Seeking help: Adding two much larger drives to my DS920+

[WWBiscuit]

To preface everything that follows, I'm pretty much a NOOB in all things NAS.

I have the DS920+ and have been using only two slots, taken up by a pair of Ironwolf 8TB drives. I chose RAID1, as mirroring seemed the most straightforward thing to do. However, I'm running too close to storage capacity and decided that I would just buy a couple of much larger drives to fill slots three and four in my NAS.

Of course, I've subsequently realised that it just ain't as simple as adding more drives! I've splurged on 2 Ironwolf Pro 18TB drives, and now wonder what I should do with them. My rather simplistic vision was to just insert these into the remaining slots, set up another storage pool and have them mirror each other using RAID1. So, the two new larger drives would not be interacting with the original pair of smaller drives at all. I would have two pairs of mirrored drives: a pair of 8TBs and a pair of 18TBs.

Now, I'm not sure if that is preferable or even doable. Using the RAID calculator, it looks like all I end up with is a whole lot of unused space - 20TB of it to be exact - and using RAID1 will not increase my available space at all.

So, I would greatly appreciate some advice about what to do with these two new 18TB drives. Do I exchange them for smaller drives? Or can I still use them in my DS920+? If so, what would be your suggested RAID configuration?

Many thanks in advance

 

[Rusty]

Well, you have options. Some you have already listed. The real question here is what are you expecting from the whole setup?

Do you want more space, more redundancy, multiple volumes, single volume etc?

If you go for a single large volume (SHR) you will lose 18TB of space as redundancy vs 20 that you have no with two volumes. So not a lot a difference but it's there.

You could also configure each of those 18TB drives as single basic drives, giving you 2x18 (36) storage of space and no redundancy options. Ofc, if the drive fails, backup is the only thing that will pull you out.

Mixing smaller and large drives always has a high penalty if you need redundancy. So, best to think what would be the desired outcome on your end in terms of space needed and take it from there.

 

[fredbert]

The RAID calculator is for a new build and assumes the selected drives are in one storage pool. Yes, you can have your current 2x 8TB RAID 1 and add a second RAID 1 with the 2x 18TB. That would provide a raw storage of 26TB, in base10 terms of TB (1Bx10^12 = 1 TB).

With the current RAID 1 you can migrate to RAID 5 but the four drives will all provide the storage capacity of the smallest drive = 24 TB (3x 8TB) + 8TB protection. The rest of the new drives (2x 10TB) will be wasted. Only the common band of storage in RAID is used across the drives, for actual data storage and protection, the overspill is ignored.

If you had used SHR instead of RAID 1 when you set up your NAS the you could have grown the storage pool with the new drives and had 34TB (2x 8TB + 18TB) + 18TB protection. Once you have a pair of the largest drives then all storage is used: bands of the common storage are made across two or more drives (simplistically, drives participate in bands across those drives of the same size or larger). That's SHR with one-drive protection and is very similar to RAID 5, except it can utilise more drive space with mismatched sizes.

Configuration	Raw storage	Usable storage	Protection	Wasted
RAID 1 + RAID 1 total	16TB (2x 8TB) 36TB (2x18TB) 52TB	8TB 18TB 26TB	8TB 18TB 26TB	0TB
RAID 5	52TB (2x 8TB + 2x 18TB)	24TB (2x 8TB + 8TB from a 18TB)	8TB (8TB from other 18TB)	20TB
SHR-1	52TB (2x 8TB + 2x 18TB)	34TB (2x 8TB + 18TB)	18TB	0TB

With two RAID 1 you would have to lose three drives to lose all your data, but only two with RAID 5 or SHR-1.

How you could migrate to SHR-1: by creating a new SHR-1 storage pool with the two 18TB drives you can then used Control Panel to move shared folders across to the new volume on it. For example...

The snag comes when you want to remove the original RAID 1 pool. Since it was your only volume then any packages from Package Center that you installed will be on this volume. It is possible to use SSH and the command line to move files and folders, and repoint links to the new volume locations, but it's not that simple. You can consider making a Hyper Backup task to backup theses applications and then uninstalling them and restoring them after reinstalling on the new volume. The Package Center Settings will provide a new option when you have more than one volume...

 

[dimfil]

As an (almost) noob myself, I would make a Hyper Backup set of my current data, then create a new big pool with all disks in btrfs filesystem and SHR1 raid type. That would give me 8+8+18 TB of usable space.
Then I would restore the data on the new pool.