Hello, Thanks Sitsofe Wheeler for sharing useful info. The terminologies(ioengine, iodepth, etc) used in fio is bit overwhelming. Quite not clearly grasp how it works. Can you recommend any blogs or books to get deep understanding of the underlying concepts before dive in to fio. Thanks and Regards, Mohan On 8/26/18, Sitsofe Wheeler <sitsofe@xxxxxxxxx> wrote: > Hi, > > On Fri, 24 Aug 2018 at 11:50, Mohanraj B <bmohanraj91@xxxxxxxxx> wrote: >> >> I have few query on fio direct=0/1 option. >> >> fio version: fio-3.8-88-ge971 >> linux version: Ubuntu 18.04 4.15.0-29-generic >> >> Below are the job files that I am testing and its output snippet. >> >> Job file1: read_bypass_cache.fio >> >> [read_bypass_cache] >> rw=read >> ioengine=sync >> size=10MB >> filename=read_file >> >> Output snippet of multiple runs: >> read issued = 2560 >> Run1: sda: ios=8/0, merge=0/0, ticks=820/0, in_queue=1040, util=86.14% >> Run2: sda: ios=39/0, merge=0/0, ticks=240/0, in_queue=268, util=57.38% >> Run3: sda: ios=39/0, merge=0/0, ticks=248/0, in_queue=276, util=59.02% >> >> Above job files seems working fine, io hits the cache and not the file >> for read operation. >> >> >> Job file2: randread_bypass_cache.fio >> >> [randread_bypass_cache] >> rw=randread >> ioengine=sync >> size=10MB >> filename=randread_file >> >> Output snippet of multiple runs: >> read issued: 2560 >> sda: ios=2227/0, merge=0/0, ticks=2500/0, in_queue=2500, util=88.97% >> sda: ios=2148/7, merge=0/3, ticks=1200/76, in_queue=1276, util=80.64% >> sda: ios=2137/0, merge=0/0, ticks=1196/0, in_queue=1196, util=80.16% >> >> Why when rw=randread and direct=0(by default) is set, io still hitting >> file for read operation instead of cache? >> As 2560-2227= 333 only accessed from cache. > > How do you know the file is fully cached already and wasn't just > created before you started this job? Also be aware that by default fio > random reading hits every area equally - just in a random order (see > https://fio.readthedocs.io/en/latest/fio_doc.html#cmdoption-arg-random-distribution > )... > >> set direct=1 option in below file. >> Job file3: randread_bypass_cache_direct.fio >> >> [randread_bypass_cache] >> rw=randread >> direct=1 >> ioengine=sync >> size=10MB >> filename=randread_file_direct >> >> Output snippet of multiple runs: >> read issued: 2560 >> sda: ios=2343/0, merge=0/0, ticks=3692/0, in_queue=3692, util=92.65% >> sda: ios=2334/0, merge=0/0, ticks=1896/0, in_queue=1896, util=84.57% >> sda: ios=2365/0, merge=0/0, ticks=1940/0, in_queue=1940, util=86.57% >> >> Actually there is not much difference in sda ios when direct =0 and >> direct=1 is set rw=randread. >> Is it how io randread suppose to work? > > See above and there's also information and techniques that may be > helpful in https://www.spinics.net/lists/fio/msg05772.html . > -- > Sitsofe | http://sucs.org/~sits/ >
