Re: live migration with DRBD devices

[Michael Mey <michael.mey@to.com>]

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On Tuesday 25 October 2005 16:49, 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.
I'll give it a try, although I'm expecting it being not that successful 
because of its development state and Michael's experiences.

> 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...
So the solution would be to get Xen mounting the device in the following order 
for migration as Michael Paesold suggested:

1) create domain on target host and mount the drbd device r/o
2) do the rest of the migration, e.g. iterative copy of memory ...
3) unmount or mount the drbd device on the source host r/o
4) mount drbd device r/w on target host
5) start domain on target host

Is there any further developer's documentation for Xen 2.0.7?
Or maybe anybody can give me some hints at approx. which points I'd have to 
patch Xen to get that running? That would help me a lot saving some time for 
trying to implement that because I'm not so deep into Xen right now.

> > xc_linux_save start 8
> > [1130244324.174077] Saving memory pages: iter 1   0%
> > Saving memory pages: iter 1   0%Error when writing to state file (5)
> > Error when writing to state file (5)
> > 2790 [WRN] XFRD> Transfer errors:
> > 2790 [WRN] XFRD> state=XFR_STATE    err=1
> > 2790 [INF] XFRD> Xfr service err=1
> >
> >
> > Does anybody have an idea? I'd appreciate.
>
> Weird...  Does this work without your script?  Does migration work with a
> simpler domain?  If you boot a domain from an initrd (removing disk
> dependencies), will that migrate correctly?
>
> If it's something weird in the migration-related code, somebody else might
> have some further ideas as to what caused this...
Migration without my script works fine, so there shouldn't be anything weird 
in the migration-related code.

Regards,
Mike

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