Re: Experimental patch for inter-page delay in VACUUM

Greg Stark <gsstark(at)mit(dot)edu> writes:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>> The main problem with this is knowing which files need to be fsync'd.

> Why could the postmaster not just fsync *every* file?

You want to find, open, and fsync() every file in the database cluster
for every checkpoint? Sounds like a non-starter to me. In typical
situations I'd expect there to be lots of files that have no writes
during any given checkpoint interval (system catalogs for instance).

> I'm assuming fsync syncs writes issued by other processes on the same file,
> which isn't necessarily true though.

It was already pointed out that we can't rely on that assumption.

> Or using aio write ahead as much as you want and then just make checkpoint
> block until all the writes are completed. You don't actually need to rush them
> at all, just know when they're done.

If the objective is to avoid an i/o storm, I don't think this does it.
The system could easily delay most of the writes until the next syncer()
pass.

regards, tom lane