This page has been visited
times.
Overview of RAID Levels
RAID 0 | RAID 1 | Peer-to-Peer RAID 1 | RAID 3 | RAID 5
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
enviromnent. 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
Dual Disc Mirror
Immunity
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
Network Disc Mirror
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.
|
