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

To: "Klaus Steinberger" <klaus.steinberger@xxxxxxxxxxxxxxxxxxxxxx>, <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-users] GPLPV and LUN's on a storage
From: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Date: Tue, 27 Oct 2009 21:31:30 +1100
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Thread-topic: [Xen-users] GPLPV and LUN's on a storage
> just a information:
> I did run into big troubles running Windows HVM VM's with GPLPV driver
> LUN's on a storage system. (In my case a CX4 from EMC).

I'm not familiar with CX4... is it iSCSI or AoE or hyperSCSI or

> 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
> 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
> boots, but the LUN contains really already different data. This leds
> curious
> crashes.

That is strange... when GPLPV is running as it should, I don't think
that there are any writes done before GPLPV takes over. If there are, it
is done via the int13h BIOS interface and it writes a flag that says the
boot started which enables the next boot to present the "last boot was
not successful... safe mode?" menu.

Strange things do happen when you involve loopback or kpartx though, as
I found out recently. I use losetup with an offset to create a loopback
device so I can mount my NTFS volume under Linux (you break a lot of
systems when you do driver development :). If I forget to delete the
loopback device, all sorts of strange things happen, although I've never
seen complete corruption before. kpartx (which uses device mapper) gives
the same problems.

Fwiw, I do almost all of my testing on phy: mapped lvm volumes.


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