format.
.
.TP
.B --udevcookie \fIcookie
Use cookie for udev synchronisation.
Note: Same cookie should be used for same type of operations i.e. creation of
multiple different devices. It's not advised to combine different
operations on the single device.
.
.TP
.BR -u | --uuid " " \fIuuid
Specify the \fIuuid\fP.
.
.TP
.BR -y | --yes
Answer yes to all prompts automatically.
.
.TP
.BR -v | --verbose " [" -v | --verbose ]
Produce additional output.
.
.TP
.B --verifyudev
If udev synchronisation is enabled, verify that udev operations get performed
correctly and try to fix up the device nodes afterwards if not.
.
.TP
.B --version
Display the library and kernel driver version.
.
.SH COMMANDS
.
.HP
.CMD_CLEAR
.br
Destroys the table in the inactive table slot for device_name.
.
.HP
.CMD_CREATE
.br
Creates a device with the given name.
If \fItable\fP or \fItable_file\fP is supplied, the table is loaded and made live.
Otherwise a table is read from standard input unless \fB--notable\fP is used.
The optional \fIuuid\fP can be used in place of
device_name in subsequent dmsetup commands.
If successful the device will appear in table and for live
device the node \fI/dev/mapper/device_name\fP is created.
See below for more information on the table format.
.
.HP
.CMD_CREATE_CONCISE
.br
Creates one or more devices from a concise device specification.
Each device is specified by a comma-separated list: name, uuid, minor number, flags, comma-separated table lines.
Flags defaults to read-write (rw) or may be read-only (ro).
Uuid, minor number and flags are optional so those fields may be empty.
A semi-colon separates specifications of different devices.
Use a backslash to escape the following character, for example a comma or semi-colon in a name or table. See also CONCISE FORMAT below.
.
.HP
.CMD_DEPS
.br
Outputs a list of devices referenced by the live table for the specified
device. Device names on output can be customised by following \fIoptions\fP:
\fBdevno\fP (major and minor pair, used by default),
\fBblkdevname\fP (block device name),
\fBdevname\fP (map name for device-mapper devices, equal to blkdevname otherwise).
.
.HP
.CMD_HELP
.br
Outputs a summary of the commands available, optionally including
the list of report fields.
.
.HP
.CMD_INFO
.br
Outputs some brief information about the device in the form:
.RS
.RS
 State: SUSPENDED|ACTIVE, READ-ONLY
 Tables present: LIVE and/or INACTIVE
 Open reference count
 Last event sequence number (used by \fBwait\fP)
 Major and minor device number
 Number of targets in the live table
 UUID
.RE
.RE
.HP
.CMD_INFOLONG
.br
Output you can customise.
Fields are comma-separated and chosen from the following list:
.BR name ,
.BR major ,
.BR minor ,
.BR attr ,
.BR open ,
.BR segments ,
.BR events ,
.BR uuid .
Attributes are:
.RB ( L )ive,
.RB ( I )nactive,
.RB ( s )uspended,
.RB ( r )ead-only,
.RB read-( w )rite.
Precede the list with '\fB+\fP' to append
to the default selection of columns instead of replacing it.
Precede any sort field with '\fB-\fP' for a reverse sort on that column.
.
.HP
.CMD_LS
.br
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:
\fBdevno\fP (major and minor pair, used by default),
\fBblkdevname\fP (block device name),
\fBdevname\fP (map name for device-mapper devices, equal to blkdevname otherwise).
\fB--tree\fP displays dependencies between devices as a tree.
It accepts a comma-separate list of \fIoptions\fP.
Some specify the information displayed against each node:
.BR device / nodevice ;
.BR blkdevname ;
.BR active ", " open ", " rw ", " uuid .
Others specify how the tree is displayed:
.BR ascii ", " utf ", " vt100 ;
.BR compact ", " inverted ", " notrunc .
.
.HP
.BR load | \c
.CMD_RELOAD
.br
Loads \fItable\fP or \fItable_file\fP into the inactive table slot for device_name.
If neither is supplied, reads a table from standard input.
.
.HP
.CMD_MANGLE
.br
Ensure existing device-mapper \fIdevice_name\fP and UUID 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 \fB--manglename\fP setting. Automatic rename works only for device
names and not for device UUIDs because the kernel does not allow changing
the UUID of active devices. Any incorrect UUIDs are reported only and they
must be manually corrected by deactivating the device first and then
reactivating it with proper mangling mode used (see also \fB--manglename\fP).
.
.HP
.CMD_MEASURE
.br
Show the data that \fIdevice_name\fP would report to the IMA subsystem
if a measurement was triggered at the current time.
This is for debugging and does not actually trigger a measurement.
.
.HP
.CMD_MESSAGE
.br
Send message to target. If sector not needed use 0.
.
.HP
.CMD_MKNODES
.br
Ensure that the node in \fI/dev/mapper\fP for \fIdevice_name\fP is correct.
If no device_name is supplied, ensure that all nodes in \fI/dev/mapper\fP
correspond to mapped devices currently loaded by the device-mapper kernel
driver, adding, changing or removing nodes as necessary.
.
.HP
.CMD_REMOVE
.br
Removes a device.  It will no longer be visible to dmsetup.  Open devices
cannot be removed, but adding \fB--force\fP will replace the table with one
that fails all I/O.  \fB--deferred\fP will enable deferred removal of open
devices - the device will be removed when the last user closes it. The deferred
removal feature is supported since version 4.27.0 of the device-mapper
driver available in upstream kernel version 3.13.  (Use \fBdmsetup version\fP
to check this.)  If an attempt to remove a device fails, perhaps because a process run
from a quick udev rule temporarily opened the device, the \fB--retry\fP
option will cause the operation to be retried for a few seconds before failing.
Do NOT combine
\fB--force\fP and \fB--udevcookie\fP, as udev may start to process udev
rules in the middle of error target replacement and result in nondeterministic
result.
.
.HP
.CMD_REMOVE_ALL
.br
Attempts to remove all device definitions i.e. reset the driver.  This also runs
\fBmknodes\fP afterwards.  Use with care!  Open devices cannot be removed, but
adding \fB--force\fP will replace the table with one that fails all I/O.
\fB--deferred\fP will enable deferred removal of open devices - the device
will be removed when the last user closes it.  The deferred removal feature is
supported since version 4.27.0 of the device-mapper driver available in
upstream kernel version 3.13.
.
.HP
.CMD_RENAME
.br
Renames a device.
.
.HP
.CMD_RENAME_UUID
.br
Sets the uuid of a device that was created without a uuid.
After a uuid has been set it cannot be changed.
.
.HP
.CMD_RESUME
.br
Un-suspends a device.
If an inactive table has been loaded, it becomes live.
Postponed I/O then gets re-queued for processing.
.
.HP
.CMD_SETGEOMETRY
.br
Sets the device geometry to C/H/S.
.
.HP
.CMD_SPLITNAME
.br
Splits given \fIdevice name\fP into \fIsubsystem\fP constituents.
The default subsystem is LVM.
LVM currently generates device names by concatenating the names of the Volume
Group, Logical Volume and any internal Layer with a hyphen as separator.
Any hyphens within the names are doubled to escape them.
The precise encoding might change without notice in any future
release, so we recommend you always decode using the current version of
this command.
.HP
.CMD_STATS
.br
Manages IO statistics regions for devices.
See
.BR dmstats (8)
for more details.
.HP
.CMD_STATUS
.br
Outputs status information for each of the device's targets.
With \fB--target\fP, only information relating to the specified target type
any is displayed.  With \fB--noflush\fP, the thin target (from version 1.3.0)
doesn't commit any outstanding changes to disk before reporting its statistics.
.
.HP
.CMD_SUSPEND
.br
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 \fB--nolockfs\fP is specified.
Some targets such as recent (October 2006) versions of multipath may support
the \fB--noflush\fP option.  This lets outstanding I/O that has not yet reached the
device to remain unflushed.
.
.HP
.CMD_TABLE
.br
Outputs the current table for the device in a format that can be fed
back in using the create or load commands.
With \fB--target\fP, only information relating to the specified target type
is displayed.
Real encryption keys are suppressed in the table output for crypt and integrity
targets unless the \fB--showkeys\fP parameter is supplied. Kernel key
references prefixed with \fB:\fP are not affected by the parameter and get
displayed always (crypt target only).
With \fB--concise\fP, the output is presented concisely on a single line.
Commas then separate the name, uuid, minor device number, flags ('ro' or 'rw')
and the table (if present). Semi-colons separate devices. Backslashes escape
any commas, semi-colons or backslashes.  See CONCISE FORMAT below.
.
.HP
.CMD_TARGETS
.br
Displays the names and versions of the currently-loaded targets.
.
.HP
.CMD_UDEVCOMPLETE
.br
Wake any processes that are waiting for udev to complete processing the specified cookie.
.
.HP
.CMD_UDEVCOMPLETE_ALL
.br
Remove all cookies older than the specified number of minutes.
Any process waiting on a cookie will be resumed immediately.
.
.HP
.CMD_UDEVCOOKIES
.br
List all existing cookies. Cookies are system-wide semaphores with keys
prefixed by two predefined bytes (0x0D4D).
.
.HP
.CMD_UDEVCREATECOOKIE
.br
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
\fB--udevcookie\fP option. Alternatively, we can export this value into the environment
of the dmsetup process as \fBDM_UDEV_COOKIE\fP 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.
.
.HP
.CMD_UDEVFLAGS
.br
Parses given \fIcookie\fP 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.
.
.HP
.CMD_UDEVRELEASECOOKIE
.br
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 \fBDM_UDEV_COOKIE\fP environment variable.
.
.HP
.CMD_VERSION
.br
Outputs version information.
.
.HP
.CMD_WAIT
.br
Sleeps until the event counter for device_name exceeds event_nr.
Use \fB-v\fP to see the event number returned.
To wait until the next event is triggered, use \fBinfo\fP to find
the last event number.
With \fB--noflush\fP, the thin target (from version 1.3.0) doesn't commit
any outstanding changes to disk before reporting its statistics.
.
.HP
.CMD_WIPE_TABLE
.br
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).
.
.SH TABLE FORMAT
.
Each line of the table specifies a single target and is of the form:
.sp
.I logical_start_sector num_sectors
.B target_type
.I target_args
.sp
Simple target types and target args include:
.
.TP
.B linear \fIdestination_device start_sector
The traditional linear mapping.
.TP
.B striped \fInum_stripes chunk_size \fR[\fIdestination start_sector\fR]...
Creates a striped area.
.br
e.g. striped 2 32 /dev/hda1 0 /dev/hdb1 0
will map the first chunk (16k) as follows:
.RS
.IP
 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.
.RE
.TP
.B error
Errors any I/O that goes to this area.  Useful for testing or
for creating devices with holes in them.
.TP
.B zero
Returns blocks of zeroes on reads.  Any data written is discarded silently.
This is a block-device equivalent of the \fI/dev/zero\fP
character-device data sink described in \fBnull\fP(4).
.P
More complex targets include:
.TP
.B cache
Improves performance of a block device (eg, a spindle) by dynamically
migrating some of its data to a faster smaller device (eg, an SSD).
.TP
.B crypt
Transparent encryption of block devices using the kernel crypto API.
.TP
.B delay
Delays reads and/or writes to different devices.  Useful for testing.
.TP
.B flakey
Creates a similar mapping to the linear target but
exhibits unreliable behaviour periodically.
Useful for simulating failing devices when testing.
.TP
.B mirror
Mirrors data across two or more devices.
.TP
.B multipath
Mediates access through multiple paths to the same device.
.TP
.B raid
Offers an interface to the kernel's software raid driver, md.
.TP
.B snapshot
Supports snapshots of devices.
.TP
.BR thin ", " thin-pool
Supports thin provisioning of devices and also provides a better snapshot support.
.P
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.)
.
.SH EXAMPLES
.
.nf
# A table to join two disks together
0 1028160 linear /dev/hda 0
1028160 3903762 linear /dev/hdb 0
.P
# 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
.fi
.
.SH CONCISE FORMAT
.
A concise representation of one of more devices.
.sp
.br
- A comma separates the fields of each device.
.br
- A semi-colon separates devices.
.TP
The representation of a device takes the form:
.ad l
.nh
<name>,<uuid>,\:<minor>,<flags>,\:<table>\:[,<table>+]\:[;<name>,<uuid>,\:<minor>,<flags>,<table>\:[,<table>+]]
.hy
.ad b
.TP
The fields are:
.
.TP
.B name
The name of the device.
.TP
.B uuid
The UUID of the device (or empty).
.TP
.B minor
The minor number of the device.  If empty, the kernel assigns a suitable minor number.
.TP
.B flags
Supported flags are:
.br
.ad l
.B ro
Sets the table being loaded for the device read-only
.br
.B rw
Sets the table being loaded for the device read-write (default)
.ad b
.TP
.B table
One line of the table. See TABLE FORMAT above.
.
.SH EXAMPLES
.
.PD 0
.ad l
.nh
# A simple linear read-only device
.TP
test-linear-small,,,ro,\:0 2097152 linear /dev/loop0 0,\:2097152 2097152 linear /dev/loop1 0
.P
# Two linear devices
.TP
test-linear-small,,,,\:0 2097152 linear /dev/loop0 0;\:test-linear-large,\:,,,\:0 2097152 linear /dev/loop1 0, 2097152 2097152 linear /dev/loop2 0
.hy
.ad b
.PD
.
.SH ENVIRONMENT VARIABLES
.
.TP
.B DM_DEV_DIR
The device directory name.
Defaults to "\fI/dev\fP" and must be an absolute path.
.TP
.B DM_UDEV_COOKIE
A cookie to use for all relevant commands to synchronize with udev processing.
It is an alternative to using \fB--udevcookie\fP option.
.TP
.B DM_DEFAULT_NAME_MANGLING_MODE
A default mangling mode. Defaults to "\fB#DEFAULT_MANGLING#\fP"
and it is an alternative to using \fB--manglename\fP option.
.
.SH AUTHORS
.
Original version: Joe Thornber <thornber@redhat.com>
.
.SH SEE ALSO
.
.BR dmstats (8),
.BR udev (7),
.BR udevadm (8)
.P
LVM2 resource page:
.UR https://www.sourceware.org/lvm2
.UE
.br
Device-mapper resource page:
.UR http://sources.redhat.com/dm
.UE