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raid.doc
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1999-10-23
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-*- Mode: indented-text -*-
--- $Id: raid.doc,v 1.2 1998/10/07 23:17:42 vitus Stab $
WHAT IS RAID?
RAID (redundant array of inexpensive disks) is the name of some
standards to increase the security of the disk subsystem. Those
standards were first published by Gibson, Katz and Patterson in
1987. The idea of RAID is to use several disks to build a fast,
errorprone subsystem. Usuallay this is done by a special SCSI
controller but the possibility of software RAID exists, too.
Currently the following RAID levels are commonly available:
RAID Description Checksum data disks + chksum disks
------------------------------------------------------------------
- Chaining none 2+
0 Striping none 2+ -
1 Mirroring none 1+ 1
2 Stripe Set Hamming Code 2+ 1
3 Stripe Set XOR 2+ 1
4 Striping XOR 2+ 1
5 Striping, distributed XOR 2+ 1
Besides these 'official' RAID levels 0-5 may vendor-unique levels
exist, for example Mylex RAID 6 (combination of RAID 0 and 1) und
7, Siemens RAID 7 and many more. Every vendor has his standards
and most standards are different from standards used by other
vendors.
RAID 0 - Striping
Using two of more disks data is written alternating to all disks.
Transfer rate is usually much higher as a single disk. Access
time remains the same. RAID 0 is no real RAID as it doesn't
provide redundancy. Danger of data loss is greater than a single
disk as with a loss of a single disk the whole subsystem is
worthless. You don't need a controller to implement RAID 0, this
can by done by a piece of software. This isn't a real
professional solution but may nevertheless increase data
throughput.
RAID 1 - Mirroring
A simple way to increase data security: write all data to two or
more disks. So all disks contains exactly the same data.
Disadvantage of this very simple solution: 50 or more percent more
room for your data and --- in case of two disks --- in case both
disks report different data (write error): which disk is correct?
Some vendors name disk mirroring as disk duplexing of two or more
controller are involved. Even in case a controller fails there is
redundancy. To increase data security RAID 1 is often combined
with some other RAID level.
RAID 2 - Stripe Set
Similar to RAID 0 all data is distributed to two or more disks.
But an additional disk is used to write a hamming code. A hamming
code detects errors and may repair smaller errors. Because a
seperate disk is used to store the hamming code RAID 2 is slow.
And because modern disks use a kind of error correction code by
themself RAID 2 is now obsolete.
RAID 3 - Stripe Set
A performance-optimized aternative to RAID2. It's working on a
stripe set and write errorcorrecting to a seperate disk, too. The
difference is that a XOR-operations is used as the errorcorrecting
code. When a disk fails it's contents may be calculated from the
data of all other disks. There is no dataloss. RAID3 is fast but
it's speed is reduced by transfering small, non-continuous blocks.
Good for large transfer sizes.
RAID 4 - Sector Striping
RAID 4 does striping on basis of sector instead of bytes as RAID 3
does. There is also a seperate disk with checksum calculated by
XOR-operations. Because this seperate disk is a bottle-neck RAID
4 isn't as fast as expected, at least when writing or in case of a
disk failure (you don't need the XOR-values when reading).
RAID 5 - Sector Stripng, Distributed Checksums
This RAID level combines features from RAID 0, 3 and 4. Data is
striped in sector-sized units on several (at least three) disks.
Error correction is achieved by using XOR-operations. The
calculated checksum isn't saved on a seperate disk but distributed
along with the data sectors. Therefore there is no single disk
acting as a bottle-neck. RAID 5 combines high security and good
performance and is used for this reasons. Because the
XOR-operation over large amounts of data is still something to
take it's time RAID 5 is often implemented as hardware RAID with a
special controller using a seperate unit to calculate it.
Besides allowing to continue working with a failed disk RAID
features some additional techniques to restructure redundancy
after a failure:
Hot Swapping
Change a disk while keeping the machine up to replace a failed
device. The failed device is stopped, changed and a replacement
is automatically setup and filled to contain the data once stored
on the failed device. There is no need to shutdown a running
server. Be aware that hot swaaping need special connectors to
avoid electrical damage.
Hot Standby
Add an additional disk to the system which is used to replace a
failed device without sysop interaction. Until it is needed the
disk is stopped.
Reference
Translated from german HDDFAQ (copright: PC POWER GmbH, Holger
Ehlers@2:241/1050 aka 2:241/1020.20), available from 2:2474/424 as
hddfaq.zip).