What is RAID?

RAID level

Description

RAID 5

Best suited for transaction processing and is often used for general purpose storage, as well as for relational database, and enterprise resource system. This RAID level provides a fairly low cost per megabyte while still retaining redundancy.

RAID Level 5 stripes data at a block level across several disks and distributes parity among the disks. No single disk is devoted to parity. Because parity data is distributed on each disk, read performance can be lower than other RAID types.

Distributed parity requires all disks but one to be present to operate. If a disk fails it will reduce storage performance and should be replaced immediately, but the data loss will not occur as a result of a single disk failure.

• RAID 5 (4+1): A minimum of five disks can be allocated at a time to each pool. Because of the way parity bits are used to provide redundancy, the usable capacity for every five-disk group is approximately four disks (80%).
  
• RAID 5 (6+1): A minimum of seven disks can be allocated at a time to each pool. Because of the way parity bits are used to provide redundancy, the usable capacity for every seven-disk group is approximately six disks (86%).
  

Note: If two disks in a RAID 5 disk group fail, this will cause data loss and render any storage in the RAID group unavailable until the failed disks are replaced or the data is restored.

RAID 6

Appropriate for the same types of applications as RAID 5, but in situations where providing increased fault tolerance is importance. RAID 6 is similar to RAID-5 but includes a double parity scheme that is distributed across different disks and thus offers extremely high fault- and disk-failure tolerance.  Provides block-level striping with parity data distributed across all disks.

Arrays continue to operate with up to two failed disks. Double parity gives time to rebuild the array without the data being at risk if a single additional disk fails before the rebuild is complete.

• RAID 6 (4+2): A minimum of six disks can be allocated at a time to each pool. Because of the way parity bits are used to provide redundancy, the usable capacity for every six-disk group is approximately four disks (66%).
  

Note: If three disks in a RAID 6 disk group fail, this will cause data loss and render any storage in the RAID group unavailable until the failed disks are replaced or the data is restored.

RAID 10

Provides both high performance and reliability at medium cost, while providing  lower capacity per disk. RAID 10 may be more appropriate for applications with fast or high processing requirements, such as enterprise servers and moderate-sized database systems. 

Requires a minimum of six physical disks to implement, where two sets of three striped disks are mirrored together to provide fault tolerance. Although mirroring provides fault tolerance, if any disk is lost, it must be immediately replaced and the array rebuilt, since it this configuration cannot handle the loss of more than one disk.

• RAID 10 (3+3): A minimum of six disks can be allocated at a time to a pool, with three used strictly for mirroring. Therefore, to provide redundancy, three disks out of every six are exact duplicates of the other, and the disk usable disk capacity for every six-disk group is approximately three disks (50%) .