Date: Fri, 30 Mar 2018 14:24:23 -0400
From: Stephen Frost <sfrost@snowman.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Chapman Flack <chap@anastigmatix.net>,
	Tomas Vondra <tomas.vondra@2ndquadrant.com>,
	Michael Paquier <michael@paquier.xyz>,
	Daniel Gustafsson <daniel@yesql.se>,
	Michael Paquier <michael.paquier@gmail.com>,
	Heikki Linnakangas <hlinnaka@iki.fi>,
	Robert Haas <robertmhaas@gmail.com>,
	Bruce Momjian <bruce@momjian.us>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] AdvanceXLInsertBuffer vs. WAL segment compressibility
Message-ID: <20180330182423.GS24540@tamriel.snowman.net>
References: <46FFB930-B0F3-400A-85ED-D9948C97FADF@yesql.se>
 <5AAEE00E.9050503@anastigmatix.net>
 <21C25ED8-4ED8-4AD4-B551-3E3987ECB18F@yesql.se>
 <5AAEFF11.3070903@anastigmatix.net>
 <20180326032731.GM24540@tamriel.snowman.net>
 <20180326054354.GB2759@paquier.xyz>
 <5ABAC668.5080203@anastigmatix.net>
 <8c00b26a-d6da-5764-e67e-9c4d8128081a@2ndquadrant.com>
 <5ABD9083.5050908@anastigmatix.net>
 <12915.1522433926@sss.pgh.pa.us>
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Greetings,

* Tom Lane (tgl@sss.pgh.pa.us) wrote:
> Chapman Flack <chap@anastigmatix.net> writes:
> > On 03/27/18 22:10, Michael Paquier wrote:
> >> Here you go for one example:
> >> https://sourceforge.net/projects/pglesslog/
>=20
> > In any case, from my study of the commit, it is hard for me to see an i=
ssue.
> > The code comment says: "mark the header to indicate that WAL records
> > beginning in this page have removable backup blocks."
>=20
> Yeah, that commit just moved a flag from individual WAL records to page
> headers, arguing that it was okay to assume that the same flag value
> applies to all records on a page.  If there are no records in the page,
> it doesn't matter what you think the flag value is.
>=20
> 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.

Thanks!

Stephen

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