wal_block_size

Definition PG 18 manual

Reports the size of a WAL disk block. It is determined by the value of XLOG_BLCKSZ when building the server. The default value is 8192 bytes.

How it works

Shows the block size in the write ahead log. It is read-only and cannot be changed with SET or configuration-file edits.

It reflects XLOG_BLCKSZ, normally 8 KiB, chosen at build time and used for WAL I/O buffers.

Interpret it with block_size, wal_segment_size, segment_size. Applications and operations tooling may read wal_block_size for capability or environment detection, but cannot SET it; cross-cluster comparisons must account for build, initdb, and current-database differences.

Operational considerations

Trying to change read-only wal_block_size with SET or a configuration file.

Assuming every cluster, database, or vendor build reports the same value.

Ignoring the value in file-format or feature-compatibility checks.

Treating reported metadata as a performance target instead of an environment fact.

Workload guidance

OLAP: Not tunable. Use wal_block_size to verify format, build, or resource assumptions on analytical nodes and keep every read/write candidate compatible.

OLTP: Not tunable. Record wal_block_size as a deployment/capability fact; if it is unexpected, fix the binary, initdb, or database-creation process rather than trying SET.

SMALL: Not tunable. Preserve it in inventory and incident reports; do not custom-build or rebuild merely to change it without a demonstrated compatibility need.