RAID 5 Parity

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RAID 5 Parity

Steve Jones - SSC Editor

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March 24, 2011 at 8:06 pm

#63773

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March 25, 2011 at 2:05 am

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happycat59 One Orange Chip Points: 29389 More actions · Answer 1

Mmm, terminology is getting in my way for this one. My definition for parity (rightly or wrongly) applies at the byte level (Parity Bit).

For RAID 5, I have normally used the term "checksum" which is the value stored on 1 of the 3 disks required to store a single value,

Steve Jones - SSC Editor SSC Guru Points: 736297 More actions · Answer 2

A Better reference: http://technet.microsoft.com/en-us/library/cc938485.aspx

Checksum isn't a term I've seen associate with RAID. Almost always things are listed as parity calculations.

Evil Kraig F SSC Guru Points: 100851 More actions · Answer 3

Cat, it's for both, and same purpose. In a RAID 5, for each section of disk (I forget the break out) in say a 4 disk array, 3 have data, one has a parity to make sure the other three stay intact. Because of that parity, you can assume the data on the 'lost' disk while it's replaced, and rebuild it, due to reverse construction of the bit/bytes. Same concept, expanded further.

Lose two drives and it's game over though, as most folks are well aware. 🙂

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SQLRNNR SSC Guru Points: 281334 More actions · Answer 4

Thanks for the question.

Here is another explanation that makes some sense.

http://riceball.com/d/content/raid-5-parity-what-it-and-how-does-it-work

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UMG Developer SSChampion Points: 13482 More actions · Answer 5

UMG Developer

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March 25, 2011 at 12:42 am

#1303372

Thanks for the question Steve!

Koen Verbeeck SSC Guru Points: 259115 More actions · Answer 6

Nice question, thanks.

Luckily I had the Wikipedia page for RAID bookmarked, in case new QotD featuring RAID would arrise 🙂

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Duncan Pryde SSCertifiable Points: 7956 More actions · Answer 7

Thanks for the question. Reading the article, I now understand RAID 2, 3 and 4 as well.

But won't we all be using ZFS in a few years anyway? 😉

happycat59 One Orange Chip Points: 29389 More actions · Answer 8

Craig Farrell (3/24/2011)
Cat, it's for both, and same purpose. In a RAID 5, for each section of disk (I forget the break out) in say a 4 disk array, 3 have data, one has a parity to make sure the other three stay intact. Because of that parity, you can assume the data on the 'lost' disk while it's replaced, and rebuild it, due to reverse construction of the bit/bytes. Same concept, expanded further.
Lose two drives and it's game over though, as most folks are well aware. 🙂

I believe that the data is actually stored on 3 disks...2 have the actual data and the 3rd has the exclusive OR of the other 2 disks. Using this, if you loose any one of the 3 disks that store the data you are after, the other 2 have enough information to give to retrieve the data. This basic pattern is used regardless of the number of disks in the array. When there are more than 3 disks, each chunk of data is still stored on 3 of the disks. The controller manages the allocation of disk space to ensure that all disks on the array are utilised.

Steve - whilst you may not have heard of checksum (Vs parity) in relation to RAID 5 , try searching google for "RAID 5 checksum". I do agree that you can view this as a "parity" but I am one of them dinosaurs who have been around since the days of punch cards, paper tape and magnetic tape. Parity back then was more to do with detecting error.

SanDroid SSChampion Points: 10068 More actions · Answer 9

Nice question. Love these easy harware quesitons, used to make money and impress friends with good knowledge about RAID and how the differant levels performed and why.

These days I am more interested in the default block size used when writing and reading data to the Disk Volume.

Cliff Jones SSChampion Points: 10547 More actions · Answer 10

Duncan Pryde (3/25/2011)
Thanks for the question. Reading the article, I now understand RAID 2, 3 and 4 as well.
But won't we all be using ZFS in a few years anyway? 😉

Thanks for the link, this is interesting.

UMG Developer SSChampion Points: 13482 More actions · Answer 11

happycat59 (3/25/2011)
Steve - whilst you may not have heard of checksum (Vs parity) in relation to RAID 5 , try searching google for "RAID 5 checksum". I do agree that you can view this as a "parity" but I am one of them dinosaurs who have been around since the days of punch cards, paper tape and magnetic tape. Parity back then was more to do with detecting error.

Isn't that still what the parity/checksum in RAID5 is for to detect and correct errors?

M&M SSC-Insane Points: 21700 More actions · Answer 12

M&M

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March 25, 2011 at 10:29 pm

#1303930

Good one 🙂

M&M

Gobikannan SSCrazy Points: 2735 More actions · Answer 13

I learned some thing form this question.

I referred : http://download.oracle.com/docs/cd/E19494-01/820-1260-15/appendixf.html#50548797_98008

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Gobikannan