Re: [DRBD-user] kvm, drbd, elevator, rotational - quite an interesting co-operation

[Javier Guerra <javier@guerrag.com>]

Lars Ellenberg wrote:
> On Thu, Jul 02, 2009 at 11:55:05PM +0400, Michael Tokarev wrote:
> > drbd: what's the difference in write pattern on secondary and
> >   primary nodes?  Why `rotational' flag makes very big difference
> >   on secondary and no difference whatsoever on primary?
> 
> not much.
> disk IO on Primary is usually submitted in the context of the
> submitter (vm subsystem, filesystem or the process itself)
> whereas on Secondary, IO is naturally submitted just by the
> DRBD receiver and worker threads.

just like with KVM itself, using several worker threads against a single IO device makes performance heavily dependent on a sensible elevator algorithm.  ideally, there should be only one worker thread for each thread/process originating the initial write.  unfortunately DRBD, being a block/level protocol, might have a hard time unraveling which writes belong to which process.  maybe just merging adjacent (in block address space, not in time) write operations would keep most of the relationships.

-- 
Javier