Gregory Stark wrote:
> "Bruce Momjian" <bruce@momjian.us> writes:
>
>> I agree it index cleanup isn't > 50% of vacuum. I was trying to figure
>> out how small, and it seems about 15% of the total table, which means if
>> we have bitmap vacuum, we can conceivably reduce vacuum load by perhaps
>> 80%, assuming 5% of the table is scanned.
>
> Actually no. A while back I did experiments to see how fast reading a file
> sequentially was compared to reading the same file sequentially but skipping
> x% of the blocks randomly. The results were surprising (to me) and depressing.
> The breakeven point was about 7%.
Note that with uniformly random updates, you have dirtied every page of
the table until you get anywhere near 5% of dead space. So we have to
assume non-uniform distribution of update for the DSM to be of any help.
And if we assume non-uniform distribution, it's a good bet that the
blocks that need vacuuming are also not randomly distributed. In fact,
they might very well all be in one cluster, so that scanning that
cluster is indeed sequential I/O.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
