1.    21 Dec 2016 #1
    Join Date : Dec 2015
    Posts : 6,736
    Windows10

    Slow writing to physical disks in Hyper-V


    If I copy a file from C drive in a hyper-v install to a separate partition on the vhd, I get approx 100MB/s which is pretty much the maximum hdd speed that contains the vhd.

    However, when I copy a file to an external physical drive, it is very slow @ approx 5 MB/s.

    Any suggestions how to fix?
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  2.    21 Dec 2016 #2
    Join Date : Feb 2015
    Bamberg Germany
    Posts : 18,443
    Win10 Pro, Win10 Pro N, Win10 Home, Win10 Pro Insider Fast Ring, Windows 8.1 Pro, Ubuntu

    Martin, how's the speed if you try copying through your network, from VM->Host?
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  3.    21 Dec 2016 #3
    Join Date : Dec 2015
    Posts : 6,736
    Windows10
    Thread Starter

    Quote Originally Posted by Cliff S View Post
    Martin, how's the speed if you try copying through your network, from VM->Host?
    Hi Cliff.

    I quoted wrong speed for copying in vhd to another partition in same vhd - approx 30-40 MB/s which is reasonable when reading and writing to same hdd. I originally quoted the initial burst speed i.e when initially ram caching but it quickly dropped to 30-40 MB/s as I would expect but my brain was not in gear when I quoted figures.

    It took me a few minutes to set up vm drive with right sharing permissions as it is a while since I have done this. It copied also at around 30-40 MB/s i.e. same speed. Many thanks for this suggestion - sorted my issue!


    Any idea why direct copying in VM to attached physical device should be so slow?
      My ComputerSystem Spec
  4.    21 Dec 2016 #4
    Join Date : Feb 2015
    Bamberg Germany
    Posts : 18,443
    Win10 Pro, Win10 Pro N, Win10 Home, Win10 Pro Insider Fast Ring, Windows 8.1 Pro, Ubuntu

    Quote Originally Posted by cereberus View Post
    Hi Cliff.

    I quoted wrong speed for copying in vhd to another partition in same vhd - approx 30-40 MB/s which is reasonable when reading and writing to same hdd. I originally quoted the initial burst speed i.e when initially ram caching but it quickly dropped to 30-40 MB/s as I would expect but my brain was not in gear when I quoted figures.

    It took me a few minutes to set up vm drive with right sharing permissions as it is a while since I have done this. It copied also at around 30-40 MB/s i.e. same speed. Many thanks for this suggestion - sorted my issue!


    Any idea why direct copying in VM to attached physical device should be so slow?
    Not really, it's just that I generally do file transfers between Windows machines, VMs and such using my home network, because I have File Explorer open anyhow.
      My ComputersSystem Spec
  5.    22 Dec 2016 #5
    Join Date : Dec 2015
    Posts : 6,736
    Windows10
    Thread Starter

    Quote Originally Posted by Cliff S View Post
    Not really, it's just that I generally do file transfers between Windows machines, VMs and such using my home network, because I have File Explorer open anyhow.
    Thanks - it was more for curiosity (I'll bet our resident guru K will know ).

    You have solved my problem - slightly less elegant but it does the job well.

    I like to create image backups of the install using Macrium (more space efficient than just copying whole VM), but it was taking forever to write to external host drive but was reasonably fast to VM internal drive. But then copying the image to host drive was taking forever. So now I can copy the image to host relatively quickly now.

    Many thanks again.
      My ComputerSystem Spec

 


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