Overview of RAID Levels

Background

The Berkeley engineers defined 5 different RAID levels. These levels are not ratings, but rather classifications of functionality. Different RAID levels offer dramatic differences in performance, data availability and data integrity depending on the specific I/O environment. There is no single RAID level that is perfect for all users.

RAID 0: Striping

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.

Advantages:

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

Disadvantages:

• Significantly reduced data availability

RAID 1: Shadowing/Mirroring/Duplexing

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.

Advantages:

• Excellent data availabiliy
• Higher read performance than a single disk

Disadvantages:

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

Peer-to-Peer RAID 1: Mirroring over Networks

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.

Advantages

• Excellent data availability
• Improved system uptime rating

Disadvantages

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

RAID 3: Striping and Parity

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.

Advantages:

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

Disadvantages:

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

RAID 5: Striping and Parity

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.

Advantages:

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

Disadvantages:

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