> > Aslo, I known that direct ios can be quite slow with ceph, > > maybe can you try without --direct=1 > I can, but that is not the test case here. For the record that pushes it to 12k IOPS, with the journal SSDs reaching about 30% utilization and the actual OSDs up to 5%. So much better, but still quite some capacity for improvement. > and also enable rbd_cache > > ceph.conf > [client] > rbd cache = true > I have that set of course, as well as specifically "writeback" for the KVM instance in question. Interestingly I see no difference at all with a KVM instance that is set explicitly to "none", but that's not part of this particular inquiry either. Christian > > > > ----- Mail original ----- > > De: "Christian Balzer" <chibi at gol.com> > ?: "Gregory Farnum" <greg at inktank.com>, ceph-users at lists.ceph.com > Envoy?: Jeudi 8 Mai 2014 04:49:16 > Objet: Re: Slow IOPS on RBD compared to journal and backing > devices > > On Wed, 7 May 2014 18:37:48 -0700 Gregory Farnum wrote: > > > On Wed, May 7, 2014 at 5:57 PM, Christian Balzer <chibi at gol.com> > > wrote: > > > > > > Hello, > > > > > > ceph 0.72 on Debian Jessie, 2 storage nodes with 2 OSDs each. The > > > journals are on (separate) DC 3700s, the actual OSDs are RAID6 > > > behind an Areca 1882 with 4GB of cache. > > > > > > Running this fio: > > > > > > fio --size=400m --ioengine=libaio --invalidate=1 --direct=1 > > > --numjobs=1 --rw=randwrite --name=fiojob --blocksize=4k > > > --iodepth=128 > > > > > > results in: > > > > > > 30k IOPS on the journal SSD (as expected) > > > 110k IOPS on the OSD (it fits neatly into the cache, no surprise > > > there) 3200 IOPS from a VM using userspace RBD > > > 2900 IOPS from a host kernelspace mounted RBD > > > > > > When running the fio from the VM RBD the utilization of the journals > > > is about 20% (2400 IOPS) and the OSDs are bored at 2% (1500 IOPS > > > after some obvious merging). > > > The OSD processes are quite busy, reading well over 200% on atop, > > > but the system is not CPU or otherwise resource starved at that > > > moment. > > > > > > Running multiple instances of this test from several VMs on > > > different hosts changes nothing, as in the aggregated IOPS for the > > > whole cluster will still be around 3200 IOPS. > > > > > > Now clearly RBD has to deal with latency here, but the network is > > > IPoIB with the associated low latency and the journal SSDs are the > > > (consistently) fasted ones around. > > > > > > I guess what I am wondering about is if this is normal and to be > > > expected or if not where all that potential performance got lost. > > > > Hmm, with 128 IOs at a time (I believe I'm reading that correctly?) > Yes, but going down to 32 doesn't change things one iota. > Also note the multiple instances I mention up there, so that would be > 256 IOs at a time, coming from different hosts over different links and > nothing changes. > > > that's about 40ms of latency per op (for userspace RBD), which seems > > awfully long. You should check what your client-side objecter settings > > are; it might be limiting you to fewer outstanding ops than that. > > Googling for client-side objecter gives a few hits on ceph devel and > bugs and nothing at all as far as configuration options are concerned. > Care to enlighten me where one can find those? > > Also note the kernelspace (3.13 if it matters) speed, which is very much > in the same (junior league) ballpark. > > > If > > it's available to you, testing with Firefly or even master would be > > interesting ? there's some performance work that should reduce > > latencies. > > > Not an option, this is going into production next week. > > > But a well-tuned (or even default-tuned, I thought) Ceph cluster > > certainly doesn't require 40ms/op, so you should probably run a wider > > array of experiments to try and figure out where it's coming from. > > I think we can rule out the network, NPtcp gives me: > --- > 56: 4096 bytes 1546 times --> 979.22 Mbps in 31.91 usec > --- > > For comparison at about 512KB it reaches maximum throughput and still > isn't that laggy: > --- > 98: 524288 bytes 121 times --> 9700.57 Mbps in 412.35 usec > --- > > So with the network performing as well as my lengthy experience with > IPoIB led me to believe, what else is there to look at? > The storage nodes perform just as expected, indicated by the local fio > tests. > > That pretty much leaves only Ceph/RBD to look at and I'm not really sure > what experiments I should run on that. ^o^ > > Regards, > > Christian > > > -Greg > > Software Engineer #42 @ http://inktank.com | http://ceph.com > > > > -- Christian Balzer Network/Systems Engineer chibi at gol.com Global OnLine Japan/Fusion Communications http://www.gol.com/
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