pg_walsummary

Convert a WAL summary file to text

$ pg_walsummary [path/to/file]

$ pg_walsummary [[-i|--individual]] [path/to/file]

$ pg_walsummary [[-q|--quiet]] [path/to/file]

pg_walsummary [OPTION]...

-D DATADIR, --datadir=DATADIR
    Specifies the path to the PostgreSQL data directory of the target cluster. This directory's WAL files will be analyzed and compared against the source.

-S CONNSTR, --source-server=CONNSTR
    Provides the connection string for the running source PostgreSQL server. This allows pg_walsummary to connect and retrieve WAL information from the reference server. The format is a standard PostgreSQL connection string (e.g., "host=localhost port=5432 user=postgres").

-V, --version
    Output version information, then exit.

-?, --help
    Show help about pg_walsummary command line arguments, then exit.

pg_walsummary is a specialized utility within the PostgreSQL ecosystem, primarily designed as an internal helper for the pg_rewind command. Its core function is to analyze and summarize the differences in Write-Ahead Log (WAL) files between a target PostgreSQL data directory (e.g., a former primary or a standby) and a running source PostgreSQL server (the new primary).

When executed, pg_walsummary connects to the source server and inspects the WAL segments present in the specified target data directory. It then computes a summary indicating which WAL files are missing from the target, which are present but no longer needed by the source, and which need to be truncated or replayed. This summary is crucial for pg_rewind to efficiently synchronize the target cluster with the source, making it possible to convert a divergent cluster back into a standby or a new primary with minimal data transfer.

While it can be executed standalone, its output is primarily intended for programmatic consumption by pg_rewind rather than direct human interpretation, as it provides a structured list of WAL actions.

pg_walsummary is a PostgreSQL-specific utility, not a general Linux command, and is typically found in the PostgreSQL installation's bin directory. It's primarily an internal helper tool for pg_rewind and its output is designed for machine consumption, not direct user interpretation. It requires a running PostgreSQL source server to connect to and proper file system permissions to access the target data directory. It only summarizes WAL differences; it does not modify any files or directories itself.

Although it can be run manually, pg_walsummary is almost exclusively invoked internally by the pg_rewind command. Users typically interact directly with pg_rewind, which then orchestrates the use of pg_walsummary to gather necessary WAL comparison data before proceeding with file synchronization.

The output of pg_walsummary consists of a structured list, usually in a format readable by pg_rewind, indicating specific WAL segments to be copied, truncated, or removed. This detailed summary allows pg_rewalsummary to perform a precise and minimal synchronization.

pg_walsummary was introduced as an integral component alongside the pg_rewind utility. Both were initially developed and added to PostgreSQL in version 9.4, which was released in December 2014. Its development aimed to provide a more efficient and less resource-intensive method for quickly resynchronizing PostgreSQL clusters after a failover, compared to traditional full base backups.

pg_rewind(1), pg_basebackup(1), initdb(1), psql(1)