Hi, On Mon, Sep 22, 2025 at 8:26 AM Jeff Davis <pg...@j-davis.com> wrote: > > On Sat, 2025-09-13 at 22:04 -0700, Bharath Rupireddy wrote: > > Thanks for looking at this. Yes, the WAL writers can zero out flushed > > buffers before WALReadFromBuffers gets to them. However, > > WALReadFromBuffers was intentionally designed as an opportunistic > > optimization - it's a "try this first, quickly" approach before > > falling back to reading from WAL files. > > IIRC, one motivation (perhaps the primary motivation?) was to make it > possible to read buffers before they are flushed. It was always > possible to read already-flushed buffers. > > The benefit of reading unflushed buffers is that we can replicate the > WAL sooner (though it can't be replayed until the primary flushes it). > Is that right?
Right. Reading unflushed WAL buffers for replication was one of the motivations. But, in general, WALReadFromBuffers has more benefits since it lets WAL buffers act as a cache for reads, avoiding the need to re-read WAL from disk for (both physical and logical) replication. For example, it makes the use of direct I/O for WAL more realistic and can provide significant performance benefits [1]. [1] https://www.postgresql.org/message-id/20230114203403.4zpg72fw2qb34awf%40awork3.anarazel.de -- Bharath Rupireddy PostgreSQL Contributors Team RDS Open Source Databases Amazon Web Services: https://aws.amazon.com
