I'm using Windows 10 Enterprise. I have four identical 3TB disks set up in a Storage Space pool. Each disk shows up as providing 2.72 TB to the pool. I create a storage space with REFS and resiliency as Parity.

Coming from a RAID-5 world, I set the size to (N-1)*drive_size = 8.16 TB. When I proceeded to copy data to the disk, I was surprised when it ran out of space after 7.26 TB. I guess there are a few terms I don't understand.

Under Storage Spaces, it shows:

  • WDC 3TBx4 (F:)
  • Parity
  • 8.16 TB
  • Using 10.8 TB pool capacity

In Windows Explorer, it shows:

  • WDC 3TBx4 (F:)
  • 915 GB free of 8.15 TB.

I guess what I don't understand is, when creating the pool and space in the first place, what these definitions are:

  • Total pool capacity
  • Available pool capacity
  • Size (maximum) <---------------- something I can set
  • Including resiliency

Since I don't want to delete the 4x3TB space and start over, I tried the experiment with 4x5GB VHD files (sadly, I couldn't try 3GB disks since they apparently must be at least 4 GB). With 5 GB disks:

  • Total pool capacity: 17 GB
  • Available pool capacity: 16 GB
  • Size (maximum): defaults to 8 GB, but I can set it to whatever I want
  • Including resiliency: 12 GB

Can someone please explain how the math works?

  • The apparently uncapped size is because it's thin provisioned, see arstechnica.com/information-technology/2012/10/… (that's a good primer in general, though do keep in mind it's targeted towards Win8 and some of the complaints, like rebalancing, are solved in Win10) – Bob Jan 23 '18 at 6:28
  • Also the "Using 10.8 TB pool capacity" basically means your 4 x 2.72 TB = 10.88 TB is full. Possibly due to overhead, see community.spiceworks.com/topic/…. – Bob Jan 23 '18 at 6:29
  • I did a bunch of experiments and the results were not promising. I ended up creating pools and spaces on the command line in Powershell after looking at this thread: social.technet.microsoft.com/Forums/windows/en-US/… Even after manually setting the number of columns (which makes sense) and interleave and stripes (for which I couldn't find a good definition), there was still confusing amounts of overhead. I gave up on the idea and ended up using ZFS on Linux via a VM instead. – stuyguy Jan 31 '18 at 5:52

As in many cases, Microsoft expect us to use this without knowing how space is managed. And we don't. Practically, instead of 8.16TB usable space you get around 1 TB less. Some of its space to create a write back cache, some is used for not yet known reasons.

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