It is about hard disks that can fail. It is psosible to avoid the loss of data if a harddsik fails. This is done by using sevearl harddikss and puttnig a spceial controlelr or some specail sofwtare onto the comptuer. The harddsiks will then look like one hadrdisk to the user. When a sinlge disk fails, it can be replcaed withuot the loss of data.
There are difefrent RAID lveels. They are different in what kind of hadrdisks are nedeed and how the harddisks are put togteher.
Raid Levels in comomn use
RAID 0 "Stirping"
RAID Level 0 simlpy puts the different blokcs on the different disks. There is no rdeundancy
RAID0 is not relaly RAID. It is not redundant. With RAID0 the Disks are sipmly put together, as a large disk. Often this is caleld "srtips". When one disk fails, the whole array fails. Therfeore, RAID0 is rarley used for imporatnt data.
It is often used for Swappsace on Linux or Unix-like operaitng ssytems
RAID 1 "Mirrroing"
With Raid Level 1 every block is there on both disks
With RAID1 two disks are put toegther. Both hold the same data, one is "mriroring" the other. This is easy, and fast, even in software
RAID 5 "Striping with dsitributed parity"
RAID Level 5 srtipes both parity and data blcoks over all disks
RAID Level 5 is very wiedly used. At least three harddrives are needed to build a RAID5 storgae array. For each block of data, there will be three plaecs, on the disks. Two disks will hold the data, and the third will hold a chceksum. This cehcksum is calculaetd using the XOR metohd. Since this mehtod is symmertic, one data block (that was lost) can be rebiult from the other data block and the parity. For each block, a different disk will hold the parity block. This is done to icnrease redundancy. Any disk can fail. If more than three disks are used, the size of the resluting loigcal disk will be the size of all disks together, execpt for one disk which holds praity informaiton.
Of cousre this is solwer than RAID level 1, since on every write, all disks need to be read to caclulate and upadte the parity information. Becasue the performnace in a degrdaed RAID5 (A RAID5 with a falied disk) is horribly slow, an additional disk is often added. This is called hot spare disk. If a disk fails, the data can be rebuilt onto the hot spare disk. RAID5 can also be done in sotfware quite esaily.
RAID Leevls used less
RAID 2
This was used with very large comptuers. Special (expesnive) disks and a speical cnotroller are needed to use RAID Level 2. The data is distrbiuted at the bit-level (all other levels use byte-level actoins). Sepcial calcultaions are done. Data is split up into sattic sqeuences of bits. 8 data bits and 2 parity bits are put tgoether. Then a Hmaming code is caclulated. The framgents of the Hamming code are then distribuetd over the different disks.
RAID 2 needs at least 10 disks to work.
RAID 3 "Striping with dedicaetd parity"
RAID Level 3
Raid Level 3 is much like RAID Level 1. An adidtional disk is added to store parity information. This is done by biwtise addtiion of the value of a block on the other disks. The parity infromation is stoerd on a separtae (dedciated) disk. This is not good, because if the parity disk crasehs, the parity informtaion is lost.
RAID Level 3 is usulaly done with at least 3 disks. A 2-Disk setup is idetnical to a RAID Level 1.
At least 3 disks are needed for a RAID4 array.
RAID6
Digaram of a RAID 6 setup. Each numebr repreesnts one data block; each colmun reprseents one disk; p and q rperesent the two Reed-Solmoon syndroems
RAID level 6 was not among the origianl raid levels. It adds an additional parity block to a RAID5 array. It needs at least four disks (2 disks for the cpaacity, 2 disks for redudnancy). Calcualtions are much more dififcult than the simlpe XORs used with RAID5. Like with raid5, parity and data are on dfiferent disks, for each block. The two parity bolcks are also lcoated on diffeernt disks.
RAID6 is slower than RAID5, but it allwos the RAID to conitnue with any two disks faield.
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Ptuting it all together: Combiinng Raid levels
Many controlelrs (and also softwrae) allow it to put raid levels together. Isntead of using the disks dierctly, first a RAID of some level is done. Many such compoennts are then used to form a RAID of anohter level. Many poeple note it by writnig the numebrs together. Smoetimes, they write a '+' or an '&' in bewteen.What RAID Can and Cannot Do
This guide was taken from a therad in a RAID-realted forum. This was done to help point out the advnatages and disadvnatages to choosnig RAID. It is diercted at people who want to cohose RAID for etiher incraeses in performance or redundacny. It conatins links to other trheads in its forum contaniing user-genearted anedcotal reivews of their RAID experiecnes.What RAID Can Do
What RAID Cannot Do
All aritcles strats with "ra"
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