Re: -F option

newsreader(at)mediaone(dot)net writes:
> What I would like to know is what 'sometime later' means.
> Is it one hour? 30 seconds? 30 minutes? 24 hours?

On a typical unix setup it's the cycle length of your syncer daemon
(typically 30 seconds), plus however long it physically takes the
OS to push the data out to the drive and then the drive to get around
to writing it. The nearby estimate of 1 minute sounds good to me as
a (fairly conservative) upper bound, at least under normal conditions.

The standard advice about -F is that it's cool if you trust your OS,
your hardware, and your UPS. You do *not* need to worry about Postgres
crashes --- the backend will write the data to the kernel at commit
in any case. The only question is whether we try to encourage the
kernel to push the data down to disk before we report that the
transaction has been committed.

There is a long thread on pghackers recently to the effect that even
without -F, you are at the mercy of disk drive and power supply
failures, because fsync() only guarantees that the kernel has given the
data to the disk drive; modern disk drives may buffer the data for
awhile before they plop it down onto the platter. So, you probably
want a UPS in any case. Beyond that, how many kernel crashes and
hardware failures have you seen lately?

> My usage is frequent lookup of small pieces of information,
> ocassionally insert of small pieces of information and even less
> frequent update of small pieces of information.

OTOH, if you are not doing a lot of insert/update/delete then -F gains
little performance anyway...

regards, tom lane