# understanding RAID Level 5

Just wanted to clear my understanding on RAID 5, suppose we have 100 GB of data to be stored in SAN in RAID level 5, does the actual storage means 100 *5= 500 GB ?, or it is the 100 GB only which is stored in different 5 dsiks? please make me understand. Thanks

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This is actually a very confusing question. Are you asking how data is stored in a RAID 5? How a RAID 5 works? How to calculate volume space in a RAID 5? See my answer below for what I think you are actually asking... –  Jes Jul 1 '10 at 13:56
Smells like homework... –  Chris S Nov 10 '10 at 1:49

If I'm reading your question correctly, you are asking how the 100GB of data would be stored on the RAID 5?

RAID 5 works by striping data across multiple disks (must be at least 3 disks), and also performing a parity calculation. For this reason, a RAID 5 is excellent with Read speeds, but slow for Writes.

Your 100GB of data will be written across all disks in a RAID 5. In addition to that, a parity calculation will be performed across all of the data, and distributed across all of the disks in the array. This way, if any single disk fails, all of the data that was on it can be re-created from the Parity information.

However, the parity calculation does increase the space required to store the data. In a 3-disk array, 100GB of data will actually consume 133GB. In a 4 disk array it would be 125GB. But that's not really important - all that's important is that you remember that in a RAID 5, your available space is N-1. 100GB will look like 100GB - the additional space required is already calculated in to the extra drive in the array.

Does that help?

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Note that the number 5 in RAID5 has nothing to do with the number of disks involved.

It's just a name of this specific setup, which distributes parity information across all disks. You need a minimum of 3 disks, but can use an unlimited number of disks theoretically (in practice, this number is constrained by RAID controller limits). Total space you get from the array corresponds to S * (n-1), where S is the size of the smallest disk, and n is the number of disks.

For example, 4 disks of 1Tb will give you 3Tb of usable space in RAID5.

Wikipedia explains how the setup works in more detail.

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I'm hoping that "4 disks of 1Tb will give you 3Gb" is a typo? –  tombull89 Nov 9 '10 at 23:04
That would be a neat setup. :) Somebody already fixed this for me, luckily. –  Groo Nov 10 '10 at 9:08

The actual storage means 100*4 because 1 disc is used to store block. For more info check out the following links. http://en.wikipedia.org/wiki/RAID http://compreviews.about.com/od/storage/l/aaRAIDPage1.htm

Greetz

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He's not talking about 100GB disks but about 100GB of data, he's not either talking about the number of disks, so I'd rather not talk about 100*4 as it could be misleading. –  Luke404 Aug 3 '10 at 5:16

It's simpler than that;

Total storage used = space + (space/disks); i.e. 100 + (100/3) = 133, meaning 44 per disk in a three disk R5 array.

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Check this one out: http://en.wikipedia.org/wiki/RAID_5#RAID_5 Available disk space is shown there.

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Wikipedia actually has a really thorough walk-through on RAID technology and raid levels.