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.

src/include/storage/buf.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/buf.h,v 1.21 2007/01/05 22:19:57 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/storage/buf.h,v 1.22 2007/05/30 20:12:03 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -36,4 +36,11 @@ typedef int Buffer;
  */
 #define BufferIsLocal(buffer)	((buffer) < 0)
 
+/*
+ * Buffer access strategy objects.
+ *
+ * BufferAccessStrategyData is private to freelist.c
+ */
+typedef struct BufferAccessStrategyData *BufferAccessStrategy;
+
 #endif   /* BUF_H */