From: Kyotaro Horiguchi <horiguchi.kyotaro@lab.ntt.co.jp>
Date: Tue, 28 Feb 2017 11:39:48 +0900
Subject: [PATCH 1/2] Add WAL releaf vent for replication slots

Adds a capability to limit the number of segments kept by replication
slots by a GUC variable.
---
 src/backend/access/transam/xlog.c             | 39 +++++++++++++++++++++++++++
 src/backend/utils/misc/guc.c                  | 11 ++++++++
 src/backend/utils/misc/postgresql.conf.sample |  1 +
 src/include/access/xlog.h                     |  1 +
 4 files changed, 52 insertions(+)

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index dd028a1..cfdae39 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -105,6 +105,7 @@ int			wal_level = WAL_LEVEL_MINIMAL;
 int			CommitDelay = 0;	/* precommit delay in microseconds */
 int			CommitSiblings = 5; /* # concurrent xacts needed to sleep */
 int			wal_retrieve_retry_interval = 5000;
+int			max_slot_wal_keep_size_mb = 0;
 
 #ifdef WAL_DEBUG
 bool		XLOG_DEBUG = false;
@@ -9432,9 +9433,47 @@ KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
 	if (max_replication_slots > 0 && keep != InvalidXLogRecPtr)
 	{
 		XLogSegNo	slotSegNo;
+		int			slotlimitsegs;
+		uint64		recptroff;
+		uint64		slotlimitbytes;
+		uint64		slotlimitfragment;
+
+		recptroff = XLogSegmentOffset(recptr, wal_segment_size);
+		slotlimitbytes = 1024 * 1024 * max_slot_wal_keep_size_mb;
+		slotlimitfragment =	XLogSegmentOffset(slotlimitbytes,
+											  wal_segment_size);
+
+		/* calculate segments to keep by max_slot_wal_keep_size_mb */
+		slotlimitsegs = ConvertToXSegs(max_slot_wal_keep_size_mb,
+									   wal_segment_size);
+		/* honor the fragment */
+		if (recptroff < slotlimitfragment)
+			slotlimitsegs++;
 
 		XLByteToSeg(keep, slotSegNo, wal_segment_size);
 
+		/*
+		 * ignore slots if too many wal segments are kept.
+		 * max_slot_wal_keep_size is just accumulated on wal_keep_segments.
+		 */
+		if (max_slot_wal_keep_size_mb > 0 && slotSegNo + slotlimitsegs < segno)
+		{
+			segno = segno - slotlimitsegs; /* must be positive */
+
+			/*
+			 * warn only if the checkpoint flushes the required segment.
+			 * we assume here that *logSegNo is calculated keep location.
+			 */
+			if (slotSegNo < *logSegNo)
+				ereport(WARNING,
+					(errmsg ("restart LSN of replication slots is ignored by checkpoint"),
+					 errdetail("Some replication slots have lost required WAL segnents to continue by up to %ld segments.",
+					   (segno < *logSegNo ? segno : *logSegNo) - slotSegNo)));
+
+			/* emergency vent */
+			slotSegNo = segno;
+		}
+
 		if (slotSegNo <= 0)
 			segno = 1;
 		else if (slotSegNo < segno)
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 65372d7..511023a 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -2368,6 +2368,17 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
+		{"max_slot_wal_keep_size", PGC_SIGHUP, REPLICATION_SENDING,
+			gettext_noop("Sets the maximum size of extra WALs kept by replication slots."),
+		 NULL,
+		 GUC_UNIT_MB
+		},
+		&max_slot_wal_keep_size_mb,
+		0, 0, INT_MAX,
+		NULL, NULL, NULL
+	},
+
+	{
 		{"wal_sender_timeout", PGC_SIGHUP, REPLICATION_SENDING,
 			gettext_noop("Sets the maximum time to wait for WAL replication."),
 			NULL,
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 368b280..e76c73a 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -234,6 +234,7 @@
 #max_wal_senders = 10		# max number of walsender processes
 				# (change requires restart)
 #wal_keep_segments = 0		# in logfile segments; 0 disables
+#max_slot_wal_keep_size = 0	# measured in bytes; 0 disables
 #wal_sender_timeout = 60s	# in milliseconds; 0 disables
 
 #max_replication_slots = 10	# max number of replication slots
diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index 0f2b8bd..f0c0255 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -98,6 +98,7 @@ extern int	wal_segment_size;
 extern int	min_wal_size_mb;
 extern int	max_wal_size_mb;
 extern int	wal_keep_segments;
+extern int	max_slot_wal_keep_size_mb;
 extern int	XLOGbuffers;
 extern int	XLogArchiveTimeout;
 extern int	wal_retrieve_retry_interval;
-- 
2.9.2


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