I have the following configuration:

  • Windows Server 2012 R2 OS installed on baremetal with Hyper-V. OS is installed on RAID1 850 PRO drives.
  • Windows Server 2012 R2 Guest OS Generation 2 VM is located on Windows Storage Space Mirrored 950 PRO drives.

I ran some tests on the Windows Storage Space Mirrored 950 PRO drives using Crystal Disk. These are the following stats.

When tested on baremetal

   Sequential Read (Q= 32,T= 1) :  4178.159 MB/s
  Sequential Write (Q= 32,T= 1) :  1504.286 MB/s
  Random Read 4KiB (Q= 32,T= 1) :   460.155 MB/s [112342.5 IOPS]
 Random Write 4KiB (Q= 32,T= 1) :   266.983 MB/s [ 65181.4 IOPS]
         Sequential Read (T= 1) :  2042.397 MB/s
        Sequential Write (T= 1) :  1500.305 MB/s
   Random Read 4KiB (Q= 1,T= 1) :    40.059 MB/s [  9780.0 IOPS]
  Random Write 4KiB (Q= 1,T= 1) :    76.623 MB/s [ 18706.8 IOPS]

When tested in VM

   Sequential Read (Q= 32,T= 1) :  4635.549 MB/s
  Sequential Write (Q= 32,T= 1) :  1502.071 MB/s
  Random Read 4KiB (Q= 32,T= 1) :   181.225 MB/s [ 44244.4 IOPS]
 Random Write 4KiB (Q= 32,T= 1) :   110.272 MB/s [ 26921.9 IOPS]
         Sequential Read (T= 1) :  1624.129 MB/s
        Sequential Write (T= 1) :  1381.203 MB/s
   Random Read 4KiB (Q= 1,T= 1) :    24.528 MB/s [  5988.3 IOPS]
  Random Write 4KiB (Q= 1,T= 1) :    34.563 MB/s [  8438.2 IOPS]

I have nearly a 50% drop in performance on my Random Read/Writes. I retested a couple times to confirm values. Is this to be expected?

  • You are aware that regardless what loss you have on Hyper-V .... the random throughput numbers for the bare metal are already COMICALLY LOW?
    – TomTom
    Mar 25, 2016 at 3:44
  • @TomTom It is? I never tried Windows Storage Space Mirrored configuration or 950 PROs on windows server 2012 r2 before. I didn't know what to expect. What do you think it should be?
    – SILENT
    Mar 25, 2016 at 4:01
  • Check the spec numbers. Remember: Raid 1, both discs can read independent. And yes, storage spaces is slow - which is why everyone I know avoids it for this type of work. We all hope for Server 2016...
    – TomTom
    Mar 25, 2016 at 4:08
  • I'm not sure about the margin for error but Random Write 4KiB (Q= 1, T= 1) is 50% lower than what it's speced to be. I'll tried it without Windows Storage Spaces with similar results. I even tried on my spare 950 PRO drives with similar results. Not sure where the bottle neck is.
    – SILENT
    Mar 25, 2016 at 5:17
  • Also remember hyper-v is "never" T=1 - you normally have multiple VM's running around. But yeah, that is what I mean - your numbers look too low from the start.
    – TomTom
    Mar 25, 2016 at 6:53

1 Answer 1


First of all it's a good idea to use DiskSPD rather than anything else. Just because Microsoft uses DiskSPD during their internal QA process.


Second this is known issue with Hyper-V. It's even present in more or less the same way in Windows Server 2016. In a nutshell there's typically a 2x - 4x I/O performance drop if your underlying storage has very low latency. You can try a couple of workarounds to improve your conditions:

Use multiple volumes. Don't allocate multiple CSVs on top of a single virtual disk or namespace but rather make sure local mount points (if you run VMs right from DAS), CSVs or SMB3 shares end on top of a different logical volumes! You'll have to have a single VM with multiple VHD files or multiple VMs to "aggregate" combined performance this way. If you have one VM with a single attached VHD this approach won't help obviously.

Use "middle man". This is sort of an incremental improvement within the first approach listed. You get multiple logical volumes, wrap some Windows-based software-defined storage on top and allow it to aggregate I/O performance for your single VM with a single VHD attached. It can happen if you run single Oracle RAC or SQL Server VM and it's exactly the one you want high IOPS for.

Disable I/O scheduler. Set registry key ...


... to zero! P.S. Don't forget to reboot as nothing changes on-the-fly.

This one may or may not work. We get ±20% boost on a pair of Samsung 850 Pro SSDs in RAID0.

Hope something is going to work for you. Please update with your results for future lurkers :)

  • 2
    Could you elaborate a bit more on "middle man" approach? What SDS solutions would be a good practice apart from Storage Spaces itself? Can I combine Storage Spaces with a 3rd party SDS to boost performance/get redundancy/extra features? We have Samsung EVO's as well but in RAID 5 and would like to squeeze the maximum from them.
    – Strepsils
    Mar 31, 2016 at 9:36

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