Am Mittwoch, 3. August 2011 schrieb Martin Steigerwald: > Am Mittwoch, 3. August 2011 schrieb Martin Steigerwald: > > Am Mittwoch, 3. August 2011 schrieben Sie: > > > Martin Steigerwald <Martin@xxxxxxxxxxxx> writes: > [...] > > > Does using iodepth > 1 need ioengine=libaio? Let´s see the manpage: > > iodepth=int > > > > Number of I/O units to keep in flight against the > > file. Note that increasing iodepth beyond 1 will > > not affect synchronous ioengines (except for small > > degress when verify_async is in use). Even async > > engines my impose OS restrictions causing the > > desired depth not to be achieved. This may happen > > on Linux when using libaio and not setting > > direct=1, since buffered IO is not async on that > > OS. Keep an eye on the IO depth distribution in > > the fio output to verify that the achieved depth > > is as expected. Default: 1. > > > > Okay, yes, it does. I start getting a hang on it. Its a bit puzzling > > to have two concepts of synchronous I/O around: > > > > 1) synchronous system call interfaces aka fio I/O engine > > > > 2) synchronous I/O requests aka O_SYNC > > But isn´t this a case for iodepth=1 if buffered I/O on Linux is > synchronous? I bet most regular applications except some databases use > buffered I/O. Thanks a lot for your answers, Jens, Jeff, DongJin. Now what about the above one? In what cases is iodepth > 1 relevant, when Linux buffered I/O is synchronous? For mutiple threads or processes? One process / thread can only submit one I/O at a time with synchronous system call I/O, but the function returns when the stuff is in the page cache. So first why can´t Linux use iodepth > 1 when there is lots of stuff in the page cache to be written out? That should help the single process case. On the mutiple process/threadsa case Linux gets several I/O requests from mutiple processes/threads and thus iodepth > 1 does make sense? Maybe it helps getting clear where in the stack iodepth is located at, is it process / thread systemcall pagecache blocklayer iodepth device driver device ? If so, why can´t Linux not make use of iodepth > 1 with synchronous system call I/O? Or is it further up on the system call level? But then what sense would it make there, when using system calls that are asynchronous already? (Is that ordering above correct at all?) -- Martin 'Helios' Steigerwald - http://www.Lichtvoll.de GPG: 03B0 0D6C 0040 0710 4AFA B82F 991B EAAC A599 84C7 -- 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
