On Sat, Nov 26, 2022 at 02:08:02PM +0800, Yu Kuai wrote: > Hi, Ming > > 在 2022/11/26 10:18, Ming Lei 写道: > > > > If you want aggressive merge on sequential IO workload, the queue depth need > > to be a bit less, then more requests can be staggered into scheduler queue, > > and merge chance is increased. > > But if nr_requests >= queue_depth, it seems to me elevator will have no > effect, no request can be merged or sorted by scheduler, right? Yeah. If nr_requests <= queue_depth, every request can be queued to driver/device, so requests won't be merged by scheduler. But plug merge still works if IOs are submitted as batch. > > > > If you want good perf on random IO perf, the queue depth needs to > > be deep enough to have enough parallelism for saturating SSD internal. > > > > But we don't recognize sequential/random IO pattern, and usually fixed > > queue depth is used. > > Is it possible to use none elevator and set large queue_depth if nvme is > used in this case? Yeah, if the storage is SSD, usually none with bigger queue_depth should help, and usually 256 should be enough to saturate one single SSD for one well implemented driver. Thanks Ming
