Hyper-V VMs loose their VHDs under high load


I've set up a Win Srv2012 with Hyper-V with a 7TB iSCSI NAS attached to the Hyper-V host. It's visible as an E: drive on the Hyper-V host.

I've then created a 3TB VHD on the E: drive and assigned it to the host as a SCSI drive.
Whenever the Virtual machine access this new VHD a lot - could be copying 1 TB of data to the new VHD or even just formatting the new VHD from within the Virtual Machine, it suddenly disappears from within the Virtual Machine. Not even a rescan can detect the SCSI VHD again.

The disk is still listed as attached in the VM Settings until  you reboot the VM - then its gone and all that is left is the SCSI controller.

There are no errors on the event log(s).

The Hyper-V host is running on a Dell Desktop machine with a Core i7 CPU, 32Gb RAM and a local 480Gb Intel SSD. There are no issues with the local disk.

Any help or directions would be much appreciated. I'm out of ideas myself (except bying a "real" $10K server for this, but it may be a bit of an overkill for this project.
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Svet PaperovIT ManagerCommented:
There might be an issue with the network connection to the iSCSI NAS or with the NAS.

To improve iSCSI communication between the host and the NAS, you need to:
- Use an iSCSI-dedicated NICs on the host and on the NAS in separated IP segment  
- Enable Jumbo Frames and Flow Control on all network devices on the iSCSI path
- Enable TCP and IP Rx/Tx offload on the iSCSI-dedicated host’s NIC

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TheGuru42Author Commented:
iSCSI was running on a dedicated physical LAN
Jumbo Frames has been enabled now (4K frames - largest supported by the NIC)
Flow control enabled on all devices
TCP and RX/TX offload was already enabled on host NIC.

I'm giving it a try now. Let's see in some hours when the disk-abuse has finished :)

However. I might should have added that I tried with a completely different setup before using the iSCSI NAS. I had a RAID5 box attached directly to the Hyper-V server through the eSATA interface and the exact same thing happened back then.

Thanks for your help so far.

Br, Bjorn
TheGuru42Author Commented:
It worked like a charm!

Thank you so much for your help.

You saved me $10K ;-)
I'm affraid you of AV-Storm. I had familiar problems couple of monthes ago on FC connected storage. The CSVs are were stopped with their accoding VMs.
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Windows Server 2012

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