Can we match disk sizes on RAID 5, after the fact?

[forbrich]

Hi - new DS NAS owner here.

I set up my new DS920+ to RAID 5 using 2x 10TB and 2x 8TB disks (since my drive supplier only had those in stock at the time). This config effectively sees 4x 8 TB drives, losing 2 TB from each of the 10 TB drives.

I have since purchased two more 8 TB drives. (I will use the 10 TB drives elsewhere.)

My questions:
Is it possible to swap the 10 TBs out for 8 TB ones, one at a time, using the RAID 5 / btrfs rebuild capability? Any concerns?
Or will it be better to save the data elsewhere, and reinstall from scratch with 4x 8 TB drives?

Thanks

 

[Coop777]

since you have drives of different sizes...why not consider SHR instead of traditional RAID?

 

[forbrich]

Not looking for alternatives, but thanks for the idea.

 

[Coop777]

you can't reduce disk sizes with traditional raid. You must use disks of equal to or greater than the size that RAID is configured with.

 

[Telos]

Is it possible to swap the 10 TBs out for 8 TB ones, one at a time, using the RAID 5 / btrfs rebuild capability? Any concerns?

If RAID5 (not SHR), you can replace the 10 TB drives with 8TB drives, presuming I understood correctly that your current pool size is appx 24 TB (21.82 TiB).

As with any drive swap, ensure your backups are current before proceeding. I also recommend a data scrub to ensure storage integrity, if your system is btrfs formatted.

 

[forbrich]

Thanks Telos.

Yes, the current pool is approximately 24TB, 21.8TiB.

-- post merged: 3. Mar 2023 --

you can't reduce disk sizes with traditional raid. You must use disks of equal to or greater than the size that RAID is configured with.

Thanks Coop777

I'm not trying to reduce the size of the pool.

In RAID 5, the smallest size is used across the board - my disk sizes are 8 TB and 10 TB, therefore the pool uses 8 TB from each disk.

You say

You must use disks of equal to or greater than the size that RAID is configured with.'

and I'm asking whether the RAID thinks it is configured with 4x 8 TB disks, or if it knows two of the disks are 10 TB with 2 TB wasted.

 

[forbrich]

Went ahead and swapped the first 10 TB drive out and put in the (minimum RAID size) 8 TB drive. It works.

in this configuration (RAID 5, mixed disk sizes) it IS possible to swap out larger disks for smaller ones, down to the size of the smallest existing disk. The pool is placed in a degraded state, and the system requests that you manually invoke Repair from the Storage panel. During the repair, the new disk is detected, repair procedure works well, and seems to automatically invoke a complete scrub.

As Telos suggests, a backup of [critical] data is recommended before starting this.

Bottom line: I will end up with a uniform 4-disk RAID 5 system, and will have recovered the unused space from the larger disks.

Thanks for your help.