RAID Levels

RAID (Redundant Array of Independent Disks) is a set of methods (or levels) for storing data on a set of disks to improve performance, reliability, or both [Patterson].

RAID levels 0, 1 and 5 are most common. RAID is often implemented in hardware controllers. A RAID controller appears to a host computer just as any other storage device would. Behind the hardware RAID controller is a group of disk drives. Depending on which RAID level the controller is configured for, it will store data on the disks in different ways.

The common RAID levels have the following characteristics:

RAID-0 improves performance by striping data blocks across the individual disks. Because all drives are ideally being used in parallel, the overall performance is the combination of the performance of each disk. Striping also reduces the chance that one disk will be subject to much more load than others (a ``hot-spot''). There is no reliability or redundancy added by this level.
RAID-1 is also known as mirroring. The total usable storage space is half the physical storage capacity because half the disks are mirrored on the other half. Reliability is maximized because every disk has a duplicate. If a disk fails, its mirrored copy is synced to a spare or replacement disk. The device continues operating during recovery. If implemented in hardware, RAID 1 slightly improves performance too.
RAID-5 increases reliability allowing any one disk to fail without losing information. The space overhead is also low. Exclusive OR or ``parity'' values are calculated for each block stripe within the disk group. Blocks containing the parity values are interleaved among all the disks. If one disk fails, the data which was on that disk can be reconstructed using the data and parity blocks from the operational disks.

The RAID levels can also be implemented in the host's software on any collection of individual disks. RAID-0 (striping) and RAID-1 (mirroring) are the simplest to implement in a host driver and many volume drivers only support these two levels. Recovery procedures are the most difficult aspect of software RAID. Performance of software RAID may be slower than hardware RAID for a couple reasons. Software RAID levels one and higher often require more data to be transfered between hosts and storage than would be required for hardware RAID. For example, the host must make two writes to separate disks when maintaining mirrors whereas the data would be written only once to a hardware RAID device. In addition to the extra I/O to maintain parity, RAID-5 XOR calculations require extra CPU cycles.

Recommended Links

RAID - Wikipedia, the free encyclopedia

How To Set Up Software RAID1 On A Running LVM System (Incl. GRUB Configuration) (Fedora 8) HowtoForge - Linux Howtos and Tutorials

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Last modified: September 11, 2009