Storage Spaces, Server 2012 R2 - Pool shows 99% full but is not?

My office server is running Server 2012  Datacenter R2, fully patched. I setup a Storage Pool using 2 Western Digital Red 3 TB drives. Configuration is Mirrored, Thin Provisioned with a 5 TB capacity. Everything is working fine, but recently, when I look at the Storage Pool, it says that it is 99.6% used and only has 25 GB of free space!? Have not changed anything that I can think of.

However, the 2 Volumes in the pool show that they have the following:
Volume D: 1.25 TB Capacity and 816 GB Free
Volume E: 3.75 TB capacity and 2.82 TB Free

I've tried various powershell commands to trim, defrag, slabconsolidate.
I also moved  142 GB of files off the Storage Pool - and the free space remained at 25 GB?!

Nothing helps - and I am not by any means a storage spaces or powershell expert - just trying things that I've discovered in my research.

This screenshot shows what, to  me, doesn't make sense. Disks with lots of free space and a storage pool that is almost full!:
Am I missing some important concept or is my Storage Pool somehow corrupt, etc.? Any ideas on how to repair or make sense of what I'm seeing?
Sal SoricePresidentAsked:
Who is Participating?
Seth SimmonsConnect With a Mentor Sr. Systems AdministratorCommented:
this is normal though it does sound misleading
the storage pool usage reflects how much is allocated in terms of volumes, not what is actually used in the file system
in your case, most of your 5+tb is allocated for volumes; doesn't reflect file system usage

as an example, if you have a 10tb space and you create 5 1tb volumes but don't put data yet, your storage pool will show 50% used since you created 5tb worth of volumes regardless of how much is used in the file system
David Johnson, CD, MVPConnect With a Mentor OwnerCommented:
since it is mirrored you only have 3TB of space and what you are seeing is correct since you have allocated 4.98 TB.
Gerald ConnollyConnect With a Mentor Commented:
Using 2*3TB disks in a Mirror (aka RAID-1) gives 3TB useable.

You have provisioned 5TB of virtual space on 3TB of physical volume.

You need to add 2 more drives and convert Volume to RAID10  to allow for filling up your Virtual volume.
Sal SoricePresidentAuthor Commented:
Thanks all for the comments! I'm still trying to get the concept clear, but, thanks to your help, I think(!) I've got a better grasp. Still a bit murky on some details (see below).

From what you've offered I think Storage Spaces is trying to tell me I've got the following (feel free to add to the discussion if I've got anything wrong):
> (2) physical 3TB disks in the pool (with 2.73  of available space on each)
> A Pool with a physical capacity of 5.46 TB (ie, 2 x 2.73)
> A virtual disk, thin provisioned to 5 TB
> Of the 5TB virtual disk capacity, 2.72 TB of it is allocated

That last one is where I am still a bit confused.

The physical disks show as a capacity of 2.73 TB each, used space of 2.72 TB and 6.75 GB free. Not sure why, but I'd guess that the 6.75 GB is what is left after file system overhead, etc.

The virtual disk shows as a capacity of 5 TB (which I set to allow for growth) and an 'Allocation' of 2.72 TB. I assume that by 'Allocation', Storage Spaces is telling me that I've only given it 2.72 TB of physical space to work with and that I'd need to give it more (physical disks) if I want to reach my 5 TB capacity. Screenshot:
I'm going to mark this question as answered and award points but welcome any additional comments or corrections regarding what I've said above so that others can learn as much as possible from this question. Thanks!!
Gerald ConnollyCommented:
No you still haven't grasped what mirror is about.

You have two disks in a RAID-1 configuration (aka Mirroring). These two disks are presented as a single virtual volume to your system, any write to this volume means a write to both disks, so you only have the physical capacity of one disk. This means that if one disk fails your data is still safe (but at risk)  on the other.

Just remember that the only certainties in life are Death & Taxes and that HDD's will fail!!!
Sal SoricePresidentAuthor Commented:

Thanks for your reply. I thoroughly understand what RAID1 / Mirror means (and RAID0, 5, 10, etc. - have used them many times (in hardware RAID setups)). It is the 'Allocation' part that left me a bit confused...
Gerald ConnollyCommented:
Sorry Scion, it was this bit that made me think you didnt understand what RAID-1 was "> A Pool with a physical capacity of 5.46 TB (ie, 2 x 2.73)"
Sal SoricePresidentAuthor Commented:
Gerald, thanks for the update. I  may be wrong (have been many times before!), so please correct me if need be! I interpret the pool 'physical capacity' to be what you could 'theoretically' use - ie, with (2) 2.73 TB drives you could get up to 5.46 TB of storage IF you went with RAID 0.
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

All Courses

From novice to tech pro — start learning today.