Re: initdb and fsync

On Wed, 2012-03-14 at 10:26 +0100, Andres Freund wrote:
> On Wednesday, March 14, 2012 05:23:03 AM Jeff Davis wrote:
> > On Tue, 2012-03-13 at 09:42 +0100, Andres Freund wrote:
> > > for recursively everything in dir:
> > > posix_fadvise(fd, POSIX_FADV_DONTNEED);
> > >
> > > for recursively everything in dir:
> > > fsync(fd);
> >
> > Wow, that made a huge difference!
> >
> > no sync: ~ 1.0s
> > sync: ~10.0s
> > fadvise+sync: ~ 1.3s

I take that back. There was something wrong with my test -- fadvise
helps, but it only takes it from ~10s to ~6.5s. Not quite as good as I
hoped.

> Well, while the positive effect of this are rather large it also has the bad
> effect of pushing the whole new database out of the cache. Which is not so nice
> if you want to run tests on it seconds later.

I was unable to see a regression when it comes to starting it up after
the fadvise+fsync. My test just started the server, created a table,
then stopped the server. It was actually a hair faster with the
directory that had been fadvise'd and then fsync'd, but I assume that
was noise. Regardless, this doesn't look like an issue.

> How are the results with sync_file_range(fd, 0, 0,
> SYNC_FILE_RANGE_WRITE)?

That is much faster than using fadvise. It goes down to ~2s.

Unfortunately, that's non-portable. Any other ideas? 6.5s a little on
the annoying side (and causes some disconcerting sounds to come from my
disk), especially when we _know_ it can be done in 2s.

Anyway, updated patch attached.

Regards,
Jeff Davis