On Fri, Jan 29, 2010 at 1:56 PM, Greg Stark <gsstark@xxxxxxx> wrote: > On Tue, Jan 19, 2010 at 3:25 PM, Tom Lane <tgl@xxxxxxxxxxxxx> wrote: >> That function *seriously* needs documentation, in particular the fact >> that it's a no-op on machines without the right kernel call. The name >> you've chosen is very bad for those semantics. I'd pick something >> else myself. Maybe "pg_start_data_flush" or something like that? >> > > I would like to make one token argument in favour of the name I > picked. If it doesn't convince I'll change it since we can always > revisit the API down the road. > > I envision having two function calls, pg_fsync_start() and > pg_fsync_finish(). The latter will wait until the data synced in the > first call is actually synced. The fall-back if there's no > implementation of this would be for fsync_start() to be a noop (or > something unreliable like posix_fadvise) and fsync_finish() to just be > a regular fsync. > > I think we can accomplish this with sync_file_range() but I need to > read up on how it actually works a bit more. In this case it doesn't > make a difference since when we call fsync_finish() it's going to be > for the entire file and nothing else will have been writing to these > files. But for wal writing and checkpointing it might have very > different performance characteristics. > > The big objection to this is that then we don't really have an api for > FADV_DONT_NEED which is more about cache policy than about syncing to > disk. So for example a sequential scan might want to indicate that it > isn't planning on reading the buffers it's churning through but > doesn't want to force them to be written sooner than otherwise and is > never going to call fsync_finish(). I took a look at this patch today and I agree with Tom that pg_fsync_start() is a very confusing name. I don't know what the right name is, but this doesn't fsync so I don't think it shuld have fsync in the name. Maybe something like pg_advise_abandon() or pg_abandon_cache(). The current name is really wishful thinking: you're hoping that it will make the kernel start the fsync, but it might not. I think pg_start_data_flush() is similarly optimistic. ...Robert -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
