Re: [Drbd-dev] [PATCH 0/3] Implement {begin|after}-resync-source handlers

[Philipp Reisner <philipp.reisner@linbit.com>]

[..]
> > * Are you going to use them in your organisation?
> 
> That too, but better said we're going to use them in Ganeti (opensource,
> cluster-based virtualization manager). Background: Ganeti uses only the
> low-level drbdsetup interface, and not the drbdadm one, since we manage
> the minor numbers assignments dynamically. This dynamic part hits us,
> because we cannot make easy 'resync-after' rules (especially as on
> bigger Ganeti clusters, like 20-40 nodes, you could get easily into
> circular dependencies if you're not careful.
> 
> So the plan is to write a custom usermode_helper (right now Ganeti
> documentations says to use /bin/true :), so we're not losing any
> functionality by moving to a custom helper), that will simply restrict
> DRBD to a single sync (either incoming or outgoing) per machine, in
> order to not overload any machine's I/O resources. When a begin-sync
> request comes in, either source or target, the helper will look if any
> other minors are undergoing syncs, and if so will drbdsetup pause-sync
> the minor - I hope that this is safe to do from the callback, my few
> tests show it so. And when a after-resync-* requests comes, we look if
> there are any paused syncs (on our end), and if so then we'll unpause
> one.
> 
> Now, if drbd (the kernel driver) would have a setting like 'max_syncs',
> then we wouldn't have to implement anything in Ganeti, but that is
> probably more work in the kernel - which is harder, compared with a
> simple userspace shell script.
> 
> Hope my explanation makes sense…

Hi Iustin,

Yes, makes sense. It will go in, after the 8.3.8 release dust settled.

Best,
 Phil
-- 
: Dipl-Ing Philipp Reisner
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