[Qemu-devel] [Bug 1292234] Re: qcow2 image corruption in trusty (qemu 1.7 and 2.0 candidate)

[Chris J Arges]

Serge,
So I was able to just compile my own qemu and test with that.

I did attempt a reverse bisect, and was able to reproduce as early as v1.1 and 
also reproduce on master HEAD.
v1.0 was inconclusive because qcow2 format I made with the newer binary seemed 
to be incompatible with v1.0; however from Jamies testing this seems to be a 
working version; so I'd say somewhere between v1.0.0, v1.1.0 lies the original 
change that enabled this issue. As I've been unable to reproduce this without 
virsh, reverse bisecting and using older qemu versions is a bit challenging as 
machine types change, features virsh wants to use aren't available, etc.

Another interesting thing I tested today was I was able to reproduce
with ext4 with extents disabled; maybe that gives more clues. Just to
make sure I wasn't crazy, mkfs'd the partition to vanilla ext4 and
iterated for most of the afternoon with no failures.

My next steps are going to be enabling verbose output for qcow2, looking
more deeply into what gets corrupted in the file, and  turning on host
filesystem debugging.

--chris

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

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

Status in QEMU:
  New
Status in qemu package in Ubuntu:
  Confirmed

Bug description:
  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-
  3ubuntu5.4 from Ubuntu 13.10.

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