Raid 1 for 4 drives

If you have apps installed, you'll also need to uninstall them before destroying the volume (reinstalling them after the new ones are created).

You'd have 50% more space with RAID-5 and a single volume (assuming all disks are the same size), and you could easily put the information in different shares.  

The main benefits of two RAID-1 volumes are that the extra redundancy gives somewhat more protection from disk failures, and that data recovery (if you ever need it) would be simpler.

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jhsg wrote:
Thanks Stephen. I've got 2x 4tb drives for 1&2 and 2x 2tb drives for 3&4. Not sure I have any other option than what I'm trying to do?

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The default XRAID would give you an 8 TB volume.  Your approach would give a 4 TB volume and a 2 TB volume (2 TB less space).

One limitation of XRAID is that you can't add smaller disks to the 2x4TB array.  You can add 4 TB drives to a 2x2TB array though. (disks you replace can be the same size as the one you removed.  New disks need to be at least as large as the biggest existing disk).

If you have no data on them (or if you are willing to restore the data from backup), you can do a factory reset (or initial factory install) with all drives in place.  That will give you the full 8 TB.

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jhsg wrote:
I don't understand how you can have full redundancy if you haven't got double the space of where the data is kept?

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That's because you don't understand RAID-5.  But the short answer is that you would in fact have redundancy that would allow any drive's content to be rebuilt from the remaining drives.

Here's the key idea:  Imagine you have 3 numbers (A, B, and C) that you'd stored.  A is on drive 1, B is on drive 2, and C is on drive 3. Now let's store S=A+B+C on drive 4.

If one of the drives is replaced, you can reconstruct the missing number.  Depending on the drive you replace, it's one of these equations:

Drive 1: A=S-B-C

Drive 2: B=S-A-C

Drive 3: C=S-A-B

Drive 4: S=A+B+C

There's a bit more to it, but that's the key idea for RAID-5.

In your scenario, the drives are of unequal size.  So xraid divides the 4 TB drives into 2 TB pieces.

The two "bottom" 2 TB pieces is used (with the 2 TB drives) to create a 4x2TB RAID-5 array that works like I described above.  That is a bottom RAID "layer".

The two "top" 2 TB pieces are used to create a top 2 TB RAID-1 mirror.  

The NAS then puts the BTRFS file system on top of both layers, so you only see one volume.

If you upgrade one of the remaining 2 TB drives to 4 TB later on, the NAS will convert the top layer to RAID-5.