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On Tue, Sep 22, 2015 at 09:29:46AM -0600, Chris Friesen wrote:
> >>There is also work on post-copy migration in QEMU. Normally with live
> >>migration, the guest doesn't start executing on the target host until
> >>migration has transferred all data. There are many workloads where that
> >>doesn't work, as the guest is dirtying data too quickly, With post-copy you
> >>can start running the guest on the target at any time, and when it faults
> >>on a missing page that will be pulled from the source host. This is
> >>slightly more fragile as you risk loosing the guest entirely if the source
> >>host dies before migration finally completes. It does guarantee that
> >>migration will succeed no matter what workload is in the guest. This is
> >>probably Nxxxx cycle material.
> 
> It seems to me that the ideal solution would be to start doing pre-copy
> migration, then if that doesn't converge with the specified downtime value
> then maybe have the option to just cut over to the destination and do a
> post-copy migration of the remaining data.

Yes, that is precisely what the QEMU developers working on this
featue suggest we should do. The lazy page faulting on the target
host has a performance hit on the guest, so you definitely need
to give a little time for pre-copy to start off with, and then
switch to post-copy once some benchmark is reached, or if progress
info shows the transfer is not making progress.

Regards,
Daniel
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