Re: Spread checkpoint sync

Kevin Grittner wrote:
> Robert Haas <robertmhaas@gmail.com> wrote:
>  
> > I also think Bruce's idea of calling fsync() on each relation just
> > *before* we start writing the pages from that relation might have
> > some merit.
>  
> What bothers me about that is that you may have a lot of the same
> dirty pages in the OS cache as the PostgreSQL cache, and you've just
> ensured that the OS will write those *twice*.  I'm pretty sure that
> the reason the aggressive background writer settings we use have not
> caused any noticeable increase in OS disk writes is that many
> PostgreSQL writes of the same buffer keep an OS buffer page from
> becoming stale enough to get flushed until PostgreSQL writes to it
> taper off.  Calling fsync() right before doing "one last push" of
> the data could be really pessimal for some workloads.

OK, maybe my idea needs to be adjusted and we should trigger an early
fsync if non-fsync writes are coming in for blocks _other_ than the ones
we already wrote for that checkpoint.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +