View source for Device mapper

{{Short description|Software framework of Linux kernel}}
The '''device mapper''' is a [[Software framework|framework]] provided by the [[Linux kernel]] for mapping physical [[block device]]s onto higher-level ''virtual block devices''.  It forms the foundation of the [[logical volume manager]] (LVM), software [[RAID]]s and [[dm-crypt]] disk encryption, and offers additional features such as file system [[Snapshot (computer storage)|snapshots]].<ref name="redhat-dm">{{cite web
 | url = https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/device_mapper.html
 | title = Logical Volume Manager Administration, Appendix A. The Device Mapper
 | access-date = 2013-09-29
 | publisher = [[Red Hat]]
}}</ref>

Device mapper works by passing data from a virtual block device, which is provided by the device mapper itself, to another block device.  Data can be also modified in transition, which is performed, for example, in the case of device mapper providing [[disk encryption]] or simulation of unreliable hardware behavior.

This article focuses on the device mapper implementation in the Linux kernel, but the device mapper functionality is also available in both [[NetBSD]] and [[DragonFly BSD]].<ref>{{cite web
 | url = http://netbsd.gw.com/cgi-bin/man-cgi?dm+4+NetBSD-current
 | title = NetBSD Kernel Interfaces Manual: dm(4)
 | date = 2008-08-30
 | access-date = 2015-01-25
 | website = netbsd.gw.com
 | archive-date = 2014-07-19
 | archive-url = https://web.archive.org/web/20140719152701/http://netbsd.gw.com/cgi-bin/man-cgi?dm+4+NetBSD-current
 | url-status = dead
 }}</ref><ref>{{cite web
 | url = http://leaf.dragonflybsd.org/cgi/web-man?command=dm&section=4
 | title = DragonFly On-Line Manual Pages: dm(4)
 | date = 2010-07-28 | access-date = 2014-06-06
 | website = dragonflybsd.org
}}</ref>

== Usage ==
Applications (like [[LVM2]] and [[Enterprise Volume Management System]] (EVMS)) that need to create new mapped devices talk to the device mapper via the <code>libdevmapper.so</code> [[Library (computer science)|shared library]], which in turn issues [[ioctl]]s to the <code>/dev/mapper/control</code> [[device node]].<ref>{{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 }}</ref>  Configuration of the device mapper can be also examined and configured interactively{{mdashb}}or from [[shell script]]s{{mdashb}}by using the {{man|8|dmsetup|man.cx||inline}} utility.<ref name="dmsetup-man">{{cite web
 | url = http://man.cx/?page=dmsetup%288%29
 | title = dmsetup(8) - Linux man page
 | access-date = 2013-10-22
 | publisher = man.cx
}}</ref><ref>{{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]]
}}</ref>

Both of these two userspace components have their source code maintained alongside the LVM2 source.<ref>{{cite web
 | url = http://www.sourceware.org/dm/
 | title = Device-mapper Resource Page
 | access-date = 2013-09-29
 | publisher = sourceware.org
}}</ref>

== Features ==
[[File:The Linux Storage Stack Diagram.svg|thumb|right|upright=2.0|The position of the device mapper targets within various layers of the [[Linux kernel]]'s storage stack.<ref>{{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
}}</ref>]]

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 (Linux)|Crypto API]].<ref name="redhat-dm" />

{{As of|2014}}, the following mapping targets are available:<ref name="redhat-dm" /><ref name="dmsetup-man" />

* ''[[dm-cache|cache]]''{{snd}} allows creation of [[hybrid volume]]s, by using [[solid-state drive]]s (SSDs) as caches for [[hard disk drive]]s (HDDs)
* ''clone''{{snd}} will permit usage before a transfer is complete.
* ''[[dm-crypt|crypt]]''{{snd}} provides data encryption, by using the Linux kernel's [[Crypto API (Linux)|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<ref>{{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
}}</ref>
* ''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 [[Disk mirroring|mirrored]] logical device, while providing data redundancy
* ''[[Linux DM Multipath|multipath]]''{{snd}} supports the mapping of [[Multipath I/O|multipathed]] devices, through usage of their path groups
* ''raid''{{snd}} offers an interface to the Linux kernel's [[software RAID]] driver ([[mdadm|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 <code>/dev/zero</code>, all reads return blocks of zeros, and writes are discarded

== {{Anchor|DM-LOG-WRITES|DM-VERITY}}Applications ==
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]]/[[Linux Unified Key Setup|LUKS]]{{snd}} mapping target that provides volume encryption
* [[dm-cache]]{{snd}} mapping target that allows creation of [[Hybrid drive#DUALDRIVE|hybrid volumes]]
* dm-integrity{{snd}} mapping target that provides [[data integrity]], either using checksumming or cryptographic verification,<ref>{{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
}}</ref><ref>{{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]]
}}</ref> also used with LUKS<ref>{{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
}}</ref>
* 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<ref>{{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]]
}}</ref>
* 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<ref>{{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]]
}}</ref>
* {{man|8|dmraid|man.cx||inline}}{{snd}} provides access to [[RAID|"fake" RAID]] configurations via the device mapper
* [[Linux DM Multipath|DM Multipath]]{{snd}} provides I/O failover and load-balancing of block devices within the Linux kernel
* [[Docker (software)|Docker]]{{snd}} uses device mapper to create copy-on-write storage for software containers
* [[DRBD]] (Distributed Replicated Block Device)
* [[Enterprise Volume Management System|EVMS]] (deprecated)
* {{man|8|kpartx|man.cx||inline}}{{snd}} utility called from hotplug upon device maps creation and deletion
* [[LVM2]]{{snd}} [[logical volume management|logical volume manager]] for the Linux kernel
* [[VeraCrypt]] - Linux version of [[TrueCrypt]]
*VDO - Virtual Data Optimizer

== References ==
{{Reflist|30em}}

== External links ==
* [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}}

{{Linux kernel}}
{{Linux}}
{{Operating system}}

[[Category:Device mapper| ]]
[[Category:Red Hat software]]