Re: WAL sync behaviour

On Thu, 2005-11-10 at 10:39, Tom Lane wrote:
> Scott Marlowe <smarlowe(at)g2switchworks(dot)com> writes:
> > On Thu, 2005-11-10 at 08:43, Michael Stone wrote:
> >> There's no reason to use a journaled filesystem for the wal. Use ext2 in
> >> preference to ext3.
>
> > Not from what I understood. Ext2 can't guarantee that your data will
> > even be there in any form after a crash. I believe only metadata
> > journaling is needed though.
>
> No, Mike is right: for WAL you shouldn't need any journaling. This is
> because we zero out *and fsync* an entire WAL file before we ever
> consider putting live WAL data in it. During live use of a WAL file,
> its metadata is not changing. As long as the filesystem follows
> the minimal rule of syncing metadata about a file when it fsyncs the
> file, all the live WAL files should survive crashes OK.
>
> We can afford to do this mainly because WAL files can normally be
> recycled instead of created afresh, so the zero-out overhead doesn't
> get paid during normal operation.
>
> You do need metadata journaling for all non-WAL PG files, since we don't
> fsync them every time we extend them; which means the filesystem could
> lose track of which disk blocks belong to such a file, if it's not
> journaled.

Thanks for the clarification! Nice to know I can setup an ext2
partition for my WAL files then. Is this in the docs anywhere?