From 28915c7db4da4eaffb8eb0dc5c04c87dcf345eb6 Mon Sep 17 00:00:00 2001
From: Simon Riggs <simon@2ndQuadrant.com>
Date: Fri, 1 Sep 2017 07:55:00 +0100
Subject: [PATCH] Add note about diskspace usage of pg_commit_ts
Author: Thomas Munro
---
doc/src/sgml/maintenance.sgml | 30 +++++++++++++++++-------------
1 file changed, 17 insertions(+), 13 deletions(-)
diff --git a/doc/src/sgml/maintenance.sgml b/doc/src/sgml/maintenance.sgml
index fe1e0ed2b3d..616aece6c0e 100644
--- a/doc/src/sgml/maintenance.sgml
+++ b/doc/src/sgml/maintenance.sgml
@@ -526,19 +526,23 @@
<para>
The sole disadvantage of increasing <varname>autovacuum_freeze_max_age</>
- (and <varname>vacuum_freeze_table_age</> along with it)
- is that the <filename>pg_xact</> subdirectory of the database cluster
- will take more space, because it must store the commit status of all
- transactions back to the <varname>autovacuum_freeze_max_age</> horizon.
- The commit status uses two bits per transaction, so if
- <varname>autovacuum_freeze_max_age</> is set to its maximum allowed
- value of two billion, <filename>pg_xact</> can be expected to
- grow to about half a gigabyte. If this is trivial compared to your
- total database size, setting <varname>autovacuum_freeze_max_age</> to
- its maximum allowed value is recommended. Otherwise, set it depending
- on what you are willing to allow for <filename>pg_xact</> storage.
- (The default, 200 million transactions, translates to about 50MB of
- <filename>pg_xact</> storage.)
+ (and <varname>vacuum_freeze_table_age</> along with it) is that
+ the <filename>pg_xact</> and <filename>pg_commit_ts</filename>
+ subdirectories of the database cluster will take more space, because it
+ must store the commit status and (if <varname>track_commit_timestamp</> is
+ enabled) timestamp of all transactions back to
+ the <varname>autovacuum_freeze_max_age</> horizon. The commit status uses
+ two bits per transaction, so if
+ <varname>autovacuum_freeze_max_age</> is set to its maximum allowed value
+ of two billion, <filename>pg_xact</> can be expected to grow to about half
+ a gigabyte and <filename>pg_commit_ts</filename> to about 20GB. If this
+ is trivial compared to your total database size,
+ setting <varname>autovacuum_freeze_max_age</> to its maximum allowed value
+ is recommended. Otherwise, set it depending on what you are willing to
+ allow for <filename>pg_xact</> and <filename>pg_commit_ts</> storage.
+ (The default, 200 million transactions, translates to about 50MB
+ of <filename>pg_xact</> storage and about 2GB of <filename>pg_commit_ts</>
+ storage.)
</para>
<para>