Increase max_wal_size's default from 128MB to 1GB.

The introduction of min_wal_size & max_wal_size in 88e98230 makes it
feasible to increase the default upper bound in checkpoint
size. Previously raising the default would lead to a increased disk
footprint, even if more segments weren't beneficial.  The low default of
checkpoint size is one of common performance problem users have thus
increasing the default makes sense.  Setups where the increase in
maximum disk usage is a problem will very likely have to run with a
modified configuration anyway.

Discussion: 54F4EFB8.40202@agliodbs.com,
    CA+TgmoZEAgX5oMGJOHVj8L7XOkAe05Gnf45rP40m-K3FhZRVKg@mail.gmail.com

Author: Josh Berkus, after a discussion involving lots of people.

doc/src/sgml/config.sgml

@@ -2430,7 +2430,7 @@ include_dir 'conf.d'
         checkpoints. This is a soft limit; WAL size can exceed
         <varname>max_wal_size</> under special circumstances, like
         under heavy load, a failing <varname>archive_command</>, or a high
-        <varname>wal_keep_segments</> setting. The default is 128 MB.
+        <varname>wal_keep_segments</> setting. The default is 1 GB.
         Increasing this parameter can increase the amount of time needed for
         crash recovery.
         This parameter can only be set in the <filename>postgresql.conf</>

doc/src/sgml/wal.sgml

@@ -475,7 +475,7 @@
    linkend="guc-checkpoint-timeout"> seconds, or if
    <xref linkend="guc-max-wal-size"> is about to be exceeded,
    whichever comes first.
-   The default settings are 5 minutes and 128 MB, respectively.
+   The default settings are 5 minutes and 1 GB, respectively.
    If no WAL has been written since the previous checkpoint, new checkpoints
    will be skipped even if <varname>checkpoint_timeout</> has passed.
    (If WAL archiving is being used and you want to put a lower limit on how

src/backend/access/transam/xlog.c

@@ -79,7 +79,7 @@ extern uint32 bootstrap_data_checksum_version;
 
 
 /* User-settable parameters */
-int			max_wal_size = 8;		/* 128 MB */
+int			max_wal_size = 64;		/* 1 GB */
 int			min_wal_size = 5;		/* 80 MB */
 int			wal_keep_segments = 0;
 int			XLOGbuffers = -1;

src/backend/utils/misc/guc.c

@@ -2191,7 +2191,7 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_XSEGS
 		},
 		&max_wal_size,
-		8, 2, INT_MAX,
+		64, 2, INT_MAX,
 		NULL, assign_max_wal_size, NULL
 	},
 

src/backend/utils/misc/postgresql.conf.sample

@@ -199,7 +199,7 @@
 # - Checkpoints -
 
 #checkpoint_timeout = 5min		# range 30s-1h
-#max_wal_size = 128MB			# in logfile segments
+#max_wal_size = 1GB
 #min_wal_size = 80MB
 #checkpoint_completion_target = 0.5	# checkpoint target duration, 0.0 - 1.0
 #checkpoint_warning = 30s		# 0 disables