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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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?
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.
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.