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 availability
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 written 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.
