Avoid archiving XLOG_RUNNING_XACTS on idle server

If archive_timeout > 0 we should avoid logging XLOG_RUNNING_XACTS if idle.

Bug 13685 reported by Laurence Rowe, investigated in detail by Michael Paquier,
though this is not his proposed fix.
20151016203031.3019.72930@wrigleys.postgresql.org

Simple non-invasive patch to allow later backpatch to 9.4 and 9.5

diff --git a/src/backend/postmaster/bgwriter.c b/src/backend/postmaster/bgwriter.c
index 00f03d8acbe7ba54b4fc91aa28c80f2c508850fb..654e4f8bd5762802111a5e06233862cff9c95cfa 100644 (file)
--- a/src/backend/postmaster/bgwriter.c
+++ b/src/backend/postmaster/bgwriter.c
@@ -330,8 +330,11 @@ BackgroundWriterMain(void)
            if (now >= timeout &&
                last_snapshot_lsn != GetXLogInsertRecPtr())
            {
-               last_snapshot_lsn = LogStandbySnapshot();
+               XLogRecPtr log_standby_lsn = LogStandbySnapshot();
+
                last_snapshot_ts = now;
+               if (!XLogRecPtrIsInvalid(log_standby_lsn))
+                   last_snapshot_lsn = log_standby_lsn;
            }
        }
 

diff --git a/src/backend/storage/ipc/standby.c b/src/backend/storage/ipc/standby.c
index 6a9bf842d3923affc66e251dcd277afcab1ec7e9..8b9b438ca929c1cae5ced8985d91cd45e5a5d294 100644 (file)
--- a/src/backend/storage/ipc/standby.c
+++ b/src/backend/storage/ipc/standby.c
@@ -902,6 +902,7 @@ LogStandbySnapshot(void)
    RunningTransactions running;
    xl_standby_lock *locks;
    int         nlocks;
+   static bool last_snapshot_overflowed = false;
 
    Assert(XLogStandbyInfoActive());
 
@@ -932,8 +933,28 @@ LogStandbySnapshot(void)
     * only a shared lock.
     */
    if (wal_level < WAL_LEVEL_LOGICAL)
+   {
        LWLockRelease(ProcArrayLock);
 
+       /*
+        * Don't bother to log anything if nothing is happening, if we are
+        * using archive_timeout > 0 and we didn't overflow snapshot last time.
+        *
+        * This ensures that we don't issue an empty WAL record, which can
+        * be annoying when used in conjunction with archive timeout.
+        */
+       if (running->xcnt == 0 &&
+           nlocks == 0 &&
+           XLogArchiveTimeout > 0 &&
+           !last_snapshot_overflowed)
+       {
+           LWLockRelease(XidGenLock);
+           return InvalidXLogRecPtr;
+       }
+
+       last_snapshot_overflowed = running->subxid_overflow;
+   }
+
    recptr = LogCurrentRunningXacts(running);
 
    /* Release lock if we kept it longer ... */