Likely it won't since the OSD is already coalescing journal writes. FWIW, I ran through a bunch of tests using seekwatcher and blktrace at 4k, 128k, and 4m IO sizes on a 4 OSD cluster (3x replication) to get a feel for what the IO patterns are like for the dm-cache developers. I included both the raw blktrace data and seekwatcher graphs here: http://nhm.ceph.com/firefly_blktrace/ there are some interesting patterns but they aren't too easy to spot (I don't know why the Chris decided to use blue and green by default!) Mark On 09/22/2014 04:32 PM, Robert LeBlanc wrote: > We are still in the middle of testing things, but so far we have had > more improvement with SSD journals than the OSD cached with bcache (five > OSDs fronted by one SSD). We still have yet to test if adding a bcache > layer in addition to the SSD journals provides any additional improvements. > > Robert LeBlanc > > On Sun, Sep 14, 2014 at 6:13 PM, Mark Nelson <mark.nelson at inktank.com > <mailto:mark.nelson at inktank.com>> wrote: > > On 09/14/2014 05:11 PM, Andrei Mikhailovsky wrote: > > Hello guys, > > Was wondering if anyone uses or done some testing with using > bcache or > enhanceio caching in front of ceph osds? > > I've got a small cluster of 2 osd servers, 16 osds in total and > 4 ssds > for journals. I've recently purchased four additional ssds to be > used > for ceph cache pool, but i've found performance of guest vms to be > slower with the cache pool for many benchmarks. The write > performance > has slightly improved, but the read performance has suffered a > lot (as > much as 60% in some tests). > > Therefore, I am planning to scrap the cache pool (at least until it > matures) and use either bcache or enhanceio instead. > > > We're actually looking at dm-cache a bit right now. (and talking > some of the developers about the challenges they are facing to help > improve our own cache tiering) No meaningful benchmarks of dm-cache > yet though. Bcache, enhanceio, and flashcache all look interesting > too. Regarding the cache pool: we've got a couple of ideas that > should help improve performance, especially for reads. There are > definitely advantages to keeping cache local to the node though. I > think some form of local node caching could be pretty useful going > forward. > > > Thanks > > Andrei > > > _________________________________________________ > ceph-users mailing list > ceph-users at lists.ceph.com <mailto:ceph-users at lists.ceph.com> > http://lists.ceph.com/__listinfo.cgi/ceph-users-ceph.__com > <http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com> > > > _________________________________________________ > ceph-users mailing list > ceph-users at lists.ceph.com <mailto:ceph-users at lists.ceph.com> > http://lists.ceph.com/__listinfo.cgi/ceph-users-ceph.__com > <http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com> > >
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