Re: [HACKERS] AdvanceXLInsertBuffer vs. WAL segment compressibility

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Stephen Frost <sfrost(at)snowman(dot)net>
Cc: Chapman Flack <chap(at)anastigmatix(dot)net>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, Daniel Gustafsson <daniel(at)yesql(dot)se>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Robert Haas <robertmhaas(at)gmail(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] AdvanceXLInsertBuffer vs. WAL segment compressibility
Date: 2018-03-30 20:21:24
Message-ID: 24353.1522441284@sss.pgh.pa.us
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Stephen Frost <sfrost(at)snowman(dot)net> writes:
> * Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
>> A potentially stronger complaint is that WAL-reading tools might fail
>> outright on a page with an invalid header, but I'd say that's a robustness
>> issue that they'd need to address anyway. There's never been any
>> guarantee that the trailing pages of a WAL segment are valid.

> Agreed, I don't buy off that tools which fall apart when reading a page
> with an invalid header should block this from moving forward- those
> tools need to be fixed to not rely on trailing/unused WAL pages to be
> valid.

Yup. Pushed with some rewriting of the comments.

I did not like the proposed test case too much, particularly not its
undocumented API change for check_pg_config, so I did not push that.
We already have test coverage for pg_switch_wal() so it doesn't seem
very critical to have more.

regards, tom lane

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