Re: Allowing WAL fsync to be done via O_SYNC

* Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> [010315 11:45] wrote:
> Alfred Perlstein <bright(at)wintelcom(dot)net> writes:
> > And since we're sorta on the topic of IO, I noticed that it looks
> > like (at least in 7.0.3) that vacuum and certain other routines
> > read files in reverse order.
>
> Vacuum does that because it's trying to push tuples down from the end
> into free space in earlier blocks. I don't see much way around that
> (nor any good reason to think that it's a critical part of vacuum's
> performance anyway). Where else have you seen such behavior?

Just vacuum, but the source is large, and I'm sort of lacking
on database-foo so I guessed that it may be done elsewhere.

You can optimize this out by implementing the read behind yourselves
sorta like this:

struct sglist *
read(fd, len)
{

if (fd.lastpos - fd.curpos <= THRESHOLD) {
fd.curpos = fd.lastpos - THRESHOLD;
len = THRESHOLD;
}

return (do_read(fd, len));
}

of course this is entirely wrong, but illustrates what
would/could help.

I would fix FreeBSD, but it's sort of a mess and beyond what
I've got time to do ATM.

--
-Alfred Perlstein - [bright(at)wintelcom(dot)net|alfred(at)freebsd(dot)org]