Drobo S Multiple Volumes - Understanding Space Remaining

I recently added Drives to my Drobo S which creates new volumes once you pass 16TB, I’m now at 35 TB (2x 10TB drives, 3x 5TB drives).

Drobo Vol 1 - Drobo Dashboard shows 16TB max capacity, sounds about right
Drobo Vol 2 - Drobo Dashboard also shows 16TB max capacity - on this I am in doubt.

Drobo Dashboard’s Capacity Tab shows 22.64 TB for Data, most of the rest is for protection, which make sense. What doesn’t make sense is that 16 + 16 = 32TB, and that’s 10TB more than 22. There’s no way both volumes have 16TB of free space, and I can’t really get an accurate read on how much remains.

Finder shows Volume 1 as having 17.59 TB of capacity, and the same for volume 2, with over 15TB available. I was wanting to use this new volume for Time Machine backups, but I don’t know how well that’s going to work if neither I nor Dashboard nor Finder can really tell how much is left in there with any degree of accuracy.

How can I get an accurate read on how much space I have on the multiple Drobo Volumes?

Please post a screenshot of the 2 volumes on your Drobo, taken from the top of your Drobo Dashboard menu. Your 2 volumes should each show up as 1 liner with max capacity and used.

I believe this is what you’re referring to

Forget the “16 + 16 = 32TB” storage space, those are just volume sizes, not actual storage available. This is called ‘thin-provisioning’, and involves using virtualization technology to give the appearance of having more physical resources than are actually available. This makes it easier for your computer’s OS to think it has stable drive capacity while in reality it will fluctuate depending on different drives.

The only way to get ‘actual’ space used is by looking at your Drobo dashboard… or by looking at the blue lights on the front panel. Each light represents about 10% usage.

Hope this helps.

You also need to allow for lost capacity for RAID redundancy. Easiest way to figure space available is to add the capacity of all your drives, then subtract your largest one (two if using dual disk redundancy). In this case you have about 25TB of usable drive space…give or take.

I’m not familiar with setting up Drobo with Time Machine, but maybe this will help…

Your ‘DroboVol2’ is available for data. The Drobo Dashboard takes precedence over any Finder information on your volumes as the Mac does not report the Drobo volume structure correctly.

As an aside, your ‘Drobo’ Volume is at 84% and you will likely get a warning soon, or better, look to move some data from ‘Drobo’ to ‘DroboVol2’.

My question remains unanswered though I do appreciate your help.

My Drobo says I have 6.71 TB free for storage. I don’t know if that’s
6.71 TB Free - Vol1 at 2.45 TB Free (16-13.55TB used) = 4.26TB Vol 2? How it’s allocated is something I can only guess at, it’s unclear.

I also feel it shouldn’t be this difficult to figure out. I get that it’s an older product but still it’s supposed to be a product that’s designed to be around for a while. Even if Drobo got calculations wrong in the beginning a firmware / software update should make this simpler to understand. I heard a rumor once that computers are good with math.

As for the Time Machine page, that does not appear to be compatible with my Drobo S, as I am not presented with the “Enable Backup” Button. I’m on Drobo Dashboard 3.5 which is the latest as far as I can tell.

I’m interested to pursue this investigation further.
Are you able to share a screenshot of your both your Capacity (Pie chart) and Usage (Ribbon Bar) from the Drobo Dashboard?

Just now seeing this message, here are the screenshots -

Last question.
Going back you your posts, you mentioned that you had recently added new drives.
Can you advise what your array structure (HDD capacities) was before the recent additions and what new drives were added?

Previously - 4x 5TB drives, 1x 2TB drive
Currently - 3x 5TB drives, 2x 10TB drives

So I took out 1 of the 5TB drives and the only 2 TB drive and replaced them with 2x 10TB drives.