Re: [Xen-devel] live migration with DRBD devices

[Michael Paesold <mpaesold@xxxxxx>]

Mark Williamson wrote:
> > I want to do live migration of domains using DRBD block devices for
> > synchronisation between the two Xen hosts with Xen 2.0.7.
> >
> > I've written a script ( block-drbd ) which binds / unbinds DRBD devices for
> > specific domains (with DRBD, only one host can write on a drbd device at
> > the same time). This script sets the current host as master (bind) or
> > secondary (unbind) in a DRBD "Netword-RAID"
>
> Yep - btw this is fixed with DRBD 8 (still in development), which supports 
> multiple writers.
> How are you handling the sequencing here, though?  Move the device to the new 
> host at *unbind* time from the origin?  I'm thinking that the DRBD script 
> will get run at the destination whilst the origin domain is still running and 
> using its disk...
Correct.

And it cannot work with drbd 0.7 currently without hacking Xen to change 
block device setup. What would be needed is that Xen should open the new 
block device read-only (there is a hack in drbd 0.7 that would allow 
this) and only when the source domain is shutdown reopen the block 
device r/w.
Xen want's to open the block device rw as soon as it does the setup of 
the destination domain. This may work currectly with drbd 0.8 since it 
supports multi-master.
I have done testing with the current development trunk of drbd some time 
ago. It did not work for me. Migration crashed the guest.
Best Regards,
Michael Paesold

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