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

Robert Haas <robertmhaas(at)gmail(dot)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