From d23256ab8c8ce76a671be5415b5a51f3ce9fd8f1 Mon Sep 17 00:00:00 2001 From: Christoph Berg Date: Fri, 31 Jul 2026 14:24:23 +0200 Subject: [PATCH v4] Change wal_compression=on to the first of lz4, zstd, pglz Previously, wal_compression=on was an alias for pglz, with the idea that users could make an informed choice to pick a better option. But in practice, users either don't want to make that choice (they just want WAL compression), can't really pick the best option (they are setting the default in an environment where someone else is running the workload), or simply don't know that there are other options (previously, "on" was the only choice available). In all these cases, mapping "on" to something that isn't pglz is better. It is strictly slower and does not compress better. We pick lz4 here. Users can still make the informed choice and switch to zstd if their workload works better there, but lz4 is still better than pglz in that case. Like for the default TOAST compression, the actual default "on" value depends on if PostgreSQL was compiled with the compression algorithms. The first of lz4, zstd, pglz is picked. (Only the latter is always available, but practically, builds will have at least one of the better algorithms.) --- doc/src/sgml/config.sgml | 12 ++++++------ src/backend/utils/misc/guc_tables.c | 8 ++++---- src/backend/utils/misc/postgresql.conf.sample | 3 ++- src/include/access/xlog.h | 13 +++++++++++++ 4 files changed, 25 insertions(+), 11 deletions(-) diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index aa7b1bd75d2..39ec64a611a 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -3667,12 +3667,12 @@ include_dir 'conf.d' is on, during a base backup, etc.). A compressed page image will be decompressed during WAL replay. - The supported methods are pglz, - lz4 (if PostgreSQL - was compiled with ) and - zstd (if PostgreSQL - was compiled with ). - The value on is a historical spelling of pglz. + The supported methods are off, on, + lz4 (if PostgreSQL was compiled with ), + zstd (if PostgreSQL was compiled with ), and + pglz. + The value on selects the first of lz4, + zstd, pglz that is available. The default value is off. Only superusers and users with the appropriate SET privilege can change this setting. diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c index 1ec460b6a82..e0b21053287 100644 --- a/src/backend/utils/misc/guc_tables.c +++ b/src/backend/utils/misc/guc_tables.c @@ -484,13 +484,13 @@ static const struct config_enum_entry wal_compression_options[] = { #ifdef USE_ZSTD {"zstd", WAL_COMPRESSION_ZSTD, false}, #endif - {"on", WAL_COMPRESSION_PGLZ, false}, + {"on", DEFAULT_WAL_COMPRESSION, false}, {"off", WAL_COMPRESSION_NONE, false}, - {"true", WAL_COMPRESSION_PGLZ, true}, + {"true", DEFAULT_WAL_COMPRESSION, true}, {"false", WAL_COMPRESSION_NONE, true}, - {"yes", WAL_COMPRESSION_PGLZ, true}, + {"yes", DEFAULT_WAL_COMPRESSION, true}, {"no", WAL_COMPRESSION_NONE, true}, - {"1", WAL_COMPRESSION_PGLZ, true}, + {"1", DEFAULT_WAL_COMPRESSION, true}, {"0", WAL_COMPRESSION_NONE, true}, {NULL, 0, false} }; diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample index 7958653077b..ff108e3d3a5 100644 --- a/src/backend/utils/misc/postgresql.conf.sample +++ b/src/backend/utils/misc/postgresql.conf.sample @@ -262,7 +262,8 @@ #wal_log_hints = off # also do full page writes of non-critical updates # (change requires restart) #wal_compression = off # enables compression of full-page writes; - # off, pglz (or "on"), lz4, or zstd + # off, on, lz4, zstd, or pglz (on = lz4 if + # available, else zstd if available, else pglz) #wal_init_zero = on # zero-fill new WAL files #wal_recycle = on # recycle WAL files #wal_buffers = -1 # min 32kB, -1 sets based on shared_buffers diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h index 4dd98624204..8daf24d93cf 100644 --- a/src/include/access/xlog.h +++ b/src/include/access/xlog.h @@ -87,6 +87,19 @@ typedef enum WalCompression WAL_COMPRESSION_ZSTD, } WalCompression; +/* + * Choose an appropriate default WAL compression method for wal_compression=on. + * Prefer lz4 when compiled in; otherwise use zstd if available, falling back + * to pglz. + */ +#if defined(USE_LZ4) +#define DEFAULT_WAL_COMPRESSION WAL_COMPRESSION_LZ4 +#elif defined(USE_ZSTD) +#define DEFAULT_WAL_COMPRESSION WAL_COMPRESSION_ZSTD +#else +#define DEFAULT_WAL_COMPRESSION WAL_COMPRESSION_PGLZ +#endif + /* Recovery states */ typedef enum RecoveryState { -- 2.53.0