RAID devices are virtual devices created from two or more real block devices. This allows multiple devices (typically disk drives or partitions there-of) to be combined into a single device to hold (for example) a single filesystem. Some RAID levels include redundancy and so can survive some degree of device failure.
Linux Software RAID devices are implemented through the md (Multiple Devices) device driver.
Currently, Linux supports LINEAR md devices, RAID0 (striping), RAID1 (mirroring), RAID4, RAID5, RAID6, and MULTIPATH.
MULTIPATH is not a Software RAID mechanism, but does involve multiple devices. For MULTIPATH each device is a path to one common physical storage device.
mdadm is a program that can be used to create, manage, and monitor MD devices. As such it provides a similar set of functionality to the raidtools packages. The key differences between mdadm and raidtools are:
* mdadm is a single program and not a collection of programs.
* mdadm can perform (almost) all of its functions without having a configuration file and does not use one by default. Also mdadm helps with management of the configuration file.
* mdadm can provide information about your arrays (through Query, Detail, and Examine) that raidtools cannot. mdadm does not use /etc/raidtab, the raidtools configuration file, at all. It has a different configuration file with a different format and an different purpose.
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