Re: Allowing WAL fsync to be done via O_SYNC

Peter Eisentraut <peter_e@gmx.net> writes:
>> Peter, what do you think about configuration-dependent defaults for
>> GUC variables?

> We have plenty of those already, but we should avoid a variable whose
> specification is:

> "The default is 'on' if your system defines one of the macros O_SYNC,
> O_DSYNC, O_FSYNC, and if O_SYNC and O_DSYNC are distinct, otherwise the
> default is 'off'."

Unfortunately, I think that's just about what the default would need to
be.  What alternative do you have to offer?

> The net result of this would be that the average user would have
> absolutely no clue what the default on his machine is.

Sure he would.  Fire up the software and do "SHOW wal_use_fsync"
(or whatever we call it).  I think the documentation could just say
"the default is platform-dependent".

> Additionally consider that maybe O_SYNC and O_DSYNC have different values
> but the kernel treats them the same anyway.  We really shouldn't try to
> guess that far.

Well, that's exactly *why* we need an overridable default.  Or would you
like to try to do some performance measurements in configure?
        regards, tom lane