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?

Martin, how's the speed if you try copying through your network, from VM->Host?

Cliff S said:

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?

cereberus said:

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.

Cliff S said:

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.