Skip checkpoints, archiving on idle systems.

Some background activity (like checkpoints, archive timeout, standby
snapshots) is not supposed to happen on an idle system. Unfortunately
so far it was not easy to determine when a system is idle, which
defeated some of the attempts to avoid redundant activity on an idle
system.

To make that easier, allow to make individual WAL insertions as not
being "important". By checking whether any important activity happened
since the last time an activity was performed, it now is easy to check
whether some action needs to be repeated.

Use the new facility for checkpoints, archive timeout and standby
snapshots.

The lack of a facility causes some issues in older releases, but in my
opinion the consequences (superflous checkpoints / archived segments)
aren't grave enough to warrant backpatching.

Author: Michael Paquier, editorialized by Andres Freund
Reviewed-By: Andres Freund, David Steele, Amit Kapila, Kyotaro HORIGUCHI
Bug: #13685
Discussion:
    https://www.postgresql.org/message-id/20151016203031.3019.72930@wrigleys.postgresql.org
    https://www.postgresql.org/message-id/CAB7nPqQcPqxEM3S735Bd2RzApNqSNJVietAC=6kfkYv_45dKwA@mail.gmail.com
Backpatch: -

1 files changed, 16 insertions, 8 deletions

diff --git a/src/backend/access/transam/xloginsert.c b/src/backend/access/transam/xloginsert.c
index 3cd273b19f2..24e35a38453 100644
--- a/src/backend/access/transam/xloginsert.c
+++ b/src/backend/access/transam/xloginsert.c
@@ -73,8 +73,8 @@ static XLogRecData *mainrdata_head;
 static XLogRecData *mainrdata_last = (XLogRecData *) &mainrdata_head;
 static uint32 mainrdata_len;	/* total # of bytes in chain */
 
-/* Should the in-progress insertion log the origin? */
-static bool include_origin = false;
+/* flags for the in-progress insertion */
+static uint8 curinsert_flags = 0;
 
 /*
  * These are used to hold the record header while constructing a record.
@@ -201,7 +201,7 @@ XLogResetInsertion(void)
 	max_registered_block_id = 0;
 	mainrdata_len = 0;
 	mainrdata_last = (XLogRecData *) &mainrdata_head;
-	include_origin = false;
+	curinsert_flags = 0;
 	begininsert_called = false;
 }
 
@@ -384,13 +384,20 @@ XLogRegisterBufData(uint8 block_id, char *data, int len)
 }
 
 /*
- * Should this record include the replication origin if one is set up?
+ * Set insert status flags for the upcoming WAL record.
+ *
+ * The flags that can be used here are:
+ * - XLOG_INCLUDE_ORIGIN, to determine if the replication origin should be
+ *   included in the record.
+ * - XLOG_MARK_UNIMPORTANT, to signal that the record is not important for
+ *   durability, which allows to avoid triggering WAL archiving and other
+ *   background activity.
  */
 void
-XLogIncludeOrigin(void)
+XLogSetRecordFlags(uint8 flags)
 {
 	Assert(begininsert_called);
-	include_origin = true;
+	curinsert_flags = flags;
 }
 
 /*
@@ -450,7 +457,7 @@ XLogInsert(RmgrId rmid, uint8 info)
 		rdt = XLogRecordAssemble(rmid, info, RedoRecPtr, doPageWrites,
 								 &fpw_lsn);
 
-		EndPos = XLogInsertRecord(rdt, fpw_lsn);
+		EndPos = XLogInsertRecord(rdt, fpw_lsn, curinsert_flags);
 	} while (EndPos == InvalidXLogRecPtr);
 
 	XLogResetInsertion();
@@ -701,7 +708,8 @@ XLogRecordAssemble(RmgrId rmid, uint8 info,
 	}
 
 	/* followed by the record's origin, if any */
-	if (include_origin && replorigin_session_origin != InvalidRepOriginId)
+	if ((curinsert_flags & XLOG_INCLUDE_ORIGIN) &&
+		replorigin_session_origin != InvalidRepOriginId)
 	{
 		*(scratch++) = XLR_BLOCK_ID_ORIGIN;
 		memcpy(scratch, &replorigin_session_origin, sizeof(replorigin_session_origin));