Linux MD RAID

mdadm is a GNU/Linux utility used to manage and monitor software RAID devices. It is used in modern GNU/Linux distributions in place of older software RAID utilities such as raidtools2 or raidtools.

mdadm is free software maintained and copyrighted to Neil Brown of SUSE, and licensed under the terms of version 2 or later of the GNU General Public License.

The name is derived from the md (multiple device) device nodes it administers or manages, and it replaced a previous utility mdctl. The original name was "Mirror Disk", but was changed as more functions were added. The name is now understood to be short for Multiple Disk or Device Administration.

Linux software RAID configurations can include anything presented to the Linux kernel as a block device. This includes whole hard drives (for example, /dev/sda), and their partitions (for example, /dev/sda1).

Note that RAID 10 is distinct from RAID 0+1, which consists of a top-level RAID 1 mirror composed of high-performance RAID 0 stripes directly across the physical hard disks. A single-drive failure in a RAID 10 configuration results in one of the lower-level mirrors entering degraded mode, but the top-level stripe performing normally (except for the performance hit). A single-drive failure in a RAID 0+1 configuration results in one of the lower-level stripes completely failing, and the top-level mirror entering degraded mode. Which of the two setups is preferable depends on the details of the application in question, such as whether or not spare disks are available, and how they should be spun up.

The original (standard) form of names for md devices is /dev/md<n>, where <n> is a number between 0 and 99. More recent kernels have support for names such as /dev/md/Home. Under 2.4.x kernels and earlier these two were the only options. Both of them are non-partitionable.

Since 2.6.x kernels, a new type of MD device was introduced, a partitionable array. The device names were modified by changing md to md_d. The partitions were identified by adding p<m>; thus /dev/md/md_d2p3 for example. Since version 2.6.28 of the Linux kernel mainline, non-partitionable arrays can be partitioned, the partitions being referred to in the same way as for partitionable arrays – for example, /dev/md/md1p2.