On Mon, Sep 23, 2019 at 06:06:46PM +0300, Konstantin Khlebnikov wrote: > On 23/09/2019 17.52, Tejun Heo wrote: > > Hello, Konstantin. > > > > On Fri, Sep 20, 2019 at 10:39:33AM +0300, Konstantin Khlebnikov wrote: > > > With vm.dirty_write_behind 1 or 2 files are written even faster and > > > > Is the faster speed reproducible? I don't quite understand why this > > would be. > > Writing to disk simply starts earlier. Stupid question: how is this any different to simply winding down our dirty writeback and throttling thresholds like so: # echo $((100 * 1000 * 1000)) > /proc/sys/vm/dirty_background_bytes to start background writeback when there's 100MB of dirty pages in memory, and then: # echo $((200 * 1000 * 1000)) > /proc/sys/vm/dirty_bytes So that writers are directly throttled at 200MB of dirty pages in memory? This effectively gives us global writebehind behaviour with a 100-200MB cache write burst for initial writes. ANd, really such strict writebehind behaviour is going to cause all sorts of unintended problesm with filesystems because there will be adverse interactions with delayed allocation. We need a substantial amount of dirty data to be cached for writeback for fragmentation minimisation algorithms to be able to do their job.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
