Rename wal_keep_segments to wal_keep_size.

max_slot_wal_keep_size that was added in v13 and wal_keep_segments are
the GUC parameters to specify how much WAL files to retain for
the standby servers. While max_slot_wal_keep_size accepts the number of
bytes of WAL files, wal_keep_segments accepts the number of WAL files.
This difference of setting units between those similar parameters could
be confusing to users.

To alleviate this situation, this commit renames wal_keep_segments to
wal_keep_size, and make users specify the WAL size in it instead of
the number of WAL files.

There was also the idea to rename max_slot_wal_keep_size to
max_slot_wal_keep_segments, in the discussion. But we have been moving
away from measuring in segments, for example, checkpoint_segments was
replaced by max_wal_size. So we concluded to rename wal_keep_segments
to wal_keep_size.

Back-patch to v13 where max_slot_wal_keep_size was added.

Author: Fujii Masao
Reviewed-by: Álvaro Herrera, Kyotaro Horiguchi, David Steele
Discussion: https://postgr.es/m/574b4ea3-e0f9-b175-ead2-ebea7faea855@oss.nttdata.com

4 files changed, 32 insertions, 24 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index a518a7182d5..37dfcfbfa65 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -88,7 +88,7 @@ extern uint32 bootstrap_data_checksum_version;
 /* User-settable parameters */
 int			max_wal_size_mb = 1024; /* 1 GB */
 int			min_wal_size_mb = 80;	/* 80 MB */
-int			wal_keep_segments = 0;
+int			wal_keep_size_mb = 0;
 int			XLOGbuffers = -1;
 int			XLogArchiveTimeout = 0;
 int			XLogArchiveMode = ARCHIVE_MODE_OFF;
@@ -9523,7 +9523,7 @@ GetWALAvailability(XLogRecPtr targetLSN)
 
 	/*
 	 * Calculate the oldest segment currently reserved by all slots,
-	 * considering wal_keep_segments and max_slot_wal_keep_size.  Initialize
+	 * considering wal_keep_size and max_slot_wal_keep_size.  Initialize
 	 * oldestSlotSeg to the current segment.
 	 */
 	currpos = GetXLogWriteRecPtr();
@@ -9574,9 +9574,9 @@ GetWALAvailability(XLogRecPtr targetLSN)
 
 /*
  * Retreat *logSegNo to the last segment that we need to retain because of
- * either wal_keep_segments or replication slots.
+ * either wal_keep_size or replication slots.
  *
- * This is calculated by subtracting wal_keep_segments from the given xlog
+ * This is calculated by subtracting wal_keep_size from the given xlog
  * location, recptr and by making sure that that result is below the
  * requirement of replication slots.  For the latter criterion we do consider
  * the effects of max_slot_wal_keep_size: reserve at most that much space back
@@ -9614,14 +9614,20 @@ KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
 		}
 	}
 
-	/* but, keep at least wal_keep_segments if that's set */
-	if (wal_keep_segments > 0 && currSegNo - segno < wal_keep_segments)
+	/* but, keep at least wal_keep_size if that's set */
+	if (wal_keep_size_mb > 0)
 	{
-		/* avoid underflow, don't go below 1 */
-		if (currSegNo <= wal_keep_segments)
-			segno = 1;
-		else
-			segno = currSegNo - wal_keep_segments;
+		uint64		keep_segs;
+
+		keep_segs = ConvertToXSegs(wal_keep_size_mb, wal_segment_size);
+		if (currSegNo - segno < keep_segs)
+		{
+			/* avoid underflow, don't go below 1 */
+			if (currSegNo <= keep_segs)
+				segno = 1;
+			else
+				segno = currSegNo - keep_segs;
+		}
 	}
 
 	/* don't delete WAL segments newer than the calculated segment */
@@ -11336,7 +11342,7 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 	 * If archiving is enabled, wait for all the required WAL files to be
 	 * archived before returning. If archiving isn't enabled, the required WAL
 	 * needs to be transported via streaming replication (hopefully with
-	 * wal_keep_segments set high enough), or some more exotic mechanism like
+	 * wal_keep_size set high enough), or some more exotic mechanism like
 	 * polling and copying files from pg_wal with script. We have no knowledge
 	 * of those mechanisms, so it's up to the user to ensure that he gets all
 	 * the required WAL.
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index 9fe147bf44e..f88694672fb 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -413,19 +413,20 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 		else
 		{
 			XLogSegNo   targetSeg;
-			XLogSegNo   keepSegs;
+			uint64   slotKeepSegs;
+			uint64   keepSegs;
 			XLogSegNo   failSeg;
 			XLogRecPtr  failLSN;
 
 			XLByteToSeg(slot_contents.data.restart_lsn, targetSeg, wal_segment_size);
 
-			/* determine how many segments slots can be kept by slots ... */
-			keepSegs = XLogMBVarToSegs(max_slot_wal_keep_size_mb, wal_segment_size);
-			/* ... and override by wal_keep_segments as needed */
-			keepSegs = Max(keepSegs, wal_keep_segments);
+			/* determine how many segments slots can be kept by slots */
+			slotKeepSegs = XLogMBVarToSegs(max_slot_wal_keep_size_mb, wal_segment_size);
+			/* ditto for wal_keep_size */
+			keepSegs = XLogMBVarToSegs(wal_keep_size_mb, wal_segment_size);
 
 			/* if currpos reaches failLSN, we lose our segment */
-			failSeg = targetSeg + keepSegs + 1;
+			failSeg = targetSeg + Max(slotKeepSegs, keepSegs) + 1;
 			XLogSegNoOffsetToRecPtr(failSeg, 0, wal_segment_size, failLSN);
 
 			values[i++] = Int64GetDatum(failLSN - currlsn);
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 9f5e223920c..40dfe6dcd69 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -2640,12 +2640,13 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		{"wal_keep_segments", PGC_SIGHUP, REPLICATION_SENDING,
-			gettext_noop("Sets the number of WAL files held for standby servers."),
-			NULL
+		{"wal_keep_size", PGC_SIGHUP, REPLICATION_SENDING,
+			gettext_noop("Sets the size of WAL files held for standby servers."),
+			NULL,
+			GUC_UNIT_MB
 		},
-		&wal_keep_segments,
-		0, 0, INT_MAX,
+		&wal_keep_size_mb,
+		0, 0, MAX_KILOBYTES,
 		NULL, NULL, NULL
 	},
 
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index bd4cb8144be..ffb374152c0 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -288,7 +288,7 @@
 
 #max_wal_senders = 10		# max number of walsender processes
 				# (change requires restart)
-#wal_keep_segments = 0		# in logfile segments; 0 disables
+#wal_keep_size = 0		# in megabytes; 0 disables
 #max_slot_wal_keep_size = -1	# in megabytes; -1 disables
 #wal_sender_timeout = 60s	# in milliseconds; 0 disables