Re: Background writer process

Kurt Roeckx wrote:
> On Thu, Nov 13, 2003 at 05:39:32PM -0500, Bruce Momjian wrote:
> > Jan Wieck wrote:
> > > Bruce Momjian wrote:
> > > > He found that write() itself didn't encourage the kernel to write the
> > > > buffers to disk fast enough. I think the final solution will be to use
> > > > fsync or O_SYNC.
> > >
> > > write() alone doesn't encourage the kernel to do any physical IO at all.
> > > As long as you have enough OS buffers, it does happy write caching until
> > > you checkpoint and sync(), and then the system freezes.
> >
> > That's not completely true. Some kernels with trickle sync, meaning
> > they sync a little bit regularly rather than all at once so write() does
> > help get those shared buffers into the kernel for possible writing.
> > Also, it is possible the kernel will issue a sync() on its own.
>
> So basicly on some kernels you want them to flush their dirty
> buffers faster.
>
> I have a feeling we should more make it depend on the system how
> we ask them not to keep it in memory too long and that maybe the
> sync(), fsync() or O_SYNC could be a fallback in case it's needed
> and there are no better ways of doing it.

I think the final plan is to have a GUC variable that controls how the
kernel is _encouraged_ to write dirty buffers to disk.

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