Device mapper

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The device mapper is a framework provided by the Linux kernel for mapping physical block devices onto higher-level virtual block devices. It forms the foundation of the logical volume manager (LVM), software RAIDs and dm-crypt disk encryption, and offers additional features such as file system snapshots. [1]

Contents

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. [2] [3]

Usage

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. [4] Configuration of the device mapper can be also examined and configured interactively‍—‌or from shell scripts ‍—‌by using the dmsetup(8) utility. [5] [6]

Both of these two userspace components have their source code maintained alongside the LVM2 source. [7]

Features

The position of the device mapper targets within various layers of the Linux kernel's storage stack. The Linux Storage Stack Diagram.svg
The position of the device mapper targets within various layers of the Linux kernel's storage stack.

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. [1]

As of 2014, the following mapping targets are available: [1] [5]

Applications

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

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References

  1. 1 2 3 "Logical Volume Manager Administration, Appendix A. The Device Mapper". Red Hat . Retrieved 2013-09-29.
  2. ↑ "NetBSD Kernel Interfaces Manual: dm(4)". netbsd.gw.com. 2008-08-30. Archived from the original on 2014-07-19. Retrieved 2015-01-25.
  3. ↑ "DragonFly On-Line Manual Pages: dm(4)". dragonflybsd.org. 2010-07-28. Retrieved 2014-06-06.
  4. ↑ "libdevmapper.h". sourceware.org. Retrieved 2013-09-29.[ permanent dead link ]
  5. 1 2 "dmsetup(8) - Linux man page". man.cx. Retrieved 2013-10-22.
  6. ↑ "Logical Volume Manager Administration". Appendix A.2. The dmsetup Command. Red Hat . Retrieved 2013-09-29.
  7. ↑ "Device-mapper Resource Page". sourceware.org. Retrieved 2013-09-29.
  8. ↑ Werner Fischer; Georg Schönberger (2015-06-01). "Linux Storage Stack Diagram". Thomas-Krenn.AG. Retrieved 2015-06-08.
  9. ↑ "6. Block layer". Linux kernel 3.15. kernelnewbies.org. 2014-06-08. Retrieved 2014-06-15.
  10. ↑ "dm-integrity". cryptosetup project. 2018-04-30. Retrieved 2018-05-08.
  11. ↑ "dm-integrity target". kernel.org. 2017-05-13. Retrieved 2018-05-08.
  12. ↑ "cryptsetup - manage plain dm-crypt and LUKS encrypted volumes". 2018-01-01. Retrieved 2018-05-08.
  13. ↑ "Linux kernel documentation: Documentation/device-mapper/log-writes.txt". kernel.org. 2015-05-29. Retrieved 2015-10-13.
  14. ↑ Jonathan Corbet (2011-09-19). "dm-verity". LWN.net . Retrieved 2015-10-13.
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