[Bug 814222] [NEW] kvm cannot use vhd files over 127GB

[Matthew Mattoon]

Public bug reported:

The primary use case for using vhds with KVM is to perform a conversion
to a raw image file so that one could move from Hyper-V to Linux-KVM.
See more on this http://blog.allanglesit.com/2011/03/linux-kvm-
migrating-hyper-v-vhd-images-to-kvm/

# kvm-img convert -f raw -O vpc /root/file.vhd /root/file.img

The above works great if you have VHDs smaller than 127GB, however if it
is larger, then no error is generated during the conversion process, but
it appears to just process up to that 127GB barrier and no more.  Also
of note.  VHDs can also be run directly using KVM if they are smaller
than 127GB.  VHDs can be read and function well using virtualbox as well
as hyper-v, so I suspect the problem lies not with the VHD format (since
that has a 2TB limitation).  But instead with how qemu-kvm is
interpreting them.

BORING VERSION INFO:
# cat /etc/issue
Ubuntu 11.04 \n \l
# uname -rmiv
2.6.38-8-server #42-Ubuntu SMP Mon Apr 11 03:49:04 UTC 2011 x86_64 x86_64
# apt-cache policy kvm
kvm:
  Installed: 1:84+dfsg-0ubuntu16+0.14.0+noroms+0ubuntu4.1
  Candidate: 1:84+dfsg-0ubuntu16+0.14.0+noroms+0ubuntu4.1
  Version table:
 *** 1:84+dfsg-0ubuntu16+0.14.0+noroms+0ubuntu4.1 0
        500 http://apt.sonosite.com/ubuntu/ natty-updates/main amd64 Packages
        500 http://apt.sonosite.com/ubuntu/ natty-security/main amd64 Packages
        100 /var/lib/dpkg/status
     1:84+dfsg-0ubuntu16+0.14.0+noroms+0ubuntu4 0
        500 http://apt.sonosite.com/ubuntu/ natty/main amd64 Packages
# apt-cache policy libvirt-bin
libvirt-bin:
  Installed: 0.8.8-1ubuntu6.2
  Candidate: 0.8.8-1ubuntu6.2
  Version table:
 *** 0.8.8-1ubuntu6.2 0
        500 http://apt.sonosite.com/ubuntu/ natty-updates/main amd64 Packages
        500 http://apt.sonosite.com/ubuntu/ natty-security/main amd64 Packages
        100 /var/lib/dpkg/status
     0.8.8-1ubuntu6 0
        500 http://apt.sonosite.com/ubuntu/ natty/main amd64 Packages

qemu-img version 0.14.0

# vboxmanage -v
4.0.12r72916


REPRODUCTION STEPS (requires Windows 7 or Windows 2008 R2 with < 1GB of free space)

##  WINDOWS  MACHINE  ##

Use Computer Management > Disk Management
-Create 2 VHD files, both dynamically expanding 120GB and 140GB respectively.
-Do not initialize  or format.

These files will need to be transferred to an Ubuntu KVM machine (pscp
is what I used but usb would work as well).

##  UBUNTU KVM MACHINE  ##

# ls *.vhd
120g-dyn.vhd  140g-dyn.vhd
# kvm-img info 120g-dyn.vhd 
image: 120g-dyn.vhd
file format: vpc
virtual size: 120G (128847052800 bytes)
disk size: 244K
# kvm-img info 140g-dyn.vhd 
image: 140g-dyn.vhd
file format: vpc
virtual size: 127G (136899993600 bytes)
disk size: 284K
# kvm-img info 120g-dyn.vhd | grep "virtual size"
virtual size: 120G (128847052800 bytes)
# kvm-img info 140g-dyn.vhd | grep "virtual size"
virtual size: 127G (136899993600 bytes)

Regardless of how big the second vhd is I always get a virtual size of
127G

Now if we use virtualbox to view the vhds we see markedly different
results.

# VBoxManage showhdinfo 120g-dyn.vhd
UUID:                 e63681e0-ff12-4114-85de-7d13562b36db
Accessible:           yes
Logical size:         122880 MBytes
Current size on disk: 0 MBytes
Type:                 normal (base)
Storage format:       VHD
Format variant:       dynamic default
Location:             /root/120g-dyn.vhd
# VBoxManage showhdinfo 140g-dyn.vhd 
UUID:                 94531905-46b4-469f-bb44-7a7d388fb38f
Accessible:           yes
Logical size:         143360 MBytes
Current size on disk: 0 MBytes
Type:                 normal (base)
Storage format:       VHD
Format variant:       dynamic default
Location:             /root/140g-dyn.vhd

# kvm-img convert -f vpc -O raw 120g-dyn.vhd 120g-dyn.img
#
# kvm-img convert -f vpc -O raw 140g-dyn.vhd 140g-dyn.img
#

# kvm-img info 120g-dyn.img 
image: 120g-dyn.img
file format: raw
virtual size: 120G (128847052800 bytes)
disk size: 0
# kvm-img info 120g-dyn.img | grep "virtual size"
virtual size: 120G (128847052800 bytes)
# kvm-img info 140g-dyn.img 
image: 140g-dyn.img
file format: raw
virtual size: 127G (136899993600 bytes)
disk size: 0
# kvm-img info 140g-dyn.img | grep "virtual size"
virtual size: 127G (136899993600 bytes)

Notice after the conversion the raw image will the taken on the partial
geometry of the vhd, thus rendering that image invalid.

vboxmanage has a clonehd option which allows you to successfully convert
vhd to a raw image, which kvm then sees properly.

For giggles I also tested with a 140GB fixed VHD (in the same manner as
above) and it displayed the virtual size as correct, so a good work
around is to convert your VHDs to fixed, then use kvm-img to convert
them.

Keep in mind that these reproduction steps will not have a file systems
therefore no valid data, if there were for example NTFS with a text file
the problem would still occur but more importantly the guest trying to
use it would not be able to open the disk because of it being unable to
find the final sector.

So long story short I think we are dealing with 2 issues here.

1) kvm not being able to deal with dynamic VHD files larger than 127GB
2) kvm-img not generating an error when it "fails" at converting or displaying information on dynamic VHDs larger than 127GB.  The error should be something like "qemu-kvm does not support dynamic VHD files larger that 127GB..."

** Affects: qemu-kvm (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: kvm-img vhd

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Title:
  kvm cannot use vhd files over 127GB

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