On Mon, May 24, 2021 at 09:20:35PM -0700, Darrick J. Wong wrote: > <shrug> I think I'm comfortable starting with 256 for xfs to bump an > ioend to a workqueue, and 4096 pages as the limit for an iomap ioend. > If people demonstrate a need to smart-tune or manual-tune we can always > add one later. > > Though I guess I did kind of wonder if maybe a better limit for iomap > would be max_hw_sectors? Since that's the maximum size of an IO that > the kernel will for that device? > > (Hm, maybe not; my computers all have it set to 1280k, which is a > pathetic 20 pages on a 64k-page system.) I've got samsung nvme devices here that set max_hw_sectors_kb to 128kB.... But device sizes ignore that RAID devices give an optimal IO size for submissions: $ cat /sys/block/dm-0/queue/optimal_io_size 1048576 $ cat /sys/block/dm-0/queue/max_sectors_kb 128 IOWs, we might be trying to feed lots of devices through the one submission, so using a "device" limit isn't really something we should be doing. Ideally I think we should be looking at some multiple of the maximum optimal IO size that the underlying device requests if it is set, otherwise a byte limit of some kind.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
