Re: Async Commit, v21 (now: v22)

Am Dienstag, 17. Juli 2007 20:31 schrieb Simon Riggs:
> Here's the latest version. I've reviewed this to check that this does
> what I want it to do, re-written various comments and changed a few
> minor points in the code.
>
> I've also added a chunk to transam/README that describes the workings of
> the patch from a high level.
>
> Now ready for final review.

I'm not sure the following explanation is all that clear:

+    <para>
+     Asynchronous commit provides different behaviour to setting
+     <varname>fsync</varname> = off, since that is a server-wide
+     setting that will alter the behaviour of all transactions,
+     overriding the setting of <varname>synchronous_commit</varname>,
+     as well as risking much wider data loss.  With <varname>fsync</varname>
+       = off the WAL written but not fsynced, so data is lost only in case
+       of a system crash. With asynchronous commit the WAL is not written
+       to disk at all by the user, so data is lost if there is a database
+       server crash, as well as when the system crashes.
+    </para>

On the one hand, it claims that fsync off has much wider data loss
implications than asynchronous commit, on the other hand, it states that the
risk of a loss due to asynchronous commit happening is larger than fsync off.
I *think* I know what this is trying to say, but I suspect that the average
user might not be able to make a good choice of settings.

--
Peter Eisentraut
http://developer.postgresql.org/~petere/