Tom, * Tom Lane (t...@sss.pgh.pa.us) wrote: > Robert Haas <robertmh...@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
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