Re: Spread checkpoint sync

Tom Lane wrote:
> Bruce Momjian <bruce(at)momjian(dot)us> writes:
> > My trivial idea was: let's assume we checkpoint every 10 minutes, and
> > it takes 5 minutes for us to write the data to the kernel. If no one
> > else is writing to those files, we can safely wait maybe 5 more minutes
> > before issuing the fsync. If, however, hundreds of writes are coming in
> > for the same files in those final 5 minutes, we should fsync right away.
>
> Huh? I would surely hope we could assume that nobody but Postgres is
> writing the database files? Or are you considering that the bgwriter
> doesn't know exactly what the backends are doing? That's true, but
> I still maintain that we should design the bgwriter's behavior on the
> assumption that writes from backends are negligible. Certainly the
> backends aren't issuing fsyncs.

Right, no one else is writing but us. When I said "no one else" I meant
no other bgwrites writes are going to the files we wrote as part of the
checkpoint, but have not fsync'ed yet. I assume we have two write
streams --- the checkpoint writes, which we know at the start of the
checkpoint, and the bgwriter writes that are happening in an
unpredictable way based on database activity.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +