Just FYI, this is also the case with the DC S3700 based on my testing. On Mon, Dec 15, 2014 at 7:15 AM, Jens Axboe <axboe@xxxxxxxxx> wrote: > On 12/15/2014 04:48 AM, Georg Schönberger wrote: >> >> Hi Fio users and Jens, >> >> I am currently analyzing read and write latencies of Solid State Disks >> (SSDs). My assumption was that >> write latencies are usually higher than read latencies. But my tests with >> Fio on an Intel DCS3500 show >> that average latency while writing is lower than while reading (using >> fio-2.1.3): >> >> * $ sudo /usr/bin/fio --rw=randrw --name=intelDCS3500 --bs=4k --direct=1 >> --filename=/dev/sda --rwmixread=0 >> --numjobs=1 --ioengine=libaio --runtime=60 --iodepth=1 --time_based >> [...] >> lat (usec): min=30, max=1010, avg=43.32, stdev= 3.42 >> [...] >> * $ sudo /usr/bin/fio --rw=randrw --name=intelDCS3500 --bs=4k --direct=1 >> --filename=/dev/sda --rwmixread=100 >> --numjobs=1 --ioengine=libaio --runtime=60 --iodepth=1 --time_based >> [...] >> lat (usec): min=103, max=2315, avg=121.92, stdev=11.02 >> >> I have made this observation on two different machines with different >> SSDs, always producing similar results. >> Am I doing anything wrong with my tests? >> Are write latencies in general lower than read latencies? >> >> One guess I have is a SSD cache to enhance write access (in particular the >> Intel datacenter SSDs, as they have >> a "Enhanced power-loss data protection" feature). > > > Your guess is exactly right, that's what most flash based devices (worth > their salt) do. That's also why sync write latencies are mostly independent > of the type of nand used, whereas the read latency will easily reflect that. > > -- > Jens Axboe > > > -- > To unsubscribe from this list: send the line "unsubscribe fio" 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 fio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
