On 2021-Nov-24, Kyotaro Horiguchi wrote:
> At Tue, 23 Nov 2021 11:21:05 -0300, Alvaro Herrera <alvhe...@alvh.no-ip.org>
> wrote in
> > I wonder why did we make it round down rather than up. Does this mean
> > that if you have max_slot_wal_keep_size=8MB and wal segments of 16 MB,
> > the slot gets invalidated with more than zero reserved segments?
>
> That rounding behavior is the way we are generally doing on similar
> variables. It is based on the behavior of ConvertToXSegs(). So
> max_wal_size for example is rounded-up the same way.
After thinking on it some more, I think rounding down is correct. If
somebody sets max_slot_wal_keep_size=40MB, then by keeping the limit at
32 MB (round down) we're honoring that request better than if we made it
48 MB (round up). But in any case, I think it doesn't matter much:
users need a *rough* size limit, not a super-precise one. After all,
this is only verified at checkpoint time. We're probably dealing with a
few hundreds of megabytes at least, so 16 MB one way or the other don't
change things much.
Here's a proposed patch.
--
Álvaro Herrera 39°49'30"S 73°17'W — https://www.EnterpriseDB.com/
"Las mujeres son como hondas: mientras más resistencia tienen,
más lejos puedes llegar con ellas" (Jonas Nightingale, Leap of Faith)
>From bfbc099a1ea5a4eec57d5d25c0dbb616e8724182 Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvhe...@alvh.no-ip.org>
Date: Thu, 25 Nov 2021 11:29:41 -0300
Subject: [PATCH] Document units for max_slot_wal_keep_size
---
doc/src/sgml/config.sgml | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 3f806740d5..918d7ebd01 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -4175,6 +4175,11 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"' # Windows
to continue replication due to removal of required WAL files. You
can see the WAL availability of replication slots
in <link linkend="view-pg-replication-slots">pg_replication_slots</link>.
+ The value is rounded down to an integer number of WAL files
+ according to <varname>wal_segment_size</varname>.
+ If this value is specified without units, it is taken as megabytes.
+ This parameter can only be set in the
+ <filename>postgresql.conf</filename> file or on the server command line.
</para>
</listitem>
</varlistentry>
--
2.30.2
