On 23 July 2013 16:59, Gregory Farnum <g...@inktank.com> wrote: > On Tue, Jul 23, 2013 at 8:54 AM, Matthew Walster <matt...@walster.org> > wrote: > > I've got a relatively small Ceph cluster I'm playing with at the moment, > and > > against advice, I'm running the MONs on the OSDs. > > > > Ideally, I'd like to have half the OSDs in a different facility and > > therefore have one MON in each facility. > > > > Which gives the problem of where to put that pesky third MON... > > > > Is there any likely performance hit if I was to put the third MON in a > > totally different datacenter, potentially in a different city? I know > OSDs > > need to be able to complete a transaction across all nodes in a placement > > group before finishing a write, but what about MONs? > > > > As I understand it, the MONs just describe the topology, so surely having > > them 5ms away shouldn't be that big of a deal? > > > > Just wondered what people's thoughts were. > > 5 milliseconds of latency should not be a problem for the monitor > workload. You'll want to be sure and direct all the monitor clients to > the monitor in their local DC though[1] or you'll get a lot more > bandwidth going across DCs than is necessary. >
That's fantastic, thanks. I'm assuming that 5ms is probably too much for the OSDs -- do we have any idea/data as to the effect of latency on OSDs if they were split over a similar distance? Or even a spread - 0.5ms, 1ms, 2ms etc. Obviously this is a bit theoretical as you'd not want to have data that could be local pulled from a far away data center over your expensive links when in-datacenter would clearly be the better choice. Matthew Walster​​
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