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VMs on Windows Server 2012 Hang

I have a Windows Server 2012 (not R2) that has the following specs: Two Intel Processors (from task manager: 2.39Ghz; 2 Sockets, 8 Cores, 8 virtual processors, Virtualization Enabled); 64GB RAM, 4 Network Cards (only using 2 - one for virtual network).  I have 3 virtual servers, all running Windows Server 2012 (not R2).  2 of the VMs have 4GB each, the third has 16GB.

Task manager for host consistently shows less than 20% CPU utilization, 50% Memory Usage, and the network cards consistently show less than 50Kbps.  There is 350GB free space on drive C and 350 GB free on drive F (where the VHDs are).  Once a month I do a Windows Update on the VMs and the Host and restart each.

This environment has been running well for about 9 months.  Problem is that about a month or so ago, about once a week the VMs hang.  In Hyper-V Manager they show 0% CPU utilization.  If I click Connect, just shows a blank window.  I cannot remote into them via Remote Desktop Connection or access them in any manner.  I cannot shut them down via the shutdown button in Hyper-V so the only option is to restart the host.

A couple times this happened I was copying backups from the host to an external drive.  I noticed that the memory allocation went way up and stayed there for a while.  At that time, I had more memory allocated to two of the VMs (8GB each instead of 4GB).  I was hoping that by dropping the VM’s memory from 8GB to 4GB and not doing copies during the day that this would fix the issue.  However, it happened a few more times during the day.

I don’t see anything in the event logs that indicate a problem.  There has been an ongoing issue with the server in that one of the fans ran at a high rpm and made noise so I removed it (there are 4 remaining in the server).  It’s been like that though for about 5 months and the room where the server is has A/C so it keeps fairly cool.

Suggestions on what might be causing this and/or where to look for information?  Thanks!
Windows Server 2012Virtualization

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8/22/2022 - Mon
Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)

Is the host server, coming under CPU or Datastore (disk) pressure or bottleneck, when you copy off, to I assume USB external disks?

I didn't see anything when doing the copy that would indicated a CPU or disk issue.  Only the memory usage went extremely high.  Now though the VMs are hanging even when not doing a file copy.
Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)

on the host ?
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William Peck

Yes, I'm talking about the host.  Since I can't access the VMs when this happens, I can't get into their Task Manager to see anything.
Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)

Does the Task Manager on the Host, show any high CPU peaks, when copying a file?

what's the size of the file?

No, the CPU usage doesn't change much just much higher memory usage.  File is 100GB.
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The issue seemed to resolve on its own.