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Subject: Re: Replication slot WAL reservation
From: Christophe Pettus <xof@thebuild.com>
In-Reply-To: <CAGAwPgR=B9VTDwKkHZRHjYTmQzvJ+30mK-uyc7MMw6Tf9s9Fsw@mail.gmail.com>
Date: Tue, 25 Mar 2025 10:13:35 -0700
Cc: pgsql-general@postgresql.org
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Message-Id: <E08677AC-AE92-40A1-9C46-9C8BA4529FE7@thebuild.com>
References: <CAGAwPgR=B9VTDwKkHZRHjYTmQzvJ+30mK-uyc7MMw6Tf9s9Fsw@mail.gmail.com>
To: Phillip Diffley <phillip6402@gmail.com>
Archived-At: <https://www.postgresql.org/message-id/E08677AC-AE92-40A1-9C46-9C8BA4529FE7%40thebuild.com>
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Missed this question!

> On Mar 25, 2025, at 09:56, Phillip Diffley <phillip6402@gmail.com> =
wrote:
> But when processing data from a replication slot, we confirm rows that =
have been processed and can be deleted from the WAL based on the LSN =
(eg. with pg_replication_slot_advance). How does postgres identify what =
parts of the WAL can be freed?

Basically, if no part of the system "needs" a particular LSN position, =
the segments that include that LSN position and earlier can be free.

The various things that can "need" a particular LSN point are:

1. Replication slots, if the other side has not confirmed that it has =
received it (under whatever synchronous commit rules that slot is =
operating under).
2. The wal_keep_size setting.
3. The max_wal_size setting.
4. The archive_command, if a WAL segment hasn't been successfully =
archived yet.

One thing to remember is that the WAL does *not* contain contiguous =
blocks of operations for a single transaction.  The operations are =
written to the WAL by every session as they do operations, so the WAL is =
a jumble of different transactions.  One of the jobs of the logical =
replication framework is to sort that out so it can present only the =
operations that belong to committed transactions to the output plugin.  =
(This is why there's an internal structure called the "reorder buffer": =
it reorders WAL operations into transaction blocks.)=