Hi. On Wed, Jun 17, 2020 at 10:33:51PM +0200, to...@tuxteam.de wrote: > So to test disk write speed, 'dsync' seems the way to go. When dumping > to a device, there are no metadata (am I right there?), so probably > again you want 'dsync'. > > I don't know what 'nocache' would do for writing. For reading, it seems > to make sense to avoid huge files (think a video) uselessly clobbering > your cache (you're going to look at the blocks once, right?).
Ez. Conventional dd: $ strace dd if=/dev/zero of=/tmp/1 bs=1M count=1 openat(AT_FDCWD, "/dev/zero", O_RDONLY) = 3 ... openat(AT_FDCWD, "/tmp/1", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3 <read/write is issued> dsync dd (note O_DSYNC flag): $ strace dd if=/dev/zero of=/tmp/1 bs=1M count=1 oflag=dsync openat(AT_FDCWD, "/dev/zero", O_RDONLY) = 3 ... openat(AT_FDCWD, "/tmp/1", O_WRONLY|O_CREAT|O_TRUNC|O_DSYNC, 0666) = 3 <read/write is issued> nocache dd (note fadvise64 syscall and the lack of O_DSYNC flag): $ strace dd if=/dev/zero of=/tmp/1 bs=1M count=1 oflag=nocache openat(AT_FDCWD, "/dev/zero", O_RDONLY) = 3 ... openat(AT_FDCWD, "/tmp/1", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3 ... <read/write is issued> ... fadvise64(1, 0, 1048576, POSIX_FADV_DONTNEED) = 0 Basically by "dsync" you're telling "I want chunks of this file to be written to the disk every time I finish waiting write(2) syscall, but feel free to cache it for other readers anyway", and by "nocache" you're telling "I need it to evict from the filesystem cache once I close the file but feel free to cache it before then, and I don't need to wait for the disk writes every time I call write(2)". Long story short, if you need a primitive I/O benchmark, you're better with both dsync and nocache. Reco
