Is it wise to use parity over two-way mirror in a window storage space with 3 or more disks?

Question

Windows storage spaces use slabs for the actual storage. Slabs are distributed in an (for the user) unknown fashion over the available disks. Even if you use two-way mirror and two disks you only know for sure that each disks contains all data; either an original or a copy slab. But it is not so that one disk contains all original and the other all copy slabs.

With three disks things become interesting. In two-way mirror each slab is still present on two different disks, but everything is spread out over all three. It no longer is a mirror, just a situation where every block has a copy on another disk. This setup has a one disk resiliency, and kinda resembles parity.

With four disks the slabs in a two-way mirror are spread over four disks. Still a one disk resiliency. Same as parity.

When reaching seven disks, parity will automatically give you two disk resiliency. But with two-way mirroring and some bad luck, if just the wrong two disk crash (the one with the original and copy slab), you still lose data. It seems two way mirroring will always have just 1 disk resiliency and only 50% capacity, no matter how many disk you throw at it.

So with more than two disks, it is better to use parity? You'll have more space available, but the same or better resiliency. IOW if you expect to add more disk to a storage space then 2, then always start with 3 disks using parity? (Ignoring any performance related considerations.)

Answer 1

Yes, unfortunately with 2 way mirroring your data is divided in slabs and sparsed over all discs. Each slab copied on a disk will have a copy on another disk. Considering this behaviour, you can only lose one disk.

Answer 2

I know the answer has been answered, but here's my experience. I've been using Parity for more than 6-7 years with around 5-6 disks (externals) ranging now from 2TB to 8 TB. Once every ~2 years, a drive dies. Sometime, like this year, two almost at the same time. I'm really happy so far since I haven't had any issue with loss of data. Also, most of the files are only for read-only and performance is not a requirement for me either. So, when you reach a certain threshold (I think the disk status tells you to add a new drive and becomes yellow), you should either delete some stuff or add a new drive and you won't have any issue with redundancy since disks will rebalance itself as it goes if one or two die.