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RAID6 is intended to provide fault tolerance in the event 2 disks fail.

What is the minimum number of disks required to implement RAID6?

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migrated from stackoverflow.com Mar 13 '10 at 12:22

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4 Answers 4

http://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_6

Raid 6 is basically "two parity blocks" vs Raid 5 having one.

So you need n+2 where n is the number of data disks.

The minimum is 4 disks = 2 for data, 2 for parity
I suppose you could get away with 3 disks, but why have 2 parity for 1 data?

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I don't think any controller is going to let you do a RAID 6 with only 3 disks. It will force you to do a RAID 5. If it does let you do a RAID 6 with three disks, I would stop using that RAID card right away since it can't be reliable at all. –  Urda Mar 13 '10 at 17:38
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RAID6 over three disks is in fact equivalent to RAID1... –  b0fh Mar 13 '10 at 19:07
    
@b0fh, @Urda: personally, I've never tried RAID 6 and have never heard of anyone using it until now. With 4 disks, why not RAID 10 for example? –  gbn Mar 14 '10 at 9:04
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With raid10, if you randomly lose 2 disks, there is a 1/3 chance of them being the same stripe, and you lose the whole array. Raid6 will guard you against any combination of 2 drives failing (but is computationally more expensive, and has lower parallelism as well) –  b0fh Mar 14 '10 at 12:43
    
Fyi: that link is dead (well, the fragment part anyway) and the Wikipedia page it points to no longer explicitly states the minimum number of drives required for RAID 6. Additionally, this question is now the third result on Google for "raid 6 minimum drives". This is why I don't believe sentiments like "Did you do any investigation yourself?" are particularly useful on SE. I am doing investigation myself. That's why I'm here! ;-) –  Ajedi32 Jan 30 at 15:05

It makes a lot of sense to go for a 3 disk raid 6.

Its the most efficient config that can handle 2 failures. I can assure you that its a very nervous time when you loose one disk on a raid 1 or 5 system, especially when you know that the disks were bought at the same time which increases risk dramatically. With Raid 6 you can still be quite laid back about it.

After a disk fail you pull the wrong disk out, whoops, with raid 6 its stressful but not fatal.

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Can't you just use a 3-disk RAID 1 system? –  Ajedi32 Jan 30 at 15:16
    
@Ajedi32: No, you can't, as RAID1 needs multiples of 2. –  Sven Jan 30 at 15:26
    
@Sven Isn't RAID 1 just mirroring? Why would that require multiples of 2? –  Ajedi32 Jan 30 at 15:34
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@Ajedi32: Because the disks are mirrored pairwise. What you likely have in mind is an extension called RAID 1E. –  Sven Jan 30 at 15:39
    
@Sven I think you've got it backwards (either that or the Wikipedia page you linked to does). Compare: en.wikipedia.org/wiki/Non-standard_RAID_levels#RAID_1E and en.wikipedia.org/wiki/Standard_RAID_levels#RAID_1 Based on those diagrams, it seems that with RAID 1E in a three disk array, if two disks fail then you're out of luck. With Raid 1 in a three disk array, that's not the case. –  Ajedi32 Jan 30 at 15:46

Since you need a double level of parity it should be 4 disks. You split the data on 2 disks while you calculate different parities on the other ones..

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The minimum number of drives required for RAID 6 is four.

Technically there's no reason why you couldn't set up a RAID 6 array with only three disks, but most RAID controllers don't support this. (See Why does RAID 6 require at least 4 disks?)

In a RAID 6 array with four disks, data blocks will be distributed across the drives, with two disks being used to store each data block, and two being used to store parity blocks. As you stated, with this setup you can lose up to two disks simultaneously without experiencing any data loss.

In the future, you can use this helpful table comparing various RAID levels to help you decide which RAID level is suitable for your needs.

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