Re: [ADMIN] pg_class reltuples/relpages not updated by autovacuum/vacuum

2011/5/29 Tom Lane <tgl@sss.pgh.pa.us>:
> Cédric Villemain <cedric.villemain.debian@gmail.com> writes:
>> 2011/5/29 Tom Lane <tgl@sss.pgh.pa.us>:
>>> OK, do you like the attached version of that logic?  (Other fragments
>>> of the patch as before.)
>
>> The idea was that remove only one page from the VACUUM will prevent
>> relfrozenxid update and reltuples (and relpages) update.
>> Now, I beleive that once we've skip at least one page thanks to
>> SKIP_PAGES_THRESHOLD, then we should be more agressive and remove as
>> many as possible pages from the VACUUM, tks to the VM.
>
> That would require proof, not just suggestion.  Skipping pages will
> defeat the OS read-ahead algorithm, and so could easily cost more than
> reading them.

Correct, it needs proof. Parenthesis: I did learn also that reading
the first block of a file make read-ahead have its larger window from
the beginning (the one that posix_fadvise_sequential set too), so
remove those initial reads might be counter-productive also. But this
is damn hard to benchmark because the read ahead is also influenced by
memory pressure for example.

From theory, 1. readahead algo is a bit smarter and can work with
read-with-holes (if the holes are not larger than the read-ahead
window) and 2. if holes are that large then maybe it is not so good to
keep a larger read-ahead window (which keep trashing our buffer
cache).




--
Cédric Villemain               2ndQuadrant
http://2ndQuadrant.fr/     PostgreSQL : Expertise, Formation et Support