Device mapper#Features

Applications (like LVM2 and Enterprise Volume Management System (EVMS)) that need to create new mapped devices talk to the device mapper via the libdevmapper.so shared library, which in turn issues ioctls to the /dev/mapper/control device node.{{cite web

| url = http://www.sourceware.org/cgi-bin/cvsweb.cgi/~checkout~/LVM2/libdm/libdevmapper.h?content-type=text/plain&cvsroot=lvm2

| title = libdevmapper.h

| access-date = 2013-09-29

| publisher = sourceware.org

}}{{Dead link|date=March 2023 |bot=InternetArchiveBot |fix-attempted=yes }} Configuration of the device mapper can be also examined and configured interactively{{mdashb}}or from shell scripts{{mdashb}}by using the {{man|8|dmsetup|man.cx||inline}} utility.{{cite web

| url = http://man.cx/?page=dmsetup%288%29

| title = dmsetup(8) - Linux man page

| access-date = 2013-10-22

| publisher = man.cx

}}{{cite web

| url = https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/dmsetup.html

| title = Logical Volume Manager Administration

| work = Appendix A.2. The dmsetup Command

| access-date = 2013-09-29

| publisher = Red Hat

}}

Both of these two userspace components have their source code maintained alongside the LVM2 source.{{cite web

| url = http://www.sourceware.org/dm/

| title = Device-mapper Resource Page

| access-date = 2013-09-29

| publisher = sourceware.org

}}

File:The Linux Storage Stack Diagram.svg's storage stack.{{Cite web

| url = https://www.thomas-krenn.com/en/wiki/Linux_Storage_Stack_Diagram

| title = Linux Storage Stack Diagram

| date = 2015-06-01 | access-date = 2015-06-08

| author1 = Werner Fischer | author2 = Georg Schönberger

| publisher = Thomas-Krenn.AG

}}]]

Functions provided by the device mapper include linear, striped and error mappings, as well as crypt and multipath targets. For example, two disks may be concatenated into one logical volume with a pair of linear mappings, one for each disk. As another example, crypt target encrypts the data passing through the specified device, by using the Linux kernel's Crypto API.

{{As of|2014}}, the following mapping targets are available:

• cache{{snd}} allows creation of hybrid volumes, by using solid-state drives (SSDs) as caches for hard disk drives (HDDs)
• clone{{snd}} will permit usage before a transfer is complete.
• crypt{{snd}} provides data encryption, by using the Linux kernel's Crypto API
• delay{{snd}} delays reads and/or writes to different devices (used for testing)
• era{{snd}} behaves in a way similar to the linear target, while it keeps track of blocks that were written to within a user-defined period of time{{cite web

| url = http://kernelnewbies.org/Linux_3.15#head-33ab227d4984f4be9fa92f9163e403744e2a8f66

| title = 6. Block layer | work = Linux kernel 3.15

| date = 2014-06-08 | access-date = 2014-06-15

| publisher = kernelnewbies.org

}}

• error{{snd}} simulates I/O errors for all mapped blocks (used for testing)
• flakey{{snd}} simulates periodic unreliable behaviour (used for testing)
• linear{{snd}} maps a continuous range of blocks onto another block device
• mirror{{snd}} maps a mirrored logical device, while providing data redundancy
• multipath{{snd}} supports the mapping of multipathed devices, through usage of their path groups
• raid{{snd}} offers an interface to the Linux kernel's software RAID driver (md)
• snapshot and snapshot-origin{{snd}} used for creation of LVM snapshots, as part of the underlying copy-on-write scheme
• striped{{snd}} stripes the data across physical devices, with the number of stripes and the striping chunk size as parameters
• thin {{snd}} allows creation of devices larger than the underlying physical device, physical space is allocated only when written to
• zero{{snd}} an equivalent of /dev/zero, all reads return blocks of zeros, and writes are discarded

Linux kernel features and projects built on top of the device mapper include the following:

• cryptsetup{{snd}} utility used to conveniently setup disk encryption based on dm-crypt
• dm-crypt/LUKS{{snd}} mapping target that provides volume encryption
• dm-cache{{snd}} mapping target that allows creation of hybrid volumes
• dm-integrity{{snd}} mapping target that provides data integrity, either using checksumming or cryptographic verification,{{cite web

| url = https://gitlab.com/cryptsetup/cryptsetup/wikis/DMIntegrity

| title = dm-integrity

| date = 2018-04-30 | access-date = 2018-05-08

| publisher = cryptosetup project

}}{{cite web

| url = https://www.kernel.org/doc/Documentation/device-mapper/dm-integrity.txt

| title = dm-integrity target

| date = 2017-05-13 | access-date = 2018-05-08

| publisher = kernel.org

}} also used with LUKS{{cite web

| url = https://man.cx/?page=cryptsetup(8)

| title = cryptsetup - manage plain dm-crypt and LUKS encrypted volumes

| date = 2018-01-01 | access-date = 2018-05-08

}}

• dm-log-writes{{snd}} mapping target that uses two devices, passing through the first device and logging the write operations performed to it on the second device{{cite web

| url = https://www.kernel.org/doc/Documentation/device-mapper/log-writes.txt

| title = Linux kernel documentation: Documentation/device-mapper/log-writes.txt

| date = 2015-05-29 | access-date = 2015-10-13

| publisher = kernel.org

}}

• dm-verity{{snd}} validates the data blocks contained in a file system against a list of cryptographic hash values, developed as part of the ChromiumOS project{{cite web

| url = https://lwn.net/Articles/459420/

| title = dm-verity

| date = 2011-09-19 | access-date = 2015-10-13

| author = Jonathan Corbet | publisher = LWN.net

}}

• {{man|8|dmraid|man.cx||inline}}{{snd}} provides access to "fake" RAID configurations via the device mapper
• DM Multipath{{snd}} provides I/O failover and load-balancing of block devices within the Linux kernel
• Docker{{snd}} uses device mapper to create copy-on-write storage for software containers
• DRBD (Distributed Replicated Block Device)
• EVMS (deprecated)
• {{man|8|kpartx|man.cx||inline}}{{snd}} utility called from hotplug upon device maps creation and deletion
• LVM2{{snd}} logical volume manager for the Linux kernel
• VeraCrypt - Linux version of TrueCrypt
• VDO - Virtual Data Optimizer

{{Reflist|30em}}

• [http://sources.redhat.com/dm/ Device mapper home at Red Hat] {{Webarchive|url=https://web.archive.org/web/20060315124752/http://sources.redhat.com/dm/ |date=2006-03-15 }}
• {{cite web | url=http://linuxgazette.net/114/kapil.html | title=Right To Your Own Devices | work = Linux Gazette | date = May 2005

}}{{snd}} an article illustrating the use of various device mapper targets

• [http://people.redhat.com/~heinzm/sw/dmraid/readme userspace tool to set up software RAID using various RAID metadata formats]
• [https://lwn.net/Articles/124703/ Multipath support in the device mapper], LWN.net, February 23, 2005, by Jonathan Corbet

{{Clear}}

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Category:Red Hat software