Re: Experimental patch for inter-page delay in VACUUM

Jan Wieck <JanWieck(at)Yahoo(dot)com> writes:
> That is part of the idea. The whole idea is to issue "physical" writes
> at a fairly steady rate without increasing the number of them
> substantial or interfering with the drives opinion about their order too
> much. I think O_SYNC for random access can be in conflict with write
> reordering.

Good point. But if we issue lots of writes without fsync then we still
have the problem of a write storm when the fsync finally occurs, while
if we fsync too often then we constrain the write order too much. There
will need to be some tuning here.

> How I can see the background writer operating is that he's keeping the
> buffers in the order of the LRU chain(s) clean, because those are the
> buffers that most likely get replaced soon. In my experimental ARC code
> it would traverse the T1 and T2 queues from LRU to MRU, write out n1 and
> n2 dirty buffers (n1+n2 configurable), then fsync all files that have
> been involved in that, nap depending on where he got down the queues (to
> increase the write rate when running low on clean buffers), and do it
> all over again.

You probably need one more knob here: how often to issue the fsyncs.
I'm not convinced "once per outer loop" is a sufficient answer.
Otherwise this is sounding pretty good.

regards, tom lane