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[Xen-users] GPLPV and LUN's on a storage

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Subject: [Xen-users] GPLPV and LUN's on a storage
From: Klaus Steinberger <klaus.steinberger@xxxxxxxxxxxxxxxxxxxxxx>
Date: Mon, 26 Oct 2009 15:29:03 +0100
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just a information:

I did run into big troubles running Windows HVM VM's with GPLPV driver on LUN's on a storage system. (In my case a CX4 from EMC).

What happened:

After e.g. Windows Updates the VM's rendered unbootable

What causes this:

My assumption is: Windows boots from the Qemu device until some point there it switches over from the QEMU device to the PV System device. But qemu uses the VM Caches from DOM0, but with the PV driver the LUN is accessed directly.

Now it happens after a reboot that the VM caches are preloaded from previous boots, but the LUN contains really already different data. This leds to curious crashes.

My solution to avoid that:

Dropping caches with "echo 1 > /proc/sys/vm/drop_caches"

This could also be added to the xm definition files, as they are just python:

os.system('echo 1 >/proc/sys/vm/drop_caches');

I already had a similar problem with paravirtualized linux VM's on a Redhat System and external LUN's. pygrub did show old boot entrys, different from what the VM has. Same reason. Dropping vm_caches in this case also helped. There is currently a bug open on redhat's Bug Tracker.

Bug #466681

They work on direct I/O for at least pygrub.


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