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[Qemu-devel] [Bug 1013714] Re: Data corruption after block migration (LV

From: Paolo Bonzini
Subject: [Qemu-devel] [Bug 1013714] Re: Data corruption after block migration (LV->LV)
Date: Fri, 15 Jun 2012 16:52:44 -0000

Hi, I suggest that you try a newer version. There were several fixes
that I think went only in 0.14, in particular commit
62155e2b51e3c282ddc30adbb6d7b8d3bb7c386e, commit
62155e2b51e3c282ddc30adbb6d7b8d3bb7c386e, commit
62155e2b51e3c282ddc30adbb6d7b8d3bb7c386e. RHEL6.2 doesn't have them.
With the fixes, it's quite less likely that live block migration will
eat your data.

However, we were also thinking of deprecating block migration, so we are
interesting of hearing about your setup. The replacement would be more
powerful (it would allow migrating storage separately from the VM), more
efficient (storage and RAM streams would run in parallel on different
TCP ports), and easier for us to test and maintain.

However, it would be more complicated to set the new mechanism up for
migration without shared storage. This is what live block migration
does, and it sounds like your usecase requires migration without shared
storage. Likely, a true replacement of live block migration would not be
ready in time for the next release (1.2), hence its removal would also
be delayed.

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  Data corruption after block migration (LV->LV)

Status in QEMU:

Bug description:
  We quite frequently use the live block migration feature to move VM's
  between nodes without downtime. These sometimes result in data
  corruption on the receiving end. It only happens if the VM is actually
  doing I/O (doesn't have to be all that much to trigger the issue).

  We use logical volumes and each VM has two disks.  We use cache=none
  for all VM disks.

  All guests use virtio (a mix of various Linux distro's and Windows

  We currently have two stacks in use and have seen the issue on both of

  Fedora - qemu-kvm 0.13
  Scientific Linux 6.2 (RHEL derived) - qemu-kvm package

  Even though we don't run the most recent versions of KVM I highly
  suspect this issue is still unreported and that filing a bug is
  therefore appropriate. (There doesn't seem to be any similar bug
  report in launchpad or RedHat's bugzilla and nothing related in change
  logs, release notes and  git commit logs.)

  I have no idea where to look or where to start debugging this issue,
  but if there is any way I can provide useful debug information please
  let me know.

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