Re: Experimental patch for inter-page delay in VACUUM

Jan Wieck wrote:
> > If the background cleaner has to not just write() but write/fsync or
> > write/O_SYNC, it isn't going to be able to clean them fast enough. It
> > creates a bottleneck where we didn't have one before.
> >
> > We are trying to eliminate an I/O storm during checkpoint, but the
> > solutions seem to be making the non-checkpoint times slower.
> >
>
> It looks as if you're assuming that I am making the backends unable to
> write on their own, so that they have to wait on the checkpointer. I
> never said that.
>
> If the checkpointer keeps the LRU heads clean, that lifts off write load
> from the backends. Sure, they will be able to dirty pages faster.
> Theoretically, because in practice if you have a reasonably good cache
> hitrate, they will just find already dirty buffers where they just add
> some more dust.
>
> If after all the checkpointer (doing write()+whateversync) is not able
> to keep up with the speed of buffers getting dirtied, the backends will
> have to do some write()'s again, because they will eat up the clean
> buffers at the LRU head and pass the checkpointer.

Yes, there are a couple of issues here --- first, have a background
writer to write dirty pages. This is good, no question. The bigger
question is removing sync() and using fsync() or O_SYNC for every write
--- if we do that, the backends doing private write will have to fsync
their writes too, meaning if the checkpointer can't keep up, we now have
backends doing slow writes too.

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