Re: Remove fsync ON/OFF as a visible option?

On Sat, Mar 21, 2015 at 8:54 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Stephen Frost <sfrost(at)snowman(dot)net> writes:
> > At the moment, one could look at our default postgresql.conf and the
> > "turns forced synchronization on or off" and think it's something akin
> > or somehow related to synchronous_commit (which is completely different,
> > but the options are right next to each other..).
>
> > How about a big warning around fsync and make it more indepenent from
> > the options around it?
>
> Yeah, the main SGML docs are reasonably clear about the risks of fsync,
> but postgresql.conf doesn't give you any hint that it's dangerous. Now
> I'm not entirely sure that people who frob postgresql.conf without having
> read the docs can be saved from themselves, but we could do something
> like this:
>
> # - Settings -
>
> #wal_level = minimal # minimal, archive, hot_standby,
> or logical
> # (change requires restart)
> #fsync = on # turns forced synchronization on
> or off
> + # (fsync=off is dangerous, read the
> + # (manual before using it)
> #synchronous_commit = on # synchronization level;
> # off, local, remote_write, or on
> #wal_sync_method = fsync # the default is the first option
> # supported by the operating
> system:
>
> Also, I think the short description "turns forced synchronization on or
> off" could stand improvement; it really conveys zero information. Maybe
> something like "force data to disk when committing"?
>

I agree the current description is lacking, but that proposed wording would
be a better description of synchronous_commit. It is checkpointing and
flush-WAL-before-data where fsync=off does its damage.

"Force data to disk when needed for integrity"?

Or just don't describe what it is at all, and refer to the documentation
only.

Cheers,

Jeff