On Wed, Jul 25, 2012 at 1:06 PM, Florian Haas <florian@xxxxxxxxxxx> wrote: > Hi Mehdi, > > great work! A few questions (for you, Mark, and anyone else watching > this thread) regarding the content of that wiki page: > > For the OSD tests, which OSD filesystem are you testing on? Are you > using a separate journal device? If yes, what type? > > For the RADOS benchmarks: > > # rados bench -p pbench 900 seq > ... > 611 16 17010 16994 111.241 104 1.05852 0.574897 > 612 16 17037 17021 111.236 108 1.17321 0.574932 > 613 16 17056 17040 111.178 76 1.01611 0.574903 > Total time run: 613.339616 > Total reads made: 17056 > Read size: 4194304 > Bandwidth (MB/sec): 111.234 > > Average Latency: 0.575252 > Max latency: 1.65182 > Min latency: 0.07418 > > How meaningful is it to use a (arithmetic) average here, consisting > the min and max differ by a factor of 22? Aren't we being bitten by > outliers pretty severely here, and wouldn't, say, a median be much > more useful? (Actually, would the "max latency" include the initial > hunt for a mon and the mon/osdmap exchange?) Yeah, an average isn't necessarily very useful here — it's what you get because that's easy to implement (with a sum and a counter variable, instead of binning). The inclusion of max and min latencies is an attempt to cheaply compensate for that...but if somebody wants to find/write an appropriately-licensed statistical counting library and integrate it with rados bench, then (say it with me) contributions are welcome! ;) > seekwatcher -t rbd-latency-write.trace -o rbd-latency-write.png -p 'dd > if=/dev/zero of=/dev/rbd0 bs=4M count=1000 oflag=direct' -d /dev/rbd0 > > Just making sure: are you getting the same numbers just with dd, > rather than dd invoked by seekwatcher? > > Also, for your dd latency test of 4M direct I/O reads writes, you seem > to be getting 39 and 300 ms average latency, yet further down it says > "RBD latency read/write: 28ms and 114.5ms". Any explanation for the > write latency being cut in half on what was apparently a different > test run? > > Also, were read and write caches cleared between tests? (echo 3 > > /proc/sys/vm/drop_caches) > > Cheers, > Florian > -- > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
