And for reference, we see 0-1ms on Intel NVMe SSDs for commit latency. On Wed, Jun 3, 2015 at 10:11 AM, Gregory Farnum <[email protected]> wrote:
> On Wed, Jun 3, 2015 at 5:19 AM, Xu (Simon) Chen <[email protected]> wrote: > > Hi folks, > > > > I've always been confused about the apply/commit latency numbers in "ceph > > osd perf" output. I only know for sure that when they get too high, > > performance is bad. > > > > My deployments have seen many different versions of ceph. Pre 0.80.7, > I've > > seen those numbers being pretty high. After upgrading to 0.80.7, all of a > > sudden, commit latency of all OSDs drop to 0-1ms, and apply latency > remains > > pretty low most of the time. > > > > Now I'm trying hammer in a new cluster, and even when the cluster is > doing > > nothing, I see commit latency being as high as 20ms, and apply latency > being > > 200+ms, which seems a bit off to me. > > > > Any ideas how these numbers changed over versions? > > Commit latency is how long it takes for an operation to be applied to > disk — generally speaking, how long it takes the journal to write an > entry. Apply latency is how long it takes to get applied to the > backing filesystem (which can be throttled by various things to > prevent us getting arbitrarily large amounts of dirty data). > > The speeds we can get on these haven't changed much, although they've > generally gotten a little faster for Hammer. If you've seen wide > variances I think it has more to do with the setup of each cluster or > the state of the system at the time you ran the test than anything > inherent. :) > > (For reference, the numbers I expect to see in a lightly-loaded SSD > cluster are ~2ms commit times for writes, or just a bit less. Anything > over 10 is definitely wrong, although that's close to correct for an > SSD-journaled hard drive cluster — probably more like 5-7.) > -Greg > _______________________________________________ > ceph-users mailing list > [email protected] > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > -- David Burley NOC Manager, Sr. Systems Programmer/Analyst Slashdot Media e: [email protected]
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