Depending on how the array is set up all you need to protect against is one single drive failing, the third drive contains what is known as "parity" information for the other two drives.
Effectively what you have is the following oversimplification:
- Drive 1: data block 1
- Drive 2: data block 2
- Drive 3: block of data that contains sum of data block 1 and data block 2
If data block 1 becomes corrupt then you can simply take the data on drive 2 and subtract it from the data on drive 3 to recover the information on drive 1. In this way you have resilience, but don't need as much space as a full backup and it can be extended to any number of drives and give you fault tolerance for any single drive failure.
This will be slower than just reading the original data and will not protect your data if you have two drives fail, but it does mean that you only need to use a single drive for the protection rather than having to have a backup drive for every "data" drive.
As an example for RAID 1 (mirror) you will always have one half of your total number of disks "wasted" to give you redundancy in case of failure. In the theme above (RAID 5) you could have four disks and only need to use one of them for the parity information meaning that rather than using half of your disk space for protection you are now only using one quarter leaving you three quarters for your storage.
1Don't use SHR - it will use RAID5. Don't use RAID5 either - with 4TB and 6TB disks you are simply asking for your data to be destroyed. RAID5 is a joke with big modern consumer drives. It's an enormous gamble to rebuild, and that's when you need it most. There is no protection here. This is a bad combination of disks to put into an array - just don't do it. – J... – 2017-08-15T09:58:41.010
^^ Hear, hear. It literally takes half a week at these sizes to build and rebuild. Which is no biggie when disks are new, there's no data on them anyway, and failures are a mostly theoretical thing. But when you have SMART degrading (or one disk already failed) then scrubbing the remaining disks for 4 days straight is not precisely what you dream of. – Damon – 2017-08-15T10:02:38.550
So even though this is dooable your advice is always have a spared configuration. I think synology has solved this "spare problem" with SHR-2, which has the requisite double redundancy, but will still automatically upscale my volumes if I add bigger drives. – awiebe – 2017-08-16T05:25:25.250
@awiebe No, the advice is to not use RAID5. If you have a single disk failure with RAID5 and 3-6TB consumer grade disks then your only hope to get the array back online is to swap in a good drive and wait half a week for the rebuild, crossing your fingers that you don't get a single read error on any other sector of any other drive - an outcome with such frighteningly low probability that you might as well just forget about it. RAID5 is basically a massive liability. – J... – 2017-08-16T14:22:40.960