Re: Write Ahead Logging for Hash Indexes

Tom,

* Tom Lane (tgl@sss.pgh.pa.us) wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
> > That theory seems inconsistent with how mdextend() works.  My
> > understanding is that we zero-fill the new blocks before populating
> > them with actual data precisely to avoid running out of disk space due
> > to deferred allocation at the OS level.  If we don't care about
> > failures due to deferred allocation at the OS level, we can rip that
> > logic out and improve the performance of relation extension
> > considerably.
> 
> See my reply to Stephen.  The fact that this fails to guarantee no
> ENOSPC on COW filesystems doesn't mean that it's not worth doing on
> other filesystems.  We're reducing the risk, not eliminating it,
> but reducing risk is still a worthwhile activity.

Considering how much work we end up doing to extend a relation and how
we know that's been a hotspot, I'm not entirely sure I agree that
avoiding the relativly infrequent out-of-disk-space concern when
extending the relation (instead of letting it happen when we go to
actually write data into the page) really is a good trade-off to make.

Thanks!

Stephen