RAID Overview and Data Recovery Striped and Broken RAID Array

What is RAID ?

RAID "(Redundant Array of Inexpensive Disks) is a method whereby information is spread across several disks, using techniques such as disk striping (RAID Level 0) and disk mirroring (RAID level 1) to achieve redundancy, lower latency and/or higher bandwidth for reading and/or writing, and data recoverability."

Commonly used RAID designations run from RAID 0-10, with RAID 0, RAID 1, RAID 3, and RAID 5 being the most readily available (and used).

• RAID 0 is stripping with no redundancy.
• RAID 1 is disk mirroring.
• RAID 3 uses striping and requires an extra drive for parity, used to recover a failed drive.
• RAID 5 is similar to RAID 3 but spreads the parity information over all of the drives in the set.

RAID 3 and RAID 5 arrays require a minimum of three drives. RAID drive arrays can be software or hardware based. Current hardware RAID systems usually have hot swap capability. This allows you to replace a failed drive w/o taking the system off line, which does a lot for system up time.

RAID Benefits

RAID has gained popularity because it improves I/O performance and increases storage subsystem reliability. RAID provides data security through fault tolerance and redundant data storage.

Although disk drive capabilities have improved drastically, actual performance has improved only three to four times in the last decade. Computing performance has improved over 50 times during the same time period. Implementing RAID improves the performance of the disk subsystem.

The electromechanical components of a disk subsystem operate more slowly, require more power, and generate more noise and vibration than electronic devices. These factors reduce the reliability of data stored on disks. Implementing RAID on disk subsystems improve the reliability of data.

RAID level 0 refers to striping data across multiple disks without any redundant information. RAID level 0 was not defined by the Berkeley engineers but has become a commonly used term.

Striping can be used to enhance performance in either a request rate intensive or transfer rate intensive environment. Unfortunately. striping reduces the level of data availability since a disk failure will cause the entire array to be inaccessible.

• High performance
• No cost penalty - all storage is usable
  

• Significantly reduced data availability

RAID level 1 refers to maintaining duplicate sets of all data on separate disk drives. Of the RAID levels, level 1 provides the highest data availability since two complete copies of all information are maintained. In addition. read performance may be enhanced if the array controller allows simultaneous reads from both members of a mirrored pair. During writes, there will be a minor performance penalty when compared to writing to a single disk. Higher availability will be achieved if both disks in a mirror pair are on separate I/O busses, known as duplexing.

• Very Good data availability
• Higher read performance than a single disk
  

• Used to be expensive - requires twice the desired disk space

Peer-to-peer RAID Level 1 refers to maintaining duplicate sets of all data on separate disk drives, on separate hosts or servers. Of the RAID levels, this provides the highest data availability since two complete copies of all information are maintained. During writes, there will be a minor performance penalty when compared to writing to a single disk.

• Excellent data availability
• Improved system uptime rating

• Used to be expensive - requires at least 2 disks on each host
• Moderately slower write performance

In RAID level 3, data is striped across a set of disks. In addition, parity is generated and stored on a dedicated disk. With RAID 3, data chunks are much smaller than the average I/O size and the disk spindles are synchronized to enhance throughput in transfer rate intensive environments. RAID 3 is well suited for CAD/CAM or imaging type applications. Since parity is used, a RAID 3 stripe set can withstand a single disk failure without losing data or access to data.

• Good data availability
• High performance for transfer rate intensive applications
• Cost effective - only 1 extra disk is required for parity

• Can satisfy only 1 I/O request at a time
• Poor small, random I/O performance
• Complicated

In RAID level 5, both parity and data are striped across a set of disks. Data chunks are much larger than the average I/O size. Disks are able to satisfy requests independently which provides high read performance in a request rate intensive environment. Since parity information is used, a RAID 5 stripe can withstand a single disk failure without losing data or access to data.

Unfortunately, the write performance of RAID 5 is poor. Each write requires four independent disk accesses to be completed. First old data and parity are read off of separate disks. Next the new parity is calculated. Finally, the new data and parity are wntten to separate disks. Many array vendors use write caching to compensate for the poor write performance of RAID 5.

• Average data availability
• Cost effective - only 1 extra disk is required

• Poor write performance
• No performance gain in data transfer rate intensive applications
• Complexity
• Requires special hardware.