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[Xen-users] Problem with larger (250GB) bootable virtual disk image.

To: "Xen Users" <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-users] Problem with larger (250GB) bootable virtual disk image.
From: Derek <xen@xxxxxxxxxxxxxxxx>
Date: Sat, 27 Oct 2007 16:42:36 -0600
Delivery-date: Sat, 27 Oct 2007 15:43:26 -0700
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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?


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