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Device mapper
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<h2 id="usage">Usage</h2> <p>Applications (like <a href="/facts/LVM2/HV86mJw0">LVM2</a> and <a href="/facts/Enterprise_Volume_Management_System/hg8UhqF4">Enterprise Volume Management System</a> (EVMS)) that need to create new mapped devices talk to the device mapper via the libdevmapper.so <a href="/facts/Library_(computer_science)/xRzL3P7q">shared library</a>, which in turn issues <a href="/facts/Ioctl/119Wrsz9">ioctls</a> to the /dev/mapper/control <a href="/facts/Device_node/t8ISnXft">device node</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> Configuration of the device mapper can be also examined and configured interactively—or from <a href="/facts/Shell_script/NE7YvsBD">shell scripts</a>—by using the <a href="https://man.cx/?page=dmsetup(8)">dmsetup(8)</a> utility.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a><a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p><p>Both of these two userspace components have their source code maintained alongside the LVM2 source.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p> <h2 id="features">Features</h2> <p>Functions provided by the device mapper include linear, striped and error <i>mappings,</i> as well as crypt and multipath <i>targets.</i> For example, two disks may be concatenated into one logical volume with a pair of <i>linear</i> mappings, one for each disk. As another example, <i>crypt</i> target encrypts the data passing through the specified device, by using the Linux kernel's <a href="/facts/Crypto_API_(Linux)/9TCbRQra">Crypto API</a>.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> </p><p>As of 2014<a href="https://en.wikipedia.org/w/index.php?title=Device_mapper&action=edit">[update]</a>, the following mapping targets are available:<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a><a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p> <ul><li><i><a href="/facts/Dm-cache/eSWIYqSh">cache</a></i> – allows creation of <a href="/facts/Hybrid_volume/zkmDxd7q">hybrid volumes</a>, by using <a href="/facts/Solid-state_drive/aMHBy8gU">solid-state drives</a> (SSDs) as caches for <a href="/facts/Hard_disk_drive/kekLSGSE">hard disk drives</a> (HDDs)</li> <li><i>clone</i> – will permit usage before a transfer is complete.</li> <li><i><a href="/facts/Dm-crypt/DpsfvyrW">crypt</a></i> – provides data encryption, by using the Linux kernel's <a href="/facts/Crypto_API_(Linux)/9TCbRQra">Crypto API</a></li> <li><i>delay</i> – delays reads and/or writes to different devices (used for testing)</li> <li><i>era</i> – 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<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a></li> <li><i>error</i> – simulates I/O errors for all mapped blocks (used for testing)</li> <li><i>flakey</i> – simulates periodic unreliable behaviour (used for testing)</li> <li><i>linear</i> – maps a continuous range of blocks onto another block device</li> <li><i>mirror</i> – maps a <a href="/facts/Disk_mirroring/ulg3lVVm">mirrored</a> logical device, while providing data redundancy</li> <li><i><a href="/facts/Linux_DM_Multipath/ekCnnnxF">multipath</a></i> – supports the mapping of <a href="/facts/Multipath_I%2fO/KRmpD3nm">multipathed</a> devices, through usage of their path groups</li> <li><i>raid</i> – offers an interface to the Linux kernel's <a href="/facts/Software_RAID/IpzWWgog">software RAID</a> driver (<a href="/facts/Mdadm/Nhv2GcUQ">md</a>)</li> <li><i>snapshot</i> and <i>snapshot-origin</i> – used for creation of LVM snapshots, as part of the underlying copy-on-write scheme</li> <li><i>striped</i> – stripes the data across physical devices, with the number of stripes and the striping chunk size as parameters</li> <li><i>thin</i> – allows creation of devices larger than the underlying physical device, physical space is allocated only when written to</li> <li><i>zero</i> – an equivalent of /dev/zero, all reads return blocks of zeros, and writes are discarded</li></ul> <h2 id="applications">Applications</h2> <p>Linux kernel features and projects built on top of the device mapper include the following: </p> <ul><li><a href="/facts/Cryptsetup/DpsfvyrW">cryptsetup</a> – utility used to conveniently setup disk encryption based on dm-crypt</li> <li><a href="/facts/Dm-crypt/DpsfvyrW">dm-crypt</a>/<a href="/facts/Linux_Unified_Key_Setup/mKPeeurF">LUKS</a> – mapping target that provides volume encryption</li> <li><a href="/facts/Dm-cache/eSWIYqSh">dm-cache</a> – mapping target that allows creation of <a href="/facts/Hybrid_drive/zkmDxd7q">hybrid volumes</a></li> <li>dm-integrity – mapping target that provides <a href="/facts/Data_integrity/g3XGbKIH">data integrity</a>, either using checksumming or cryptographic verification,<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a><a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> also used with LUKS<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a></li> <li>dm-log-writes – mapping target that uses two devices, passing through the first device and logging the write operations performed to it on the second device<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a></li> <li>dm-verity – validates the data blocks contained in a file system against a list of cryptographic hash values, developed as part of the <a href="/facts/ChromiumOS/rHSqcS74">ChromiumOS</a> project<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a></li> <li><a href="https://man.cx/?page=dmraid(8)">dmraid(8)</a> – provides access to <a href="/facts/RAID/IpzWWgog">"fake" RAID</a> configurations via the device mapper</li> <li><a href="/facts/Linux_DM_Multipath/ekCnnnxF">DM Multipath</a> – provides I/O failover and load-balancing of block devices within the Linux kernel</li> <li><a href="/facts/Docker_(software)/7IvvsMug">Docker</a> – uses device mapper to create copy-on-write storage for software containers</li> <li><a href="/facts/DRBD/xybfWFno">DRBD</a> (Distributed Replicated Block Device)</li> <li><a href="/facts/Enterprise_Volume_Management_System/hg8UhqF4">EVMS</a> (deprecated)</li> <li><a href="https://man.cx/?page=kpartx(8)">kpartx(8)</a> – utility called from hotplug upon device maps creation and deletion</li> <li><a href="/facts/LVM2/HV86mJw0">LVM2</a> – <a href="/facts/Logical_volume_management/ku3Ivefk">logical volume manager</a> for the Linux kernel</li> <li><a href="/facts/VeraCrypt/F6h1wIrH">VeraCrypt</a> - Linux version of <a href="/facts/TrueCrypt/vK1yL9Io">TrueCrypt</a></li> <li>VDO - Virtual Data Optimizer</li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="http://sources.redhat.com/dm/">Device mapper home at Red Hat</a> <a href="https://web.archive.org/web/20060315124752/http://sources.redhat.com/dm/">Archived</a> 2006-03-15 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a></li> <li><a href="http://linuxgazette.net/114/kapil.html">"Right To Your Own Devices"</a>. <i><a href="/facts/Linux_Gazette/fKhUnSyv">Linux Gazette</a></i>. May 2005. – an article illustrating the use of various device mapper targets</li> <li><a href="http://people.redhat.com/~heinzm/sw/dmraid/readme">userspace tool to set up software RAID using various RAID metadata formats</a></li> <li><a href="https://lwn.net/Articles/124703/">Multipath support in the device mapper</a>, <a href="/facts/LWN.net/8uTsltvM">LWN.net</a>, February 23, 2005, by Jonathan Corbet</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"Logical Volume Manager Administration, Appendix A. The Device Mapper". Red Hat. Retrieved 2013-09-29. <a href="https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/device_mapper.html" target="_blank">https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/device_mapper.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"NetBSD Kernel Interfaces Manual: dm(4)". netbsd.gw.com. 2008-08-30. Archived from the original on 2014-07-19. Retrieved 2015-01-25. <a href="https://web.archive.org/web/20140719152701/http://netbsd.gw.com/cgi-bin/man-cgi?dm+4+NetBSD-current" target="_blank">https://web.archive.org/web/20140719152701/http://netbsd.gw.com/cgi-bin/man-cgi?dm+4+NetBSD-current</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"DragonFly On-Line Manual Pages: dm(4)". dragonflybsd.org. 2010-07-28. Retrieved 2014-06-06. <a href="http://leaf.dragonflybsd.org/cgi/web-man?command=dm§ion=4" target="_blank">http://leaf.dragonflybsd.org/cgi/web-man?command=dm§ion=4</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"libdevmapper.h". sourceware.org. Retrieved 2013-09-29.[permanent dead link] <a href="http://www.sourceware.org/cgi-bin/cvsweb.cgi/~checkout~/LVM2/libdm/libdevmapper.h?content-type=text/plain&cvsroot=lvm2" target="_blank">http://www.sourceware.org/cgi-bin/cvsweb.cgi/~checkout~/LVM2/libdm/libdevmapper.h?content-type=text/plain&cvsroot=lvm2</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"dmsetup(8) - Linux man page". man.cx. Retrieved 2013-10-22. <a href="http://man.cx/?page=dmsetup%288%29" target="_blank">http://man.cx/?page=dmsetup%288%29</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"Logical Volume Manager Administration". Appendix A.2. The dmsetup Command. Red Hat. Retrieved 2013-09-29. <a href="https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/dmsetup.html" target="_blank">https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/dmsetup.html</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>"Device-mapper Resource Page". sourceware.org. Retrieved 2013-09-29. <a href="http://www.sourceware.org/dm/" target="_blank">http://www.sourceware.org/dm/</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>"Logical Volume Manager Administration, Appendix A. The Device Mapper". Red Hat. Retrieved 2013-09-29. <a href="https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/device_mapper.html" target="_blank">https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/device_mapper.html</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>"Logical Volume Manager Administration, Appendix A. The Device Mapper". Red Hat. Retrieved 2013-09-29. <a href="https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/device_mapper.html" target="_blank">https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/device_mapper.html</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>"dmsetup(8) - Linux man page". man.cx. Retrieved 2013-10-22. <a href="http://man.cx/?page=dmsetup%288%29" target="_blank">http://man.cx/?page=dmsetup%288%29</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>"6. Block layer". Linux kernel 3.15. kernelnewbies.org. 2014-06-08. Retrieved 2014-06-15. <a href="http://kernelnewbies.org/Linux_3.15#head-33ab227d4984f4be9fa92f9163e403744e2a8f66" target="_blank">http://kernelnewbies.org/Linux_3.15#head-33ab227d4984f4be9fa92f9163e403744e2a8f66</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>"dm-integrity". cryptosetup project. 2018-04-30. Retrieved 2018-05-08. <a href="https://gitlab.com/cryptsetup/cryptsetup/wikis/DMIntegrity" target="_blank">https://gitlab.com/cryptsetup/cryptsetup/wikis/DMIntegrity</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>"dm-integrity target". kernel.org. 2017-05-13. Retrieved 2018-05-08. <a href="https://www.kernel.org/doc/Documentation/device-mapper/dm-integrity.txt" target="_blank">https://www.kernel.org/doc/Documentation/device-mapper/dm-integrity.txt</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>"cryptsetup - manage plain dm-crypt and LUKS encrypted volumes". 2018-01-01. Retrieved 2018-05-08. <a href="https://man.cx/?page=cryptsetup(8)" target="_blank">https://man.cx/?page=cryptsetup(8)</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>"Linux kernel documentation: Documentation/device-mapper/log-writes.txt". kernel.org. 2015-05-29. Retrieved 2015-10-13. <a href="https://www.kernel.org/doc/Documentation/device-mapper/log-writes.txt" target="_blank">https://www.kernel.org/doc/Documentation/device-mapper/log-writes.txt</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>Jonathan Corbet (2011-09-19). "dm-verity". LWN.net. Retrieved 2015-10-13. <a href="https://lwn.net/Articles/459420/" target="_blank">https://lwn.net/Articles/459420/</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> </ol>