Re: [RFC] Should we fix postmaster to avoid slow shutdown?

Robert Haas <robertmhaas@gmail.com> writes:
> I agree.  However, in many cases, the major cost of a fast shutdown is
> getting the dirty data already in the operating system buffers down to
> disk, not in writing out shared_buffers itself.  The latter is
> probably a single-digit number of gigabytes, or maybe double-digit.
> The former might be a lot more, and the write of the pgstat file may
> back up behind it.  I've seen cases where an 8kB buffered write from
> Postgres takes tens of seconds to complete because the OS buffer cache
> is already saturated with dirty data, and the stats files could easily
> be a lot more than that.

I think this is mostly FUD, because we don't fsync the stats files.  Maybe
we should, but we don't today.  So even if we have managed to get the
system into a state where physical writes are heavily backlogged, that's
not a reason to assume that the stats collector will be unable to do its
thing promptly.  All it has to do is push a relatively small amount of
data into kernel buffers.
        regards, tom lane