Re: Experimental patch for inter-page delay in VACUUM

Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Now, if we are sure that writes will happen only in the checkpoint
> process, O_SYNC would be OK, I guess, but will we ever be sure of that?

This is a performance issue, not a correctness issue.  It's okay for
backends to wait for writes as long as it happens very infrequently.
The question is whether we can design a background dirty-buffer writer
that works well enough to make it uncommon for backends to have to
write dirty buffers for themselves.  If we can, then doing all the
writes O_SYNC would not be a problem.

(One possibility that could help improve the odds is to allow a certain
amount of slop in the LRU buffer reuse policy --- that is, if you see
the buffer at the tail of the LRU list is dirty, allow one of the next
few buffers to be taken instead, if it's clean.  Or just keep separate
lists for dirty and clean buffers.)
        regards, tom lane