On Tuesday 29 December 2009 00:06:28 Tom Lane wrote: > Andres Freund <andres@xxxxxxxxxxx> writes: > > This speeds up CREATE DATABASE from ~9 seconds to something around 0.8s > > on my laptop. Still slower than with fsync off (~0.25) but quite a > > worthy improvement. > I can't help wondering whether that's real or some kind of > platform-specific artifact. I get numbers more like 3.5s (fsync off) > vs 4.5s (fsync on) on a machine where I believe the disks aren't lying > about write-complete. It makes sense that an fsync at the end would be > a little bit faster, because it would give the kernel some additional > freedom in scheduling the required I/O, but it isn't cutting the total > I/O required at all. So I find it really hard to believe a 10x speedup. Well, a template database is about 5.5MB big here - that shouldnt take too long when written near-sequentially? As I said the real benefit only occurred after adding posix_fadvise(.., FADV_DONTNEED) which is somewhat plausible, because i.e. the directory entries don't need to get scheduled for every file and because the kernel can reorder a whole directory nearly sequentially. Without the advice it the kernel doesn't know in time that it should write that data back and it wont do it for 5 seconds by default on linux or such... I looked at the strace output - it looks sensible timewise to me. If youre interested I can give you output of that. Andres -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
