Date: Fri, 31 Jul 2026 17:50:41 +0200
From: Christoph Berg <myon@debian.org>
To: Andrey Borodin <x4mmm@yandex-team.ru>
Cc: Tomas Vondra <tomas@vondra.me>, wenhui qiu <qiuwenhuifx@gmail.com>,
	Laurenz Albe <laurenz.albe@cybertec.at>,
	Andres Freund <andres@anarazel.de>,
	Jelte Fennema-Nio <postgres@jeltef.nl>,
	Michael Paquier <michael@paquier.xyz>,
	Japin Li <japinli@hotmail.com>, Fujii Masao <masao.fujii@gmail.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>,
	John Naylor <johncnaylorls@gmail.com>
Subject: Re: WAL compression setting after PostgreSQL LZ4 default change
Message-ID: <amzEUaGtr8JsMVtR@msg.df7cb.de>
Mail-Followup-To: Christoph Berg <myon@debian.org>,
	Andrey Borodin <x4mmm@yandex-team.ru>,
	Tomas Vondra <tomas@vondra.me>, wenhui qiu <qiuwenhuifx@gmail.com>,
	Laurenz Albe <laurenz.albe@cybertec.at>,
	Andres Freund <andres@anarazel.de>,
	Jelte Fennema-Nio <postgres@jeltef.nl>,
	Michael Paquier <michael@paquier.xyz>,
	Japin Li <japinli@hotmail.com>, Fujii Masao <masao.fujii@gmail.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>,
	John Naylor <johncnaylorls@gmail.com>
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Re: Andrey Borodin
> FWIW on 19beta2, one client: a seqscan setting hint bits on a freshly
> checkpointed 380MB table with wal_log_hints on, so ~47k full-page images
> and little else.  Best of three: off 765ms, pglz 1402ms, lz4 444ms, zstd
> 797ms, writing 382/105/107/67 MB of WAL (fsync off, so the I/O saving is
> understated).  lz4 is the only setting faster than no compression at all,
> while pglz costs nearly double for the size lz4 gives away for free.  zstd
> compresses a third better but pays for it in CPU, partly by allocating a
> ZSTD_CCtx per FPI - see the nearby thread "Compression of bigger WAL
> records".  +1 for "on" meaning lz4 where available.

v4 moves lz4 first.

(I did not include any reason in the commit message about choosing lz4
over zstd yet.)

One minor bit: The patch now has

#if defined(USE_LZ4)
#define DEFAULT_WAL_COMPRESSION·WAL_COMPRESSION_LZ4
#elif defined(USE_ZSTD)
#define DEFAULT_WAL_COMPRESSION·WAL_COMPRESSION_ZSTD

when the old one had
#ifdef USE_ZSTD
#elif defined(USE_LZ4)

I left it at "#if defined(USE_LZ4)" because that looks more
symmetrical.

Christoph

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