Move SLRU_PAGES_PER_SEGMENT to pg_config_manual.h

It seems plausible that someone might want to experiment with
different values. The pressing reason though is that I'm reviewing a
patch that requires pg_upgrade to manipulate SLRU files. That patch
needs to access SLRU_PAGES_PER_SEGMENT from pg_upgrade code, and
slru.h, where SLRU_PAGES_PER_SEGMENT is currently defined, cannot be
included from frontend code. Moving it to pg_config_manual.h makes it
accessible.

Now that it's a little more likely that someone might change
SLRU_PAGES_PER_SEGMENT, add a cluster compatibility check for it.

Bump catalog version because of the new field in the control file.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://www.postgresql.org/message-id/c7a4ea90-9f7b-4953-81be-b3fcb47db057@iki.fi

1 files changed, 10 insertions, 0 deletions

diff --git a/src/include/pg_config_manual.h b/src/include/pg_config_manual.h
index 7e1aa422332..c61a9faf870 100644
--- a/src/include/pg_config_manual.h
+++ b/src/include/pg_config_manual.h
@@ -20,6 +20,16 @@
 #define DEFAULT_XLOG_SEG_SIZE	(16*1024*1024)
 
 /*
+ * SLRU segment size.  A page is the same BLCKSZ as is used everywhere else in
+ * Postgres.  The segment size can be chosen somewhat arbitrarily; we make it
+ * 32 pages by default, or 256Kb, i.e. 1M transactions for CLOG or 64K
+ * transactions for SUBTRANS.
+ *
+ * Changing this requires an initdb.
+ */
+#define SLRU_PAGES_PER_SEGMENT	32
+
+/*
  * Maximum length for identifiers (e.g. table names, column names,
  * function names).  Names actually are limited to one fewer byte than this,
  * because the length must include a trailing zero byte.