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I just had an odd thing happen with a Hyper-V server that has 4 guest VMS running on it. The host server and all guests are Server 2008 R2 SP1.

I logged into one of the guest VMs using Remote Desktop and rebooted a web application running on it, then logged out. I tried to access that service remotely and it was unresponsive. I tried other services and they were unresponsive too. I logged into the host server and opened the Hyper-V console, and all of the VMs were paused. WTF? I manually resumed them and off they went.

I'm pretty sure I didn't do anything to make those VMs pause. One I may hove done accidentally somehow, but a bunch of 4, definitely not! So how could this have happened? What would make VMs pause themselves?

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Did you try checking the event logs? This would be where I would start. –  Quinten Aug 15 '11 at 19:59
    
How long was this pause? –  surfasb Aug 15 '11 at 20:12
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1 Answer

up vote 10 down vote accepted

Your host ran out of disk space (or thought it did, can happen with flaky clusters or SANs)

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Right on the money. There was actually 2Gb free but I guess that's not much shared accross seral VMs. Looks like some bigger disks are in my future... –  Tim Long Aug 15 '11 at 20:27
    
There's some minimum percentage where Hyper-V shuts down, I'm thinking it's 95% but I could be wrong. It does so to ensure the host doesn't crash. –  Chris S Aug 15 '11 at 20:30
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Tim, I would recommend not using Hyper-V's dynamic disks feature. This is not great for performance on heavily used VMs as the VHD can become fragmented. It's a pain to have to shut down the VM to increase the disk size, but not compared to issues like the whole system stopping like you described above :) –  Quinten Aug 15 '11 at 20:32
    
@Quinten - thanks for the advice. I agree with you in principle but sometimes one has to live with what is available and the server isn't all that heavily used so the performance is OK at the moment. I'm going to order up a couple of 2Tb drives at some point and go to all fixed-size volumes. It totally makes sense. –  Tim Long Aug 15 '11 at 21:40
    
Dynamic disks can do that when they "inflate" their sizes. Plus DO NOT USE snapshotted machine for a long while or your disks will double, double and double again, it's no matter they are fixed or dynamic size. I mean in 24 hours, ie. –  Nime Cloud Aug 19 '11 at 20:05
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