Hmmmm.....this whole Storage Spaces thing ioften doesn't make sense.
I took a Windows 8 VM, and I added 3 x 4GB hard drives to it. I created a storage spaces pool with all 3 disks and that showed up as a total pool size of 9.75GB. My 3gb drives provide 3.25GB of space after overhead/metadata, etc.
So, then I create a Storage Space. I selected Resiliency of Parity and I chose to create a size of 6GB, so including resiliency, it says that it would consume 9GB of disk space. So, just a little less than my total pool capacity of 9.75GB.
Upon creation, it shows that my storage pool is using 3.50GB of 9.75GB of pool capacity
My storage space says 6.0Gb, using 2.25GB of pool capacity.
Windows explorer shows a 6GB drive E that's available (well, 5.87gb according to Windows).
So, then I copied in a 1GB ISO for Exchange Server 2010. As expected my free space in Explorer went from 5.87gb to 4.76gb free.
But looking at my Storage Pool and my Storage space...I increased to using 5.0Gb of my pool and 3.75gb of my storage space... Thus, these both grew by 1.5Gb.
So, I then copied my 1gb ISO file and pasted it onto the same drive. Therefore, I'm using 3.72GB of my 5.87GB volume...so around 2GB was consumed. And my pool increased to 6.5GB out of 9.75 and my storage space increased to 5.25gb of pool capacity.
So, at this point, WIndows explorer says I have 3.72GB free, but storage spaces shows only 750MB or so free.
So, I tried to copy that 1GB ISO file again and paste it to the storage space drive. So, now Explorer says 2.67GB free out of 5.87GB...so around 3gb is consumed...as expected.
My pool now says 8.0GB used out of 9.75GB, and my storage space is telling me I'm using 6.75Gb of pool capacity and I should add 3 MORE drives.
I copied my 1GB iso file again. So, this is the 4th copy of the file, so around 4gb of space total consumed. So, explorer shows 1.63gb free. However, the storage pool says 9.50GB of 9.75GB is consumed, and my storage space says I am using 8.25GB pool capacity, it's in ERROR state and I should add 2 drives.
So, what gives? It appears I created a 6gb volume, that after resiliency purposes would consume approx 9.0gb of my pool...which supposedly provides 9.75gb of space.
However, after copying in 4GB of data, I'm out of space. I'm unable to copy that 1GB iso file again. Even though I have 1.67GB of free space.
So, I have 12GB of physical disk space, and with a normal RAID 5 style config, that would give me around 8GB of usable space. However, after just 4GB of writes, Windows is out of space?????? That's the worst efficiency of disk space I have ever seen.