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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
