Make WAL segment size configurable at initdb time.

For performance reasons a larger segment size than the default 16MB
can be useful. A larger segment size has two main benefits: Firstly,
in setups using archiving, it makes it easier to write scripts that
can keep up with higher amounts of WAL, secondly, the WAL has to be
written and synced to disk less frequently.

But at the same time large segment size are disadvantageous for
smaller databases. So far the segment size had to be configured at
compile time, often making it unrealistic to choose one fitting to a
particularly load. Therefore change it to a initdb time setting.

This includes a breaking changes to the xlogreader.h API, which now
requires the current segment size to be configured.  For that and
similar reasons a number of binaries had to be taught how to recognize
the current segment size.

Author: Beena Emerson, editorialized by Andres Freund
Reviewed-By: Andres Freund, David Steele, Kuntal Ghosh, Michael
    Paquier, Peter Eisentraut, Robert Hass, Tushar Ahuja
Discussion: https://postgr.es/m/CAOG9ApEAcQ--1ieKbhFzXSQPw_YLmepaa4hNdnY5+ZULpt81Mw@mail.gmail.com

1 files changed, 57 insertions, 1 deletions

diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index 9d1e5d789f6..1d4a138618b 100644
--- a/src/bin/initdb/initdb.c
+++ b/src/bin/initdb/initdb.c
@@ -59,6 +59,7 @@
 #include "sys/mman.h"
 #endif
 
+#include "access/xlog_internal.h"
 #include "catalog/catalog.h"
 #include "catalog/pg_authid.h"
 #include "catalog/pg_class.h"
@@ -141,6 +142,8 @@ static bool sync_only = false;
 static bool show_setting = false;
 static bool data_checksums = false;
 static char *xlog_dir = NULL;
+static char *str_wal_segment_size_mb = NULL;
+static int	wal_segment_size_mb;
 
 
 /* internal vars */
@@ -1000,6 +1003,23 @@ test_config_settings(void)
 }
 
 /*
+ * Calculate the default wal_size with a "pretty" unit.
+ */
+static char *
+pretty_wal_size(int segment_count)
+{
+	int			sz = wal_segment_size_mb * segment_count;
+	char	   *result = pg_malloc(10);
+
+	if ((sz % 1024) == 0)
+		snprintf(result, 10, "%dGB", sz / 1024);
+	else
+		snprintf(result, 10, "%dMB", sz);
+
+	return result;
+}
+
+/*
  * set up all the config files
  */
 static void
@@ -1043,6 +1063,15 @@ setup_config(void)
 	conflines = replace_token(conflines, "#port = 5432", repltok);
 #endif
 
+	/* set default max_wal_size and min_wal_size */
+	snprintf(repltok, sizeof(repltok), "min_wal_size = %s",
+			 pretty_wal_size(DEFAULT_MIN_WAL_SEGS));
+	conflines = replace_token(conflines, "#min_wal_size = 80MB", repltok);
+
+	snprintf(repltok, sizeof(repltok), "max_wal_size = %s",
+			 pretty_wal_size(DEFAULT_MAX_WAL_SEGS));
+	conflines = replace_token(conflines, "#max_wal_size = 1GB", repltok);
+
 	snprintf(repltok, sizeof(repltok), "lc_messages = '%s'",
 			 escape_quotes(lc_messages));
 	conflines = replace_token(conflines, "#lc_messages = 'C'", repltok);
@@ -1352,8 +1381,9 @@ bootstrap_template1(void)
 	unsetenv("PGCLIENTENCODING");
 
 	snprintf(cmd, sizeof(cmd),
-			 "\"%s\" --boot -x1 %s %s %s",
+			 "\"%s\" --boot -x1 -X %u %s %s %s",
 			 backend_exec,
+			 wal_segment_size_mb * (1024 * 1024),
 			 data_checksums ? "-k" : "",
 			 boot_options,
 			 debug ? "-d 5" : "");
@@ -2293,6 +2323,7 @@ usage(const char *progname)
 	printf(_("  -U, --username=NAME       database superuser name\n"));
 	printf(_("  -W, --pwprompt            prompt for a password for the new superuser\n"));
 	printf(_("  -X, --waldir=WALDIR       location for the write-ahead log directory\n"));
+	printf(_("      --wal-segsize=SIZE    size of wal segment size\n"));
 	printf(_("\nLess commonly used options:\n"));
 	printf(_("  -d, --debug               generate lots of debugging output\n"));
 	printf(_("  -k, --data-checksums      use data page checksums\n"));
@@ -2983,6 +3014,7 @@ main(int argc, char *argv[])
 		{"no-sync", no_argument, NULL, 'N'},
 		{"sync-only", no_argument, NULL, 'S'},
 		{"waldir", required_argument, NULL, 'X'},
+		{"wal-segsize", required_argument, NULL, 12},
 		{"data-checksums", no_argument, NULL, 'k'},
 		{NULL, 0, NULL, 0}
 	};
@@ -3116,6 +3148,9 @@ main(int argc, char *argv[])
 			case 'X':
 				xlog_dir = pg_strdup(optarg);
 				break;
+			case 12:
+				str_wal_segment_size_mb = pg_strdup(optarg);
+				break;
 			default:
 				/* getopt_long already emitted a complaint */
 				fprintf(stderr, _("Try \"%s --help\" for more information.\n"),
@@ -3178,6 +3213,27 @@ main(int argc, char *argv[])
 
 	check_need_password(authmethodlocal, authmethodhost);
 
+	/* set wal segment size */
+	if (str_wal_segment_size_mb == NULL)
+		wal_segment_size_mb = (DEFAULT_XLOG_SEG_SIZE) / (1024 * 1024);
+	else
+	{
+		char	   *endptr;
+
+		/* check that the argument is a number */
+		wal_segment_size_mb = strtol(str_wal_segment_size_mb, &endptr, 10);
+
+		/* verify that wal segment size is valid */
+		if (*endptr != '\0' ||
+			!IsValidWalSegSize(wal_segment_size_mb * 1024 * 1024))
+		{
+			fprintf(stderr,
+					_("%s: --wal-segsize must be a power of two between 1 and 1024\n"),
+					progname);
+			exit(1);
+		}
+	}
+
 	get_restricted_token(progname);
 
 	setup_pgdata();