RAID: Redundant Array of Inexpensive Disks Supplemental Material not in book

RAID Redundant Array of Inexpensive Disks (or Independent Disks) proposed with five levels (RAID 1-5) in 1987 RAID employs two concepts: –Striping –Parity Concepts allowed for larger logical disks, greater speed and / or higher reliability Some initial RAID technologies are defunct (RAID 2), and other new variations have been defined (RAID 6, 0+1, 1+0, 5+0, etc.)

Striping By using multiple disks, and writing data to every disk simultaneously, can improve read and write performance Having each disk on their own channel can further improve performance Multiple disks striped together appear to the operating system as one larger logical drive Advantages: speed and large volume size

Parity By using multiple disks, and writing a parity bit for every bit written, the system can recover from a disk failure Advantage: Provides a level of fault tolerance Good examples for RAID – –

RAID 0 Not part of original RAID specification Only consists of striping (no parity) so not true RAID Can provide fastest read and write times Has weakness in a single drive failure means the entire RAID is lost –Not recommended for important, changing data –Good for fast access to large volumes of backed up, unchanging data

RAID 1 Original RAID specification for 2 disks in a mirror –No parity – as a full mirror, parity not required –No striping Provide fast read, slower write times Can recover from loss of single drive Usable data space: 1 disk out of 2

RAID 4 Original RAID specification for multiple disks in a stripe, with one disk maintaining parity Provide very fast read, slower write times Can recover from loss of single drive Usable data space: n-1 disks out of n

RAID 5 Original RAID specification for multiple disks in a stripe, with parity distributed across all disks Provide very fast read, better write times than 4 Can recover from loss of single drive Usable data space: n-1 disks out of n

RAID 6 Not in original RAID specification. For multiple disks in a stripe, with dual parity distributed across all disks Provide very fast read, slower write times Advantage over RAID 5 is dual parity can allow for loss of 2 drives. Usable data space: n-2 disks out of n

RAID 50 Not in original RAID specification. Known as nested RAID. A set of RAID 5 stripes, then brought together into a larger stripe. Provide very fast read, slower write times System can lose multiple disks, as long as only one disk in an individual RAID 5 stripe is not lost. Usable data space: n-s disks out of n, where s = number of RAID 5s present in the stripe

RAID 10 & Raid 0+1 Not in original RAID specification. Known as nested RAID. A set of mirrored disks, then brought together into a stripe. Provide very fast read, slower write times System can lose multiple disks, as long as both disks in a mirrored pair are not lost. Usable data space: n/2 disks out of n (n is even) 10 is stripping mirrored sets 0+1 is mirroring stripped sets – e.g database with remote real time backup copy

RAID ?P ?P ?P ?P ? ? ? ? P 1P 0P 0P Parity Calculation If sum of non-parity bits is even, then 0 If sum of non-parity bits is odd, then 1 Knowing this, if a disk is lost its bits can be determined from the remaining bits and the parity bit

RAID Software RAID –RAID can be implemented in software Inexpensive Lower performance, as CPU handles RAID management In Windows, to create a software RAID, disks must first be dynamic Hardware RAID –RAID can be implemented in hardware More expensive Better performance, as CPU offloads RAID management to the RAID controller Hot Spares –RAIDs can define one or more hotspares –Hotspares are disks that can be automatically pulled into the array when a disk fails

COMMON RAID These RAIDs are commonly used: RAID 0 (striping for large volumes and speed) RAID 1 (mirror for full copy of system) RAID 5 (stripe with parity, for larger volumes with redundancy) RAID 6 (when more redundancy is desired) –Becoming more common as storage cost is falling RAID 10 or 0+1 (for extensive redundancy coupled with high volume) RAID 50 (for very large volume arrays with redundancy) Everything else is almost never used!