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

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Mon, Oct 26, 2009 at 03:29:03PM +0100, Klaus Steinberger wrote:
> Hi,
> 
> 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.
> 

Do you use phy:, file: or tap:aio: ? 

-- Pasi



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