Re: Write Ahead Logging for Hash Indexes

Tom,

* Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
> Robert Haas <robertmhaas(at)gmail(dot)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