Make large sequential scans and VACUUMs work in a limited-size "ring" of

buffers, rather than blowing out the whole shared-buffer arena.  Aside from
avoiding cache spoliation, this fixes the problem that VACUUM formerly tended
to cause a WAL flush for every page it modified, because we had it hacked to
use only a single buffer.  Those flushes will now occur only once per
ring-ful.  The exact ring size, and the threshold for seqscans to switch into
the ring usage pattern, remain under debate; but the infrastructure seems
done.  The key bit of infrastructure is a new optional BufferAccessStrategy
object that can be passed to ReadBuffer operations; this replaces the former
StrategyHintVacuum API.

This patch also changes the buffer usage-count methodology a bit: we now
advance usage_count when first pinning a buffer, rather than when last
unpinning it.  To preserve the behavior that a buffer's lifetime starts to
decrease when it's released, the clock sweep code is modified to not decrement
usage_count of pinned buffers.

Work not done in this commit: teach GiST and GIN indexes to use the vacuum
BufferAccessStrategy for vacuum-driven fetches.

Original patch by Simon, reworked by Heikki and again by Tom.

1 files changed, 13 insertions, 2 deletions

diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index ad203621793..9ae83b4253e 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/storage/bufmgr.h,v 1.103 2007/05/02 23:18:03 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/storage/bufmgr.h,v 1.104 2007/05/30 20:12:03 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -19,6 +19,14 @@
 
 typedef void *Block;
 
+/* Possible arguments for GetAccessStrategy() */
+typedef enum BufferAccessStrategyType
+{
+	BAS_NORMAL,		/* Normal random access */
+	BAS_BULKREAD,	/* Large read-only scan (hint bit updates are ok) */
+	BAS_VACUUM		/* VACUUM */
+} BufferAccessStrategyType;
+
 /* in globals.c ... this duplicates miscadmin.h */
 extern DLLIMPORT int NBuffers;
 
@@ -111,6 +119,8 @@ extern DLLIMPORT int32 *LocalRefCount;
  * prototypes for functions in bufmgr.c
  */
 extern Buffer ReadBuffer(Relation reln, BlockNumber blockNum);
+extern Buffer ReadBufferWithStrategy(Relation reln, BlockNumber blockNum,
+									 BufferAccessStrategy strategy);
 extern Buffer ReadOrZeroBuffer(Relation reln, BlockNumber blockNum);
 extern void ReleaseBuffer(Buffer buffer);
 extern void UnlockReleaseBuffer(Buffer buffer);
@@ -157,6 +167,7 @@ extern void BgBufferSync(void);
 extern void AtProcExit_LocalBuffers(void);
 
 /* in freelist.c */
-extern void StrategyHintVacuum(bool vacuum_active);
+extern BufferAccessStrategy GetAccessStrategy(BufferAccessStrategyType btype);
+extern void FreeAccessStrategy(BufferAccessStrategy strategy);
 
 #endif