Jan Kara <jack@xxxxxxx> writes: > On Tue 24-01-12 14:14:14, Jeff Moyer wrote: >> Chris Mason <chris.mason@xxxxxxxxxx> writes: >> >> >> All three filesystems use the generic mpages code for reads, so they >> >> all get the same (bad) I/O patterns. Looks like we need to fix this up >> >> ASAP. >> > >> > Can you easily run btrfs through the same rig? We don't use mpages and >> > I'm curious. >> >> The readahead code was to blame, here. I wonder if we can change the >> logic there to not break larger I/Os down into smaller sized ones. >> Fengguang, doing a dd if=file of=/dev/null bs=1M results in 128K I/Os, >> when 128KB is the read_ahead_kb value. Is there any heuristic you could >> apply to not break larger I/Os up like this? Does that make sense? > Well, not breaking up I/Os would be fairly simple as ondemand_readahead() > already knows how much do we want to read. We just trim the submitted I/O to > read_ahead_kb artificially. And that is done so that you don't trash page > cache (possibly evicting pages you have not yet copied to userspace) when > there are several processes doing large reads. Do you really think applications issue large reads and then don't use the data? I mean, I've seen some bad programming, so I can believe that would be the case. Still, I'd like to think it doesn't happen. ;-) > Maybe 128 KB is a too small default these days but OTOH noone prevents you > from raising it (e.g. SLES uses 1 MB as a default). For some reason, I thought it had been bumped to 512KB by default. Must be that overactive imagination I have... Anyway, if all of the distros start bumping the default, don't you think it's time to consider bumping it upstream, too? I thought there was a lot of work put into not being too aggressive on readahead, so the downside of having a larger read_ahead_kb setting was fairly small. Cheers, Jeff -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
