Re: Background writer process

Jan Wieck wrote:
> Bruce Momjian wrote:
>
> > Kurt Roeckx wrote:
> >> On Thu, Nov 13, 2003 at 04:35:31PM -0500, Jan Wieck wrote:
> >> > For sure the sync() needs to be replaced by the discussed fsync() of
> >> > recently written files. And I think the algorithm how much and how often
> >> > to flush can be significantly improved. But after all, this does not
> >> > change the real checkpointing at all, and the general framework having a
> >> > separate process is what we probably want.
> >>
> >> Why is the sync() needed at all? My understanding was that it
> >> was only needed in case of a checkpoint.
> >
> > 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.

--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073