Reduce checkpoints and WAL traffic on low activity database server

Previously, we skipped a checkpoint if no WAL had been written since
last checkpoint, though this does not appear in user documentation.
As of now, we skip a checkpoint until we have written at least one
enough WAL to switch the next WAL file. This greatly reduces the
level of activity and number of WAL messages generated by a very
low activity server. This is safe because the purpose of a checkpoint
is to act as a starting place for a recovery, in case of crash.
This patch maintains minimal WAL volume for replay in case of crash,
thus maintaining very low crash recovery time.

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 7ac17779c88761b67bf0bf507ac8a211889cf3cf..c4bedb45da0f85b48494aae5ea98b70f65cd0a09 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7560,6 +7560,10 @@ 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;
 
@@ -7636,8 +7640,8 @@ CreateCheckPoint(int flags)
    LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
 
    /*
-    * 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
+    * 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
     * 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
@@ -7645,10 +7649,11 @@ CreateCheckPoint(int flags)
     * (Perhaps it'd make even more sense to checkpoint only when the previous
     * checkpoint record is in a different xlog page?)
     *
-    * 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.
+    * 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.
     */
    if ((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
                  CHECKPOINT_FORCE)) == 0)
@@ -7656,13 +7661,10 @@ CreateCheckPoint(int flags)
        XLogRecPtr  curInsert;
 
        INSERT_RECPTR(curInsert, Insert, Insert->curridx);
-       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)
+       XLByteToSeg(curInsert, insert_logId, insert_logSeg);
+       XLByteToSeg(ControlFile->checkPointCopy.redo, redo_logId, redo_logSeg);
+       if (insert_logId == redo_logId &&
+           insert_logSeg == redo_logSeg)
        {
            LWLockRelease(WALInsertLock);
            LWLockRelease(CheckpointLock);