Revert "Reduce checkpoints and WAL traffic on low activity database server"

This reverts commit 18fb9d8d21a28caddb72c7ffbdd7b96d52ff9724.  Per
discussion, it does not seem like a good idea to allow committed changes to
go un-checkpointed indefinitely, as could happen in a low-traffic server;
that makes us entirely reliant on the WAL stream with no redundancy that
might aid data recovery in case of disk failure.

This re-introduces the original problem of hot-standby setups generating a
small continuing stream of WAL traffic even when idle, but there are other
ways to address that without compromising crash recovery, so we'll revisit
that issue in a future release cycle.

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index bcb71c45b2a1b677d81c4634b14b6af96faffe97..0d68760e81260373f4fc077895e4c986a26e5bc7 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7646,10 +7646,6 @@ CreateCheckPoint(int flags)
    uint32      freespace;
    uint32      _logId;
    uint32      _logSeg;
-   uint32      redo_logId;
-   uint32      redo_logSeg;
-   uint32      insert_logId;
-   uint32      insert_logSeg;
    TransactionId *inCommitXids;
    int         nInCommit;
 
@@ -7726,8 +7722,8 @@ CreateCheckPoint(int flags)
    LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
 
    /*
-    * If this isn't a shutdown or forced checkpoint, and we have not switched
-    * to the next WAL file since the start of the last checkpoint, skip the
+    * If this isn't a shutdown or forced checkpoint, and we have not inserted
+    * any XLOG records since the start of the last checkpoint, skip the
     * checkpoint.  The idea here is to avoid inserting duplicate checkpoints
     * when the system is idle. That wastes log space, and more importantly it
     * exposes us to possible loss of both current and previous checkpoint
@@ -7735,11 +7731,10 @@ CreateCheckPoint(int flags)
     * (Perhaps it'd make even more sense to checkpoint only when the previous
     * checkpoint record is in a different xlog page?)
     *
-    * While holding the WALInsertLock we find the current WAL insertion point
-    * and compare that with the starting point of the last checkpoint, which
-    * is the redo pointer. We use the redo pointer because the start and end
-    * points of a checkpoint can be hundreds of files apart on large systems
-    * when checkpoint writes are spread out over time.
+    * We have to make two tests to determine that nothing has happened since
+    * the start of the last checkpoint: current insertion point must match
+    * the end of the last checkpoint record, and its redo pointer must point
+    * to itself.
     */
    if ((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
                  CHECKPOINT_FORCE)) == 0)
@@ -7747,10 +7742,13 @@ CreateCheckPoint(int flags)
        XLogRecPtr  curInsert;
 
        INSERT_RECPTR(curInsert, Insert, Insert->curridx);
-       XLByteToSeg(curInsert, insert_logId, insert_logSeg);
-       XLByteToSeg(ControlFile->checkPointCopy.redo, redo_logId, redo_logSeg);
-       if (insert_logId == redo_logId &&
-           insert_logSeg == redo_logSeg)
+       if (curInsert.xlogid == ControlFile->checkPoint.xlogid &&
+           curInsert.xrecoff == ControlFile->checkPoint.xrecoff +
+           MAXALIGN(SizeOfXLogRecord + sizeof(CheckPoint)) &&
+           ControlFile->checkPoint.xlogid ==
+           ControlFile->checkPointCopy.redo.xlogid &&
+           ControlFile->checkPoint.xrecoff ==
+           ControlFile->checkPointCopy.redo.xrecoff)
        {
            LWLockRelease(WALInsertLock);
            LWLockRelease(CheckpointLock);