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We have a brand new pair of servers for Hyper-V with fast RAID 10 SSDs in 2 arrays. On the host, using Crystal Disk Mark, I get speeds that I would expect like RND64K Q32T12 = 4712.49 MB/S while the test VM gets 21.34 MB/S on a 600GB VHDX and 52.12 MB/S on a 200GB VHDX. The virtual disks are on the same array so it is a bit strange to me that they would have different speeds.

I've searched but I have not found a specific reference to this, much less a fix for it, which is doubly strange because they system that the new servers are to replace also has this problem. Has anyone here had this issue as well?

D_B_AINT
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    VHDX - fixed size or expanding? Performance should be fixed. – TomTom Apr 20 '21 at 15:54
  • All VHDX files are fixed. I am using a 32 GB file size for the test so even if they were dynamic, it would really only slow it down the first time it ran, but this is happening consistently. – D_B_AINT Apr 20 '21 at 21:35

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Mystery solved! Sophos Endpoint Protection/Anti-Virus was causing this issue. The admins had disabled scanning however the issue did not go away until Sophos was completely removed from the Host machine.

D_B_AINT
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  • This is quite concerning... a scan exclusion for VHDX files should have been enough with any antivirus. You should raise the issue with Sophos. – Massimo Apr 21 '21 at 11:37
  • It is concerning and the issue has already been raised. Apparently it had the wrong exclusions applied from their service. Depending on the role you assign the server, Sophos is meant to automatically exclude certain services, but in this case, there was a misfire, either by Sophos or the people that provisioned the server and this is what caused the problem. I was going out of my mind trying to find some reference to the issue where the larger the VHDX file size, the slower the performance but this issue is not so present on the internet. – D_B_AINT Apr 22 '21 at 12:05