Microsoft Storage Spaces: unusable space when using drives with different size

Question

I want to join five drives (4x50GB, 1x100GB) into one pool/drive/file system using storage spaces. I do also want to use the "Mirror" option.

So, in my opinion, I should be able to get 150GB mirrored space. But I am not able to achieve this.

1st try:

• Add all drives to the pool at once.
• Pool has 296GB capacity (295GB free) => fine
• Add new virtual drive (mirror, 2-way, fixed size, max size = 126GB) => that is less than the 150GB I expected
• Now the pool still has 296GB capacity and 40GB free. This is correct, but I would like to also use those 40GB. Even if I create a new virtual drive, I get the message that there are only 0,00 Bytes available.

2nd try:

• First add the 100Gb drive and two 50GB drives
• Pool has 198GB capacity (197GB free) => fine
• Add new virtual drive (mirror, 2-way, fixed size, max size = 91GB) => that is less than the 100GB I expected
• Now the pool still has 198GB capacity and 12,5GB free.
• Now I add the two remaining 50GB drives to the pool => pool has 296GB capacity (111GB free)
• I can increase the size of the virtual drive to 143GB, now there are 6,5GB free in the pool => this is already much better than the first try, but still not perfect.

I am not able to reach a better utilization. I would be glad for ways to use all of the pool space.

The tests above were done in a virtual machine environment to demonstrate the behaviour, in real life I plan to use bigger SSDs (1x1TB, 4x512GB), so it is not like I can just buy two cheap hard drives instead to do the job.

Thanks for your help!

Answer 1

OK, I am not able to find a "perfect" solution, but setting the columns to 1 manually with Get-StoragePool POOL | Set-ResiliencySetting -Name Mirror -NumberOfColumnsDefault 1 got me down to 4GB "wasted" space out of 300GB, which is ok for me. I understand that, without an explicit setting, Storage Spaces set a value which may not be optimal for using all available space, but instead for speed. So you probably don't want to alter it if you don't have to. I still don't understand the math behind it (1 column = 4GB unusable space, how can this be 6,5GB or 12GB, maybe 2 and 4 columns?).