Re: New XLOG record indicating WAL-skipping

Fujii Masao wrote:
> On Wed, Dec 9, 2009 at 6:25 PM, Fujii Masao <masao.fujii@gmail.com> wrote:
>> Here is the patch:
>>
>> - Write an XLOG UNLOGGED record in WAL if WAL-logging is skipped for only
>>  the reason that WAL archiving is not enabled and such record has not been
>>  written yet.
>>
>> - Cause archive recovery to end if an XLOG UNLOGGED record is found during
>>  it.
>
> Here's an updated version of my "New XLOG record indicating WAL-skipping" patch.
> http://archives.postgresql.org/pgsql-hackers/2009-12/msg00788.php

Thanks!

I don't like special-casing UNLOGGED records in XLogInsert and
ReadRecord(). Those functions are complicated enough already. The
special handling from XLogInsert() (and a few other places) is only
required because the UNLOGGED records carry no payload. That's easy to
avoid, just add some payload to them, doesn't matter what it is. And I
don't think ReadRecord() is the right place to emit the errors/warnings,
that belongs naturally in xlog_redo().

It might be useful to add some information in the records telling why
WAL-logging was skipped. It might turn out to be useful in debugging.
That also conveniently adds payload to the records, to avoid the
special-casing in XLogInsert() :-).

I think it's a premature optimization to skip writing the records if
we've written in the same session already. Especially with the 'reason'
information added to the records, it's nice to have a record of each
such operation. All operations that skip WAL-logging are heavy enough
that an additional WAL record will make no difference. I can see that it
was required to avoid the flooding from heap_insert(), but we can move
the XLogSkipLogging() call from heap_insert() to heap_sync().

Attached is an updated patch, doing the above. Am I missing anything?

--
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com
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Index: src/backend/access/heap/heapam.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/access/heap/heapam.c,v
retrieving revision 1.282
diff -u -r1.282 heapam.c
--- src/backend/access/heap/heapam.c    14 Jan 2010 11:08:00 -0000    1.282
+++ src/backend/access/heap/heapam.c    15 Jan 2010 11:27:44 -0000
@@ -5074,10 +5074,16 @@
 void
 heap_sync(Relation rel)
 {
+    char reason[NAMEDATALEN + 30];
+
     /* temp tables never need fsync */
     if (rel->rd_istemp)
         return;

+    snprintf(reason, sizeof(reason), "heap inserts on \"%s\"",
+             RelationGetRelationName(rel));
+    XLogSkipLogging(reason);
+
     /* main heap */
     FlushRelationBuffers(rel);
     /* FlushRelationBuffers will have opened rd_smgr */
Index: src/backend/access/nbtree/nbtsort.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/access/nbtree/nbtsort.c,v
retrieving revision 1.122
diff -u -r1.122 nbtsort.c
--- src/backend/access/nbtree/nbtsort.c    15 Jan 2010 09:19:00 -0000    1.122
+++ src/backend/access/nbtree/nbtsort.c    15 Jan 2010 11:27:44 -0000
@@ -215,6 +215,18 @@
      */
     wstate.btws_use_wal = XLogIsNeeded() && !wstate.index->rd_istemp;

+    /*
+     * Write an XLOG UNLOGGED record if WAL-logging was skipped because
+     * WAL archiving is not enabled.
+     */
+    if (!wstate.btws_use_wal && !wstate.index->rd_istemp)
+    {
+        char reason[NAMEDATALEN + 20];
+        snprintf(reason, sizeof(reason), "b-tree build on \"%s\"",
+                 RelationGetRelationName(wstate.index));
+        XLogSkipLogging(reason);
+    }
+
     /* reserve the metapage */
     wstate.btws_pages_alloced = BTREE_METAPAGE + 1;
     wstate.btws_pages_written = 0;
Index: src/backend/access/transam/xlog.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v
retrieving revision 1.358
diff -u -r1.358 xlog.c
--- src/backend/access/transam/xlog.c    15 Jan 2010 09:19:00 -0000    1.358
+++ src/backend/access/transam/xlog.c    15 Jan 2010 11:27:45 -0000
@@ -7562,6 +7562,31 @@
 }

 /*
+ * Write an XLOG UNLOGGED record, indicating that some operation was
+ * performed on data that we fsync()'d directly to disk, skipping
+ * WAL-logging.
+ *
+ * Such operations screw up archive recovery, so we complain if we see
+ * these records during archive recovery. That shouldn't happen in a
+ * correctly configured server, but you can induce it by temporarily
+ * disabling archiving and restarting, so it's good to at least get a
+ * warning of silent data loss in such cases. These records serve no
+ * other purpose and are simply ignored during crash recovery.
+ */
+void
+XLogSkipLogging(char *reason)
+{
+    XLogRecData rdata;
+
+    rdata.buffer = InvalidBuffer;
+    rdata.data = reason;
+    rdata.len = strlen(reason) + 1;
+    rdata.next = NULL;
+
+    XLogInsert(RM_XLOG_ID, XLOG_UNLOGGED, &rdata);
+}
+
+/*
  * XLOG resource manager's routines
  *
  * Definitions of info values are in include/catalog/pg_control.h, though
@@ -7703,6 +7728,19 @@
             LWLockRelease(ControlFileLock);
         }
     }
+    else if (info == XLOG_UNLOGGED)
+    {
+        if (InArchiveRecovery)
+        {
+            /*
+             * Note: We don't print the reason string from the record,
+             * because that gets added as a line using xlog_desc()
+             */
+            ereport(WARNING,
+                    (errmsg("unlogged operation performed, data may be missing"),
+                     errhint("This can happen if you temporarily disable archive_mode without taking a new base
backup.")));
+        }
+    }
 }

 void
@@ -7752,6 +7790,12 @@
         appendStringInfo(buf, "backup end: %X/%X",
                          startpoint.xlogid, startpoint.xrecoff);
     }
+    else if (info == XLOG_UNLOGGED)
+    {
+        char *reason = rec;
+
+        appendStringInfo(buf, "unlogged operation: %s", reason);
+    }
     else
         appendStringInfo(buf, "UNKNOWN");
 }
Index: src/backend/commands/cluster.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/commands/cluster.c,v
retrieving revision 1.193
diff -u -r1.193 cluster.c
--- src/backend/commands/cluster.c    15 Jan 2010 09:19:01 -0000    1.193
+++ src/backend/commands/cluster.c    15 Jan 2010 11:27:45 -0000
@@ -821,6 +821,18 @@
      */
     use_wal = XLogIsNeeded() && !NewHeap->rd_istemp;

+    /*
+     * Write an XLOG UNLOGGED record if WAL-logging was skipped because
+     * WAL archiving is not enabled.
+     */
+    if (!use_wal && !NewHeap->rd_istemp)
+    {
+        char reason[NAMEDATALEN + 20];
+        snprintf(reason, sizeof(reason), "CLUSTER on \"%s\"",
+                 RelationGetRelationName(NewHeap));
+        XLogSkipLogging(reason);
+    }
+
     /* use_wal off requires rd_targblock be initially invalid */
     Assert(NewHeap->rd_targblock == InvalidBlockNumber);

Index: src/backend/commands/tablecmds.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/commands/tablecmds.c,v
retrieving revision 1.315
diff -u -r1.315 tablecmds.c
--- src/backend/commands/tablecmds.c    15 Jan 2010 09:19:01 -0000    1.315
+++ src/backend/commands/tablecmds.c    15 Jan 2010 11:27:45 -0000
@@ -7018,6 +7018,19 @@

     heap_close(pg_class, RowExclusiveLock);

+    /*
+     * Write an XLOG UNLOGGED record if WAL-logging was skipped because
+     * WAL archiving is not enabled.
+     */
+    if (!XLogIsNeeded() && !rel->rd_istemp)
+    {
+        char reason[NAMEDATALEN + 40];
+        snprintf(reason, sizeof(reason), "ALTER TABLE SET TABLESPACE on \"%s\"",
+                 RelationGetRelationName(rel));
+
+        XLogSkipLogging(reason);
+    }
+
     relation_close(rel, NoLock);

     /* Make sure the reltablespace change is visible */
@@ -7046,6 +7059,10 @@
     /*
      * We need to log the copied data in WAL iff WAL archiving/streaming is
      * enabled AND it's not a temp rel.
+     *
+     * Note: If you change the conditions here, update the conditions in
+     * ATExecSetTableSpace() for when an XLOG UNLOGGED record is written
+     * to match.
      */
     use_wal = XLogIsNeeded() && !istemp;

Index: src/include/access/xlog.h
===================================================================
RCS file: /cvsroot/pgsql/src/include/access/xlog.h,v
retrieving revision 1.96
diff -u -r1.96 xlog.h
--- src/include/access/xlog.h    15 Jan 2010 09:19:06 -0000    1.96
+++ src/include/access/xlog.h    15 Jan 2010 11:27:45 -0000
@@ -278,6 +278,7 @@
 extern void CreateCheckPoint(int flags);
 extern bool CreateRestartPoint(int flags);
 extern void XLogPutNextOid(Oid nextOid);
+extern void XLogSkipLogging(char *reason);
 extern XLogRecPtr GetRedoRecPtr(void);
 extern XLogRecPtr GetInsertRecPtr(void);
 extern XLogRecPtr GetWriteRecPtr(void);
Index: src/include/catalog/pg_control.h
===================================================================
RCS file: /cvsroot/pgsql/src/include/catalog/pg_control.h,v
retrieving revision 1.48
diff -u -r1.48 pg_control.h
--- src/include/catalog/pg_control.h    4 Jan 2010 12:50:50 -0000    1.48
+++ src/include/catalog/pg_control.h    15 Jan 2010 11:27:45 -0000
@@ -63,6 +63,7 @@
 #define XLOG_NEXTOID                    0x30
 #define XLOG_SWITCH                        0x40
 #define XLOG_BACKUP_END                    0x50
+#define XLOG_UNLOGGED                    0x60


 /* System status indicator */