Re: Defaulting wal_sync_method to fdatasync on Linux for 9.1?

Hi,

On Monday 08 November 2010 23:12:57 Greg Smith wrote:
> This seems to be ignoring the fact that unless you either added a
> non-volatile cache or specifically turned off all write caching on your
> drives, the results of all power-fail testing done on earlier versions
> of Linux was that it failed. The default configuration of PostgreSQL on
> Linux has been that any user who has a simple SATA drive gets unsafe
> writes, unless they go out of their way to prevent them.
Which is about *no* argument in favor of any of the options, right?

> Whatever newer kernels do by default cannot be worse. The open question
> is whether it's still broken, in which case we might as well favor the
> known buggy behavior rather than the new one, or whether everything has
> improved enough to no longer be unsafe with the new defaults.
Either I majorly misunderstand you, or ... I dont know.

There simply *is* no new implementation relevant for this discussion. Full
Stop. What changed is that O_DSYNC is defined differently from O_SYNC these days
and O_SYNC actually does what it should. Which causes pg to move open_datasync
first in the preference list doing what the option with the lowest preference
did up to now.

That does not *at all* change the earlier fdatasync() or fsync()
implementations/tests. It simply makes open_datasync the default doing what
open_sync did earlier.
For that note that open_sync was the method of *least* preference till now...
And that fdatasync() thus was the default till now. Which it is not anymore.

I don't argue *at all* that we have to test the change moving fdatasync before
open_datasync on the *other* operating systems. What I completely don't get is
all that talking about data consistency on linux. Its simply irrelevant in
that context.

Andres