[Bug 1857143] Re: VMs won't boot from external snapshots on qemu 4.2

[Thomas Huth]

Since everything now boots fine for you, I think we can close this
ticket now, right? If not, feel free to open again.

** Changed in: qemu
       Status: New => Invalid

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https://bugs.launchpad.net/bugs/1857143

Title:
  VMs won't boot from external snapshots on qemu 4.2

Status in QEMU:
  Invalid

Bug description:
  After upgrading from qemu 4.1.1-1 to 4.2.0-1, VMs that were set to use
  an external snapshot as their disk failed to boot.

  Depending on the guest OS and other VM settings the boot fails and you
  get either the "Boot failed: not a bootable drive" message or the grub
  rescue shell or the EFI shell. Downgrading back to qemu 4.1 allows the
  VMs to boot from the external snapshots without any problem and the
  disk images doesn't appear to be corrupted afterwards.

  From my testing this bug is easily reproducible. Create a VM, install
  a guest os, confirm that the VM boots the guest os without problems,
  shutdown the VM, create an external snapshot of the VM disk, set the
  VM to boot from the snapshot, try to boot the VM with qemu 4.2 and see
  it fail, try to boot it with qemu 4.1 and see it succeed.

  In my case, to test that this bug is reproducible, I used virt-manager
  to install Xubuntu 19.10 on a qcow2 disk image, and then used qemu-img
  create -f qcow2 -b base_image.qcow2 snapshot_image.qcow2 to create the
  external snapshot and edited the xml in virt-manager to point the VM's
  disk to snapshot_image.qcow2. It failed to boot with qemu 4.2, but it
  was working fine with 4.1.

  I booted this test VM off a live distro using the virtual CDROM and
  fdisk can't seem to find a partition table on the VM disk when qemu
  4.2 is used, with 4.1 it can see the partition table just fine.

  Internal snapshots don't seem to have this problem.

  I'm using Archlinux, virt-manager 2.2.1-2, libvirt 5.10.0-1, qemu
  4.2.0-1.

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