Bring some clarity to the defaults for the xxx_flush_after parameters.

Instead of confusingly stating platform-dependent defaults for these
parameters in the comments in postgresql.conf.sample (with the main
entry being a lie on Linux), teach initdb to install the correct
platform-dependent value in postgresql.conf, similarly to the way
we handle other platform-dependent defaults.  This won't do anything
for existing 9.6 installations, but since it's effectively only a
documentation improvement, that seems OK.

Since this requires initdb to have access to the default values,
move the #define's for those to pg_config_manual.h; the original
placement in bufmgr.h is unworkable because that file can't be
included by frontend programs.

Adjust the default value for wal_writer_flush_after so that it is 1MB
regardless of XLOG_BLCKSZ, conforming to what is stated in both the
SGML docs and postgresql.conf.  (We could alternatively make it scale
with XLOG_BLCKSZ, but I'm not sure I see the point.)

Copy-edit related SGML documentation.

Fabien Coelho and Tom Lane, per a gripe from Tomas Vondra.

Discussion: <[email protected]>

2 files changed, 18 insertions, 11 deletions

diff --git a/src/include/pg_config_manual.h b/src/include/pg_config_manual.h
index a2b2b614bec..96885bb9901 100644
--- a/src/include/pg_config_manual.h
+++ b/src/include/pg_config_manual.h
@@ -148,6 +148,24 @@
 #endif
 
 /*
+ * Default and maximum values for backend_flush_after, bgwriter_flush_after
+ * and checkpoint_flush_after; measured in blocks.  Currently, these are
+ * enabled by default if sync_file_range() exists, ie, only on Linux.  Perhaps
+ * we could also enable by default if we have mmap and msync(MS_ASYNC)?
+ */
+#ifdef HAVE_SYNC_FILE_RANGE
+#define DEFAULT_BACKEND_FLUSH_AFTER 0	/* never enabled by default */
+#define DEFAULT_BGWRITER_FLUSH_AFTER 64
+#define DEFAULT_CHECKPOINT_FLUSH_AFTER 32
+#else
+#define DEFAULT_BACKEND_FLUSH_AFTER 0
+#define DEFAULT_BGWRITER_FLUSH_AFTER 0
+#define DEFAULT_CHECKPOINT_FLUSH_AFTER 0
+#endif
+/* upper limit for all three variables */
+#define WRITEBACK_MAX_PENDING_FLUSHES 256
+
+/*
  * USE_SSL code should be compiled only when compiling with an SSL
  * implementation.  (Currently, only OpenSSL is supported, but we might add
  * more implementations in the future.)
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 821bee5ecea..c543ad6fdec 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -54,17 +54,6 @@ struct WritebackContext;
 extern PGDLLIMPORT int NBuffers;
 
 /* in bufmgr.c */
-#define WRITEBACK_MAX_PENDING_FLUSHES 256
-
-/* FIXME: Also default to on for mmap && msync(MS_ASYNC)? */
-#ifdef HAVE_SYNC_FILE_RANGE
-#define DEFAULT_CHECKPOINT_FLUSH_AFTER 32
-#define DEFAULT_BGWRITER_FLUSH_AFTER 64
-#else
-#define DEFAULT_CHECKPOINT_FLUSH_AFTER 0
-#define DEFAULT_BGWRITER_FLUSH_AFTER 0
-#endif   /* HAVE_SYNC_FILE_RANGE */
-
 extern bool zero_damaged_pages;
 extern int	bgwriter_lru_maxpages;
 extern double bgwriter_lru_multiplier;