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From: =?ISO-8859-1?Q?C=E9dric_Villemain?= <cedric.villemain.debian@gmail.com>
Date: Mon, 30 May 2011 01:32:54 +0200
Message-ID: <BANLkTinL6QuAm_Xf8teRZboG2Mdy3dR_vw@mail.gmail.com>
Subject: Re: [ADMIN] pg_class reltuples/relpages not updated by
	autovacuum/vacuum
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Greg Stark <gsstark@mit.edu>, Robert Haas <robertmhaas@gmail.com>,
	Kevin Grittner <Kevin.Grittner@wicourts.gov>,
	Florian Helmberger <fh@25th-floor.com>,
	Alvaro Herrera <alvherre@commandprompt.com>, pgsql-hackers@postgresql.org
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

2011/5/29 Tom Lane <tgl@sss.pgh.pa.us>:
> =3D?ISO-8859-1?Q?C=3DE9dric_Villemain?=3D <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? =A0(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. =A0Skipping 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.