Re: Disk migration

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Haydn Solomon wrote:
> I see that vmware just demonstrated migration of virtual disks. I know 
> that kvm is very young but I think has come a long way in a very short 
> period. I just wanted to get some feedback/discussion on how difficult 
> this would be to implement in kvm/qemu and if this would be anywhere 
> in future plans for development? Would this be less complicated than 
> live migration of memory?

Disk migration is super easy to implement in KVM.  There are two basic 
models you can follow to do disk migration.  In a push model, you would 
apply a similar algorithm to the memory migration algorithm whereas you 
pushed the whole disk over in chunks keeping track of what bits on disk 
have been dirtied.  You'd converge after a certain number of iterations 
and then perform the memory migration.

A pull model would immediately do the memory migration and allow QEMU to 
continue running on the source node acting as an IO server.  Any disk IO 
from the guest would either go directly to disk if the block was present 
or would be fetched from the QEMU instance on the source node.  The only 
trick here is that you'd want to continue transferring blocks even when 
the guest isn't accessing the disk.

The push model may require parallelization of the disk and memory 
convergence depending on how much disk activity the guest is doing.  
That's a pretty interesting problem but it shouldn't be too hard to solve.

The push model has the advantage of having the smallest performance 
impact over time.  The pull model has an advantage of immediately 
reducing CPU usage on the source node at the expense of a potentially 
severe degradation in performance of the guest.  Both models probably 
require roughly the same amount of time for migration completion.

Very good thing for someone looking for a fun task in KVM :-)

Regards,

Anthony Liguori



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