Minimum number of disks to implement RAID6

Question

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

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

thanks

Solution 1

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?

Solution 2

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.

Solution 3

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.

Comments

[Urda]

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.

[gbn]

@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?

[b0fh]

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)

[ny195]

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! ;-)

[ny195]

Can't you just use a 3-disk RAID 1 system?

[FooBee]

@Ajedi32: No, you can't, as RAID1 needs multiples of 2.

[ny195]

@Sven Isn't RAID 1 just mirroring? Why would that require multiples of 2?

[ny195]

@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.

[ny195]

@Sven So I did a bit of research, and it seems our misunderstanding has arisen from the differences in capabilities between software and hardware RAID. It most definitely is possible to set up a three-disk RAID 1 array, but you have to do it in software. Have a look at this thread: ubuntuforums.org/showthread.php?t=825205

[gbn]

@Ajedi32: the comment was not directed at you! A simple look at RAID anywhere would have answered this. I;m flattered to be in a Google result though

[MrMajestyk]

Generally speaking, RAID 6 is good for larger amounts of disks where disk cost is a problem. Writes to RAID 6 (and 5) setups are not very fast though. Reading is nice and speedy, so if that is your thing, RAID 6 is nice. In both the RAID 5 and 6 cases, I would add a hot spare to add resilience. If you were to spend a bit more on a setup, go for RAID 10 - it has both good read and write performance and up 50% failure tolerance. Still, a hot spare is really nice though.

[ny195]

@gbn Yes, I realize you meant that comment for the OP, and I agree that this question wasn't very well researched. I was just saying that, from the perspective of anyone other than the OP viewing this question (which, judging by the view count and Google ranking, is quite a lot of people), remarks like like that aren't very useful, and should probably go in the comments instead of the answers.

[ny195]

@gbn I think a lot of people on SE tend to forget that answers aren't just meant for the OP, but also potentially for thousands of future viewers who come from Google. We're not just answering people's questions here, we're building a giant FAQ. ;)

[hanshenrik]

if you use SNIA's definition of raid6, then the minimum number of required disks is 3. by SINA's definition, a simple raid1 with 3 disks is RAID6-compliant, any 2 disks can die and the raid is still fully operational. (SNIA=Storage Networking Industry Association)

[Darf Nader]

If RAID 6 requires 2 disks for parity, if D is the size of each disk in domr N number of disks in the RAID 6 set, I'd generally expect roughly (N-2)D usable storage in the RAID set. What I am curious about if there is a limit to a RAID 6 size or if size begins to cause performance hits. I use 8 disks in my RAID 6 set and it seems great, but I wonder what would happen if I tried 16 disks?