[dm-crypt] dm-crypt on top of DRBD for live migration

[Berengar Lehr]

We want to use LVM, dm-crypt and DRBD in a 2-machine setup for KVM.

We think, a proper setup could be something like this (dm-crypt below DRBD):


   Machine 1               Machine 2

      KVM  -> -> -> -> -> ->  KVM
       |   (live migration)    .
       |                       .
      DRBD - - - - - - - - - DRBD
       |                       |
      LVM                     LVM
       |                       |
    dm-crypt                dm-crypt
       |                       |
 Disk/Partition          Disk/Partition

The KVM guest machines should run on machine 1. Live migration to
machine 2 should be supported.

Using this setup, every write to DRBD would be (independently) crypted
on both machines,
leading to additional (unnecessary?) cpu load on machine 2 before live
migrating, and additional
cpu load on machine 1 after live migration.

Could these additional cpu loads be avoided using a setup like this
(dm-crypt in top of DRBD):


   Machine 1               Machine 2

      KVM  -> -> -> -> -> ->  KVM
       |   (live migration)    .
       |                       .(b)
    dm-crypt                dm-crypt
       |                       |(a)
      DRBD - - - - - - - - - DRBD
       |                       |
      LVM                     LVM
       |                       |
 Disk/Partition          Disk/Partition

In this setup, dm-crypt runs on both machines, too, but is not used on
machine 2 until KVM
guests send write-requests after the live migration. So crypting is
done only by one machine
at every time point.

Is such a setup safe and stable?

What about caching at points (a) or (b) on machine 2?
Can KVM read cached, outdated data from dm-crypt after live migration?

Is there a workaround?

Thank You
B. Lehr & M. Müller

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