MB-6753 Provide the timestamp of TAP backfill start

This stat is used to debug when the backfill is scheduled.

Change-Id: I50713f00419c6d20ff023c2b4ce0e0eb1fe96b70
Reviewed-on: http://review.couchbase.org/21148
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>

docs/stats.org

@@ -341,6 +341,7 @@ another colon.  For example, if your client is named, =slave1=, the
 |                           | from disk for this connection            | P  |
 | backfill_completed        | true if all items from backfill is       | P  |
 |                           | successfully transmitted to the client   | P  |
+| backfill_start_timestamp  | Timestamp of backfill start              | P  |
 | reconnects                | Number of reconnects from this client.   | P  |
 | backfill_age              | The age of the start of the backfill.    | P  |
 | ack_seqno                 | The current tap ACK sequence number.     | P  |

src/tapconnection.cc

@@ -245,7 +245,8 @@ TapProducer::TapProducer(EventuallyPersistentEngine &theEngine,
     isSeqNumRotated(false),
     numNoops(0),
     tapFlagByteorderSupport(false),
-    specificData(NULL)
+    specificData(NULL),
+    backfillTimestamp(0)
 {
     evaluateFlags();
     queue = new std::list<queued_item>;
@@ -1255,6 +1256,7 @@ void TapProducer::addStats(ADD_STAT add_stat, const void *c) {
     addStat("pending_backfill", isPendingBackfill_UNLOCKED(), add_stat, c);
     addStat("pending_disk_backfill", diskBackfillCounter > 0, add_stat, c);
     addStat("backfill_completed", isBackfillCompleted_UNLOCKED(), add_stat, c);
+    addStat("backfill_start_timestamp", backfillTimestamp, add_stat, c);
 
     addStat("queue_memory", getQueueMemory(), add_stat, c);
     addStat("queue_fill", getQueueFillTotal(), add_stat, c);
@@ -1757,6 +1759,7 @@ void TapProducer::scheduleBackfill_UNLOCKED(const std::vector<uint16_t> &vblist)
     if (new_vblist.size() > 0) {
         doRunBackfill = true;
         backfillCompleted = false;
+        backfillTimestamp = ep_real_time();
     }
 }
 

src/tapconnection.hh

@@ -1161,6 +1161,8 @@ private:
 
     //! EP-engine specific item info
     uint8_t *specificData;
+    //! Timestamp of backfill start
+    time_t backfillTimestamp;
 
     DISALLOW_COPY_AND_ASSIGN(TapProducer);
 };