Xen devel - Oct 2007 - Problem with larger (250GB) bootable virtual disk image.

Hi all,

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.

The way I made the filesystem image was:
  lvcreate -L250G d4600_disk main
  dd if=/dev/sdi of=/dev/main/d4600_disk  (Where /dev/sdi is the device file
for the 250GB disk that used to be the boot disk on the bare-metal machine
before I installed XEN).

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.

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

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?

Thanks,
Derek.


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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.
OK.
> 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.
>
<snip>
> 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!

Cheers,
Mark

-- 
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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