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[Xen-devel] [BIOS] HVM boot disk size limitations (was Re: [Xen-users] P

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Subject: [Xen-devel] [BIOS] HVM boot disk size limitations (was Re: [Xen-users] Problem with larger (250GB) bootable virtual disk image.)
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Mon, 29 Oct 2007 23:38:43 +0000
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Derek <xen@xxxxxxxxxxxxxxxx>
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> Don't know if this is a known problem, but I didn't find anything about it
> with a quick google.  I'm using Xen 3.1.0.


> I have found that if I try to create a large virtual device and boot from
> it within an HVM domU, I can't get it to work.  The size I was trying to
> use was 250GB, with most of the space (about 240GB) allocated to a bootable
> WindowsXP NTFS partition.  The failure mode was for the domU to crash and
> die with no messages other than a few lines of BIOS hardware discovery.


> I happend to notice that within the DomU SDL window, just before it dies,
> the BIOS reports the disk size as -6G (MINUS 6 Gig) rather than the
> expected 250G.   This message comes from the BIOS of the LVM partition,
> just before it says it's trying to boot the OS.  This sounds like a problem
> with signed/unsigned interpretation of the value 250GB.  So, on a hunch, I
> used gparted to resize the NTFS filesystem, and the entire disk image, to
> smaller than 128GB.  I chose 72GB.  After lvresizing /dev/main/d4600_disk,
> I was able to successfully boot windows.

Sounds like your intuition was right.

> Now, I know that HVM virtual disks of comparable size _do_ normally work, I 
> use them all the time.

Is that for non bootable for drives on HVM machines?  That's a bigger drive 
than I've ever tried myself :-)

> I suspect that what is going wrong is that the BIOS 
> used by the DomU for initial boot has some bug or limitation that prevents
> it from booting from larger disk sizes.

Yes, that sounds reasonable.

> Does anybody know of such a limitation or bug in the BIOS?  Or has anybody
> else been able to boot an HVM from a virtual disk of around 250GB, with the
> bootable partition taking up most of the disk?

I should think that it's a BIOS limitation.  I'm cross posting this onto 
xen-devel.  Lets continue discussions just on that list, since I'm confident 
from what you've said this is a limitation of the current BIOS code.

I've not heard of any changes to increase the supported sizes for a bootable 
volume, but maybe I've just not seen it mentioned.  Presumably you can work 
around this by using a smaller boot disk?  But I realise that's not really 
the point!


Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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