[Bug 1292234] [NEW] qcow2 image corruption in trusty (qemu 1.7 and 2.0 candidate)

[Jamie Strandboge]

Public bug reported:

The security team uses a tool (http://bazaar.launchpad.net/~ubuntu-
bugcontrol/ubuntu-qa-tools/master/view/head:/vm-tools/uvt) that uses
libvirt snapshots quite a bit. I noticed after upgrading to trusty some
time ago that qemu 1.7 (and the qemu 2.0 in the candidate ppa) has had
stability problems such that the disk/partition table seems to be
corrupted after removing a libvirt snapshot and then creating another
with the same name. I don't have a very simple reproducer, but had
enough that hallyn suggested I file a bug. First off:

qemu-kvm 2.0~git-20140307.4c288ac-0ubuntu2

$ cat /proc/version_signature 
Ubuntu 3.13.0-16.36-generic 3.13.5

$ qemu-img info ./forhallyn-trusty-amd64.img
image: ./forhallyn-trusty-amd64.img
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 4.0G
cluster_size: 65536
Format specific information:
    compat: 0.10

Steps to reproduce:
1. create a virtual machine. For a simplified reproducer, I used virt-manager with:
  OS type: Linux
  Version: Ubuntu 14.04
  Memory: 768
  CPUs: 1

  Select managed or existing (Browse, new volume)
    Create a new storage volume:
      qcow2
      Max capacity: 8192
      Allocation: 0

  Advanced:
    NAT
    kvm
    x86_64
    firmware: default

2. install a VM. I used trusty-desktop-amd64.iso from Jan 23 since it
seems like I can hit the bug more reliably if I have lots of updates in
a dist-upgrade. I have seen this with lucid-trusty guests that are i386
and amd64. After the install, reboot and then cleanly shutdown

3. Backup the image file somewhere since steps 1 and 2 take a while :)

4. Execute the following commands which are based on what our uvt tool
does:

$ virsh snapshot-create-as forhallyn-trusty-amd64 pristine "uvt snapshot"
$ virsh snapshot-current --name forhallyn-trusty-amd64
pristine
$ virsh start forhallyn-trusty-amd64
$ virsh snapshot-list forhallyn-trusty-amd64 # this is showing as shutoff after start, this might be different with qemu 1.5

in guest:
sudo apt-get update
sudo apt-get dist-upgrade
780 upgraded...
shutdown -h now

$ virsh snapshot-delete forhallyn-trusty-amd64 pristine --children
$ virsh snapshot-create-as forhallyn-trusty-amd64 pristine "uvt snapshot"

$ virsh start forhallyn-trusty-amd64 # this command works, but there is
often disk corruption

The idea behind the above is to create a new VM with a pristine snapshot
that we could revert later if we wanted. Instead, we boot the VM, run
apt-get dist-upgrade, cleanly shutdown and then remove the old
'pristine' snapshot and create a new 'pristine' snapshot. The intention
is to update the VM and the pristine snapshot so that when we boot the
next time, we boot from the updated VM and can revert back to the
updated VM.

After running 'virsh start' after doing snapshot-delete/snapshot-create-
as, the disk may be corrupted. This can be seen with grub failing to
find .mod files, the kernel not booting, init failing, etc.

This does not seem to be related to the machine type used. Ie, pc-
i440fx-1.5, pc-i440fx-1.7 and pc-i440fx-2.0 all fail with qemu 2.0, pc-
i440fx-1.5 and pc-i440fx-1.7 fail with qemu 1.7 and pc-i440fx-1.5 works
fine with qemu 1.5.

Only workaround I know if is to downgrade qemu to 1.5.0+dfsg-3ubuntu5.3
from Ubuntu 13.10.

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

** Summary changed:

- qcow2 image corruption in trusty (qemu 1.7)
+ qcow2 image corruption in trusty (qemu 1.7 and 2.0 candidate)

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https://bugs.launchpad.net/bugs/1292234

Title:
  qcow2 image corruption in trusty (qemu 1.7 and 2.0 candidate)

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