This patch extracts page buffer pooling and the simple

least-recently-used strategy from clog.c into slru.c.  It doesn't
change any visible behaviour and passes all regression tests plus a
TruncateCLOG test done manually.

Apart from refactoring I made a little change to SlruRecentlyUsed,
formerly ClogRecentlyUsed:  It now skips incrementing lru_counts, if
slotno is already the LRU slot, thus saving a few CPU cycles.  To make
this work, lru_counts are initialised to 1 in SimpleLruInit.

SimpleLru will be used by pg_subtrans (part of the nested transactions
project), so the main purpose of this patch is to avoid future code
duplication.

Manfred Koizar

1 files changed, 36 insertions, 792 deletions

diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c
index 3eb25f604eb..3653d05bc1e 100644
--- a/src/backend/access/transam/clog.c
+++ b/src/backend/access/transam/clog.c
@@ -13,7 +13,7 @@
  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Header: /cvsroot/pgsql/src/backend/access/transam/clog.c,v 1.15 2003/05/03 03:52:07 momjian Exp $
+ * $Header: /cvsroot/pgsql/src/backend/access/transam/clog.c,v 1.16 2003/06/11 22:37:45 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -26,15 +26,14 @@
 #include <unistd.h>
 
 #include "access/clog.h"
+#include "access/slru.h"
 #include "storage/lwlock.h"
 #include "miscadmin.h"
 
 
 /*
  * Defines for CLOG page and segment sizes.  A page is the same BLCKSZ
- * as is used everywhere else in Postgres.	The CLOG segment size can be
- * chosen somewhat arbitrarily; we make it 1 million transactions by default,
- * or 256Kb.
+ * as is used everywhere else in Postgres.
  *
  * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
  * CLOG page numbering also wraps around at 0xFFFFFFFF/CLOG_XACTS_PER_PAGE,
@@ -43,17 +42,12 @@
  * segment and page numbers in TruncateCLOG (see CLOGPagePrecedes).
  */
 
-#define CLOG_BLCKSZ			BLCKSZ
-
 /* We need two bits per xact, so four xacts fit in a byte */
 #define CLOG_BITS_PER_XACT	2
 #define CLOG_XACTS_PER_BYTE 4
-#define CLOG_XACTS_PER_PAGE (CLOG_BLCKSZ * CLOG_XACTS_PER_BYTE)
+#define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)
 #define CLOG_XACT_BITMASK	((1 << CLOG_BITS_PER_XACT) - 1)
 
-#define CLOG_XACTS_PER_SEGMENT	0x100000
-#define CLOG_PAGES_PER_SEGMENT	(CLOG_XACTS_PER_SEGMENT / CLOG_XACTS_PER_PAGE)
-
 #define TransactionIdToPage(xid)	((xid) / (TransactionId) CLOG_XACTS_PER_PAGE)
 #define TransactionIdToPgIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE)
 #define TransactionIdToByte(xid)	(TransactionIdToPgIndex(xid) / CLOG_XACTS_PER_BYTE)
@@ -63,47 +57,6 @@
 /*----------
  * Shared-memory data structures for CLOG control
  *
- * We use a simple least-recently-used scheme to manage a pool of page
- * buffers for the CLOG.  Under ordinary circumstances we expect that write
- * traffic will occur mostly to the latest CLOG page (and to the just-prior
- * page, soon after a page transition).  Read traffic will probably touch
- * a larger span of pages, but in any case a fairly small number of page
- * buffers should be sufficient.  So, we just search the buffers using plain
- * linear search; there's no need for a hashtable or anything fancy.
- * The management algorithm is straight LRU except that we will never swap
- * out the latest page (since we know it's going to be hit again eventually).
- *
- * We use an overall LWLock to protect the shared data structures, plus
- * per-buffer LWLocks that synchronize I/O for each buffer.  A process
- * that is reading in or writing out a page buffer does not hold the control
- * lock, only the per-buffer lock for the buffer it is working on.
- *
- * To change the page number or state of a buffer, one must normally hold
- * the control lock.  (The sole exception to this rule is that a writer
- * process changes the state from DIRTY to WRITE_IN_PROGRESS while holding
- * only the per-buffer lock.)  If the buffer's state is neither EMPTY nor
- * CLEAN, then there may be processes doing (or waiting to do) I/O on the
- * buffer, so the page number may not be changed, and the only allowed state
- * transition is to change WRITE_IN_PROGRESS to DIRTY after dirtying the page.
- * To do any other state transition involving a buffer with potential I/O
- * processes, one must hold both the per-buffer lock and the control lock.
- * (Note the control lock must be acquired second; do not wait on a buffer
- * lock while holding the control lock.)  A process wishing to read a page
- * marks the buffer state as READ_IN_PROGRESS, then drops the control lock,
- * acquires the per-buffer lock, and rechecks the state before proceeding.
- * This recheck takes care of the possibility that someone else already did
- * the read, while the early marking prevents someone else from trying to
- * read the same page into a different buffer.
- *
- * Note we are assuming that read and write of the state value is atomic,
- * since I/O processes may examine and change the state while not holding
- * the control lock.
- *
- * As with the regular buffer manager, it is possible for another process
- * to re-dirty a page that is currently being written out.	This is handled
- * by setting the page's state from WRITE_IN_PROGRESS to DIRTY.  The writing
- * process must notice this and not mark the page CLEAN when it's done.
- *
  * XLOG interactions: this module generates an XLOG record whenever a new
  * CLOG page is initialized to zeroes.	Other writes of CLOG come from
  * recording of transaction commit or abort in xact.c, which generates its
@@ -117,90 +70,12 @@
  *----------
  */
 
-typedef enum
-{
-	CLOG_PAGE_EMPTY,			/* CLOG buffer is not in use */
-	CLOG_PAGE_READ_IN_PROGRESS, /* CLOG page is being read in */
-	CLOG_PAGE_CLEAN,			/* CLOG page is valid and not dirty */
-	CLOG_PAGE_DIRTY,			/* CLOG page is valid but needs write */
-	CLOG_PAGE_WRITE_IN_PROGRESS /* CLOG page is being written out */
-} ClogPageStatus;
-
-/*
- * Shared-memory state for CLOG.
- */
-typedef struct ClogCtlData
-{
-	/*
-	 * Info for each buffer slot.  Page number is undefined when status is
-	 * EMPTY.  lru_count is essentially the number of operations since
-	 * last use of this page; the page with highest lru_count is the best
-	 * candidate to replace.
-	 */
-	char	   *page_buffer[NUM_CLOG_BUFFERS];
-	ClogPageStatus page_status[NUM_CLOG_BUFFERS];
-	int			page_number[NUM_CLOG_BUFFERS];
-	unsigned int page_lru_count[NUM_CLOG_BUFFERS];
-
-	/*
-	 * latest_page_number is the page number of the current end of the
-	 * CLOG; this is not critical data, since we use it only to avoid
-	 * swapping out the latest page.
-	 */
-	int			latest_page_number;
-} ClogCtlData;
-
-static ClogCtlData *ClogCtl = NULL;
-
-/*
- * ClogBufferLocks is set during CLOGShmemInit and does not change thereafter.
- * The value is automatically inherited by backends via fork, and
- * doesn't need to be in shared memory.
- */
-static LWLockId *ClogBufferLocks;		/* Per-buffer I/O locks */
-
-/*
- * ClogDir is set during CLOGShmemInit and does not change thereafter.
- * The value is automatically inherited by backends via fork, and
- * doesn't need to be in shared memory.
- */
-static char ClogDir[MAXPGPATH];
-
-#define ClogFileName(path, seg) \
-	snprintf(path, MAXPGPATH, "%s/%04X", ClogDir, seg)
-
-/*
- * Macro to mark a buffer slot "most recently used".
- */
-#define ClogRecentlyUsed(slotno)	\
-	do { \
-		int		iilru; \
-		for (iilru = 0; iilru < NUM_CLOG_BUFFERS; iilru++) \
-			ClogCtl->page_lru_count[iilru]++; \
-		ClogCtl->page_lru_count[slotno] = 0; \
-	} while (0)
-
-/* Saved info for CLOGReportIOError */
-typedef enum
-{
-	CLOG_OPEN_FAILED,
-	CLOG_CREATE_FAILED,
-	CLOG_SEEK_FAILED,
-	CLOG_READ_FAILED,
-	CLOG_WRITE_FAILED
-} ClogErrorCause;
-static ClogErrorCause clog_errcause;
-static int	clog_errno;
 
+static SlruCtlData ClogCtlData;
+static SlruCtl ClogCtl = &ClogCtlData;
+  
 
 static int	ZeroCLOGPage(int pageno, bool writeXlog);
-static int	ReadCLOGPage(int pageno, TransactionId xid);
-static void WriteCLOGPage(int slotno);
-static bool CLOGPhysicalReadPage(int pageno, int slotno);
-static bool CLOGPhysicalWritePage(int pageno, int slotno);
-static void CLOGReportIOError(int pageno, TransactionId xid);
-static int	SelectLRUCLOGPage(int pageno);
-static bool ScanCLOGDirectory(int cutoffPage, bool doDeletions);
 static bool CLOGPagePrecedes(int page1, int page2);
 static void WriteZeroPageXlogRec(int pageno);
 
@@ -217,16 +92,15 @@ TransactionIdSetStatus(TransactionId xid, XidStatus status)
 	int			pageno = TransactionIdToPage(xid);
 	int			byteno = TransactionIdToByte(xid);
 	int			bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
-	int			slotno;
 	char	   *byteptr;
 
 	Assert(status == TRANSACTION_STATUS_COMMITTED ||
 		   status == TRANSACTION_STATUS_ABORTED);
 
-	LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
+	LWLockAcquire(ClogCtl->locks->ControlLock, LW_EXCLUSIVE);
 
-	slotno = ReadCLOGPage(pageno, xid);
-	byteptr = ClogCtl->page_buffer[slotno] + byteno;
+	byteptr = SimpleLruReadPage(ClogCtl, pageno, xid, true);
+	byteptr += byteno;
 
 	/* Current state should be 0 or target state */
 	Assert(((*byteptr >> bshift) & CLOG_XACT_BITMASK) == 0 ||
@@ -234,9 +108,9 @@ TransactionIdSetStatus(TransactionId xid, XidStatus status)
 
 	*byteptr |= (status << bshift);
 
-	ClogCtl->page_status[slotno] = CLOG_PAGE_DIRTY;
+	/* ...->page_status[slotno] = CLOG_PAGE_DIRTY; already done */
 
-	LWLockRelease(CLogControlLock);
+	LWLockRelease(ClogCtl->locks->ControlLock);
 }
 
 /*
@@ -251,18 +125,17 @@ TransactionIdGetStatus(TransactionId xid)
 	int			pageno = TransactionIdToPage(xid);
 	int			byteno = TransactionIdToByte(xid);
 	int			bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
-	int			slotno;
 	char	   *byteptr;
 	XidStatus	status;
 
-	LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
+	LWLockAcquire(ClogCtl->locks->ControlLock, LW_EXCLUSIVE);
 
-	slotno = ReadCLOGPage(pageno, xid);
-	byteptr = ClogCtl->page_buffer[slotno] + byteno;
+	byteptr = SimpleLruReadPage(ClogCtl, pageno, xid, false);
+	byteptr += byteno;
 
 	status = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
 
-	LWLockRelease(CLogControlLock);
+	LWLockRelease(ClogCtl->locks->ControlLock);
 
 	return status;
 }
@@ -271,70 +144,18 @@ TransactionIdGetStatus(TransactionId xid)
 /*
  * Initialization of shared memory for CLOG
  */
+
 int
 CLOGShmemSize(void)
 {
-	return MAXALIGN(sizeof(ClogCtlData) + CLOG_BLCKSZ * NUM_CLOG_BUFFERS)
-#ifdef EXEC_BACKEND
-			+ MAXALIGN(NUM_CLOG_BUFFERS * sizeof(LWLockId))
-#endif
-	;
+	return SimpleLruShmemSize();
 }
 
-
 void
 CLOGShmemInit(void)
 {
-	bool		found;
-	int			slotno;
-
-	/* Handle ClogCtl */
-	
-	/* this must agree with space requested by CLOGShmemSize() */
-	ClogCtl = (ClogCtlData *) ShmemInitStruct("CLOG Ctl",
-				MAXALIGN(sizeof(ClogCtlData) +
-				CLOG_BLCKSZ * NUM_CLOG_BUFFERS), &found);
-
-	if (!IsUnderPostmaster)
-	/* Initialize ClogCtl shared memory area */
-	{
-		char	   *bufptr;
-
-		Assert(!found);
-
-		memset(ClogCtl, 0, sizeof(ClogCtlData));
-
-		bufptr = (char *)ClogCtl + sizeof(ClogCtlData);
-	
-		for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
-		{
-			ClogCtl->page_buffer[slotno] = bufptr;
-			ClogCtl->page_status[slotno] = CLOG_PAGE_EMPTY;
-			bufptr += CLOG_BLCKSZ;
-		}
-
-		/* ClogCtl->latest_page_number will be set later */
-	}
-	else
-		Assert(found);
-
-	/* Handle ClogBufferLocks */
-	
-#ifdef EXEC_BACKEND
-	ClogBufferLocks = (LWLockId *) ShmemInitStruct("CLOG Buffer Locks",
-						NUM_CLOG_BUFFERS * sizeof(LWLockId), &found);
-	Assert((!found && !IsUnderPostmaster) || (found && IsUnderPostmaster));
-#else
-	ClogBufferLocks = malloc(NUM_CLOG_BUFFERS * sizeof(LWLockId));
-	Assert(ClogBufferLocks);
-#endif
-
-	if (!IsUnderPostmaster)
-		for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
-			ClogBufferLocks[slotno] = LWLockAssign();
-
-	/* Init CLOG directory path */
-	snprintf(ClogDir, MAXPGPATH, "%s/pg_clog", DataDir);
+	SimpleLruInit(ClogCtl, "CLOG Ctl", "pg_clog");
+	ClogCtl->PagePrecedes = CLOGPagePrecedes;
 }
 
 /*
@@ -348,16 +169,16 @@ BootStrapCLOG(void)
 {
 	int			slotno;
 
-	LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
+	LWLockAcquire(ClogCtl->locks->ControlLock, LW_EXCLUSIVE);
 
 	/* Create and zero the first page of the commit log */
 	slotno = ZeroCLOGPage(0, false);
 
 	/* Make sure it's written out */
-	WriteCLOGPage(slotno);
-	Assert(ClogCtl->page_status[slotno] == CLOG_PAGE_CLEAN);
+	SimpleLruWritePage(ClogCtl, slotno);
+	/* Assert(ClogCtl->page_status[slotno] == CLOG_PAGE_CLEAN); */
 
-	LWLockRelease(CLogControlLock);
+	LWLockRelease(ClogCtl->locks->ControlLock);
 }
 
 /*
@@ -372,24 +193,7 @@ BootStrapCLOG(void)
 static int
 ZeroCLOGPage(int pageno, bool writeXlog)
 {
-	int			slotno;
-
-	/* Find a suitable buffer slot for the page */
-	slotno = SelectLRUCLOGPage(pageno);
-	Assert(ClogCtl->page_status[slotno] == CLOG_PAGE_EMPTY ||
-		   ClogCtl->page_status[slotno] == CLOG_PAGE_CLEAN ||
-		   ClogCtl->page_number[slotno] == pageno);
-
-	/* Mark the slot as containing this page */
-	ClogCtl->page_number[slotno] = pageno;
-	ClogCtl->page_status[slotno] = CLOG_PAGE_DIRTY;
-	ClogRecentlyUsed(slotno);
-
-	/* Set the buffer to zeroes */
-	MemSet(ClogCtl->page_buffer[slotno], 0, CLOG_BLCKSZ);
-
-	/* Assume this page is now the latest active page */
-	ClogCtl->latest_page_number = pageno;
+	int			slotno = SimpleLruZeroPage(ClogCtl, pageno);
 
 	if (writeXlog)
 		WriteZeroPageXlogRec(pageno);
@@ -398,430 +202,6 @@ ZeroCLOGPage(int pageno, bool writeXlog)
 }
 
 /*
- * Find a CLOG page in a shared buffer, reading it in if necessary.
- * The page number must correspond to an already-initialized page.
- *
- * The passed-in xid is used only for error reporting, and may be
- * InvalidTransactionId if no specific xid is associated with the action.
- *
- * Return value is the shared-buffer slot number now holding the page.
- * The buffer's LRU access info is updated.
- *
- * Control lock must be held at entry, and will be held at exit.
- */
-static int
-ReadCLOGPage(int pageno, TransactionId xid)
-{
-	/* Outer loop handles restart if we lose the buffer to someone else */
-	for (;;)
-	{
-		int			slotno;
-		bool		ok;
-
-		/* See if page already is in memory; if not, pick victim slot */
-		slotno = SelectLRUCLOGPage(pageno);
-
-		/* Did we find the page in memory? */
-		if (ClogCtl->page_number[slotno] == pageno &&
-			ClogCtl->page_status[slotno] != CLOG_PAGE_EMPTY)
-		{
-			/* If page is still being read in, we cannot use it yet */
-			if (ClogCtl->page_status[slotno] != CLOG_PAGE_READ_IN_PROGRESS)
-			{
-				/* otherwise, it's ready to use */
-				ClogRecentlyUsed(slotno);
-				return slotno;
-			}
-		}
-		else
-		{
-			/* We found no match; assert we selected a freeable slot */
-			Assert(ClogCtl->page_status[slotno] == CLOG_PAGE_EMPTY ||
-				   ClogCtl->page_status[slotno] == CLOG_PAGE_CLEAN);
-		}
-
-		/* Mark the slot read-busy (no-op if it already was) */
-		ClogCtl->page_number[slotno] = pageno;
-		ClogCtl->page_status[slotno] = CLOG_PAGE_READ_IN_PROGRESS;
-
-		/*
-		 * Temporarily mark page as recently-used to discourage
-		 * SelectLRUCLOGPage from selecting it again for someone else.
-		 */
-		ClogCtl->page_lru_count[slotno] = 0;
-
-		/* Release shared lock, grab per-buffer lock instead */
-		LWLockRelease(CLogControlLock);
-		LWLockAcquire(ClogBufferLocks[slotno], LW_EXCLUSIVE);
-
-		/*
-		 * Check to see if someone else already did the read, or took the
-		 * buffer away from us.  If so, restart from the top.
-		 */
-		if (ClogCtl->page_number[slotno] != pageno ||
-			ClogCtl->page_status[slotno] != CLOG_PAGE_READ_IN_PROGRESS)
-		{
-			LWLockRelease(ClogBufferLocks[slotno]);
-			LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
-			continue;
-		}
-
-		/* Okay, do the read */
-		ok = CLOGPhysicalReadPage(pageno, slotno);
-
-		/* Re-acquire shared control lock and update page state */
-		LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
-
-		Assert(ClogCtl->page_number[slotno] == pageno &&
-			 ClogCtl->page_status[slotno] == CLOG_PAGE_READ_IN_PROGRESS);
-
-		ClogCtl->page_status[slotno] = ok ? CLOG_PAGE_CLEAN : CLOG_PAGE_EMPTY;
-
-		LWLockRelease(ClogBufferLocks[slotno]);
-
-		/* Now it's okay to elog if we failed */
-		if (!ok)
-			CLOGReportIOError(pageno, xid);
-
-		ClogRecentlyUsed(slotno);
-		return slotno;
-	}
-}
-
-/*
- * Write a CLOG page from a shared buffer, if necessary.
- * Does nothing if the specified slot is not dirty.
- *
- * NOTE: only one write attempt is made here.  Hence, it is possible that
- * the page is still dirty at exit (if someone else re-dirtied it during
- * the write).	However, we *do* attempt a fresh write even if the page
- * is already being written; this is for checkpoints.
- *
- * Control lock must be held at entry, and will be held at exit.
- */
-static void
-WriteCLOGPage(int slotno)
-{
-	int			pageno;
-	bool		ok;
-
-	/* Do nothing if page does not need writing */
-	if (ClogCtl->page_status[slotno] != CLOG_PAGE_DIRTY &&
-		ClogCtl->page_status[slotno] != CLOG_PAGE_WRITE_IN_PROGRESS)
-		return;
-
-	pageno = ClogCtl->page_number[slotno];
-
-	/* Release shared lock, grab per-buffer lock instead */
-	LWLockRelease(CLogControlLock);
-	LWLockAcquire(ClogBufferLocks[slotno], LW_EXCLUSIVE);
-
-	/*
-	 * Check to see if someone else already did the write, or took the
-	 * buffer away from us.  If so, do nothing.  NOTE: we really should
-	 * never see WRITE_IN_PROGRESS here, since that state should only
-	 * occur while the writer is holding the buffer lock.  But accept it
-	 * so that we have a recovery path if a writer aborts.
-	 */
-	if (ClogCtl->page_number[slotno] != pageno ||
-		(ClogCtl->page_status[slotno] != CLOG_PAGE_DIRTY &&
-		 ClogCtl->page_status[slotno] != CLOG_PAGE_WRITE_IN_PROGRESS))
-	{
-		LWLockRelease(ClogBufferLocks[slotno]);
-		LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
-		return;
-	}
-
-	/*
-	 * Mark the slot write-busy.  After this point, a transaction status
-	 * update on this page will mark it dirty again.  NB: we are assuming
-	 * that read/write of the page status field is atomic, since we change
-	 * the state while not holding control lock.  However, we cannot set
-	 * this state any sooner, or we'd possibly fool a previous writer into
-	 * thinking he's successfully dumped the page when he hasn't.
-	 * (Scenario: other writer starts, page is redirtied, we come along
-	 * and set WRITE_IN_PROGRESS again, other writer completes and sets
-	 * CLEAN because redirty info has been lost, then we think it's clean
-	 * too.)
-	 */
-	ClogCtl->page_status[slotno] = CLOG_PAGE_WRITE_IN_PROGRESS;
-
-	/* Okay, do the write */
-	ok = CLOGPhysicalWritePage(pageno, slotno);
-
-	/* Re-acquire shared control lock and update page state */
-	LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
-
-	Assert(ClogCtl->page_number[slotno] == pageno &&
-		   (ClogCtl->page_status[slotno] == CLOG_PAGE_WRITE_IN_PROGRESS ||
-			ClogCtl->page_status[slotno] == CLOG_PAGE_DIRTY));
-
-	/* Cannot set CLEAN if someone re-dirtied page since write started */
-	if (ClogCtl->page_status[slotno] == CLOG_PAGE_WRITE_IN_PROGRESS)
-		ClogCtl->page_status[slotno] = ok ? CLOG_PAGE_CLEAN : CLOG_PAGE_DIRTY;
-
-	LWLockRelease(ClogBufferLocks[slotno]);
-
-	/* Now it's okay to elog if we failed */
-	if (!ok)
-		CLOGReportIOError(pageno, InvalidTransactionId);
-}
-
-/*
- * Physical read of a (previously existing) page into a buffer slot
- *
- * On failure, we cannot just elog(ERROR) since caller has put state in
- * shared memory that must be undone.  So, we return FALSE and save enough
- * info in static variables to let CLOGReportIOError make the report.
- *
- * For now, assume it's not worth keeping a file pointer open across
- * read/write operations.  We could cache one virtual file pointer ...
- */
-static bool
-CLOGPhysicalReadPage(int pageno, int slotno)
-{
-	int			segno = pageno / CLOG_PAGES_PER_SEGMENT;
-	int			rpageno = pageno % CLOG_PAGES_PER_SEGMENT;
-	int			offset = rpageno * CLOG_BLCKSZ;
-	char		path[MAXPGPATH];
-	int			fd;
-
-	ClogFileName(path, segno);
-
-	/*
-	 * In a crash-and-restart situation, it's possible for us to receive
-	 * commands to set the commit status of transactions whose bits are in
-	 * already-truncated segments of the commit log (see notes in
-	 * CLOGPhysicalWritePage).	Hence, if we are InRecovery, allow the
-	 * case where the file doesn't exist, and return zeroes instead.
-	 */
-	fd = BasicOpenFile(path, O_RDWR | PG_BINARY, S_IRUSR | S_IWUSR);
-	if (fd < 0)
-	{
-		if (errno != ENOENT || !InRecovery)
-		{
-			clog_errcause = CLOG_OPEN_FAILED;
-			clog_errno = errno;
-			return false;
-		}
-
-		elog(LOG, "clog file %s doesn't exist, reading as zeroes", path);
-		MemSet(ClogCtl->page_buffer[slotno], 0, CLOG_BLCKSZ);
-		return true;
-	}
-
-	if (lseek(fd, (off_t) offset, SEEK_SET) < 0)
-	{
-		clog_errcause = CLOG_SEEK_FAILED;
-		clog_errno = errno;
-		return false;
-	}
-
-	errno = 0;
-	if (read(fd, ClogCtl->page_buffer[slotno], CLOG_BLCKSZ) != CLOG_BLCKSZ)
-	{
-		clog_errcause = CLOG_READ_FAILED;
-		clog_errno = errno;
-		return false;
-	}
-
-	close(fd);
-	return true;
-}
-
-/*
- * Physical write of a page from a buffer slot
- *
- * On failure, we cannot just elog(ERROR) since caller has put state in
- * shared memory that must be undone.  So, we return FALSE and save enough
- * info in static variables to let CLOGReportIOError make the report.
- *
- * For now, assume it's not worth keeping a file pointer open across
- * read/write operations.  We could cache one virtual file pointer ...
- */
-static bool
-CLOGPhysicalWritePage(int pageno, int slotno)
-{
-	int			segno = pageno / CLOG_PAGES_PER_SEGMENT;
-	int			rpageno = pageno % CLOG_PAGES_PER_SEGMENT;
-	int			offset = rpageno * CLOG_BLCKSZ;
-	char		path[MAXPGPATH];
-	int			fd;
-
-	ClogFileName(path, segno);
-
-	/*
-	 * If the file doesn't already exist, we should create it.  It is
-	 * possible for this to need to happen when writing a page that's not
-	 * first in its segment; we assume the OS can cope with that.  (Note:
-	 * it might seem that it'd be okay to create files only when
-	 * ZeroCLOGPage is called for the first page of a segment.	However,
-	 * if after a crash and restart the REDO logic elects to replay the
-	 * log from a checkpoint before the latest one, then it's possible
-	 * that we will get commands to set transaction status of transactions
-	 * that have already been truncated from the commit log.  Easiest way
-	 * to deal with that is to accept references to nonexistent files here
-	 * and in CLOGPhysicalReadPage.)
-	 */
-	fd = BasicOpenFile(path, O_RDWR | PG_BINARY, S_IRUSR | S_IWUSR);
-	if (fd < 0)
-	{
-		if (errno != ENOENT)
-		{
-			clog_errcause = CLOG_OPEN_FAILED;
-			clog_errno = errno;
-			return false;
-		}
-
-		fd = BasicOpenFile(path, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
-						   S_IRUSR | S_IWUSR);
-		if (fd < 0)
-		{
-			clog_errcause = CLOG_CREATE_FAILED;
-			clog_errno = errno;
-			return false;
-		}
-	}
-
-	if (lseek(fd, (off_t) offset, SEEK_SET) < 0)
-	{
-		clog_errcause = CLOG_SEEK_FAILED;
-		clog_errno = errno;
-		return false;
-	}
-
-	errno = 0;
-	if (write(fd, ClogCtl->page_buffer[slotno], CLOG_BLCKSZ) != CLOG_BLCKSZ)
-	{
-		/* if write didn't set errno, assume problem is no disk space */
-		if (errno == 0)
-			errno = ENOSPC;
-		clog_errcause = CLOG_WRITE_FAILED;
-		clog_errno = errno;
-		return false;
-	}
-
-	close(fd);
-	return true;
-}
-
-/*
- * Issue the error message after failure of CLOGPhysicalReadPage or
- * CLOGPhysicalWritePage.  Call this after cleaning up shared-memory state.
- */
-static void
-CLOGReportIOError(int pageno, TransactionId xid)
-{
-	int			segno = pageno / CLOG_PAGES_PER_SEGMENT;
-	int			rpageno = pageno % CLOG_PAGES_PER_SEGMENT;
-	int			offset = rpageno * CLOG_BLCKSZ;
-	char		path[MAXPGPATH];
-
-	/* XXX TODO: provide xid as context in error messages */
-
-	ClogFileName(path, segno);
-	errno = clog_errno;
-	switch (clog_errcause)
-	{
-		case CLOG_OPEN_FAILED:
-			elog(ERROR, "open of %s failed: %m", path);
-			break;
-		case CLOG_CREATE_FAILED:
-			elog(ERROR, "creation of file %s failed: %m", path);
-			break;
-		case CLOG_SEEK_FAILED:
-			elog(ERROR, "lseek of clog file %u, offset %u failed: %m",
-				 segno, offset);
-			break;
-		case CLOG_READ_FAILED:
-			elog(ERROR, "read of clog file %u, offset %u failed: %m",
-				 segno, offset);
-			break;
-		case CLOG_WRITE_FAILED:
-			elog(ERROR, "write of clog file %u, offset %u failed: %m",
-				 segno, offset);
-			break;
-		default:
-			/* can't get here, we trust */
-			elog(ERROR, "unknown CLOG I/O error");
-			break;
-	}
-}
-
-/*
- * Select the slot to re-use when we need a free slot.
- *
- * The target page number is passed because we need to consider the
- * possibility that some other process reads in the target page while
- * we are doing I/O to free a slot.  Hence, check or recheck to see if
- * any slot already holds the target page, and return that slot if so.
- * Thus, the returned slot is *either* a slot already holding the pageno
- * (could be any state except EMPTY), *or* a freeable slot (state EMPTY
- * or CLEAN).
- *
- * Control lock must be held at entry, and will be held at exit.
- */
-static int
-SelectLRUCLOGPage(int pageno)
-{
-	/* Outer loop handles restart after I/O */
-	for (;;)
-	{
-		int			slotno;
-		int			bestslot = 0;
-		unsigned int bestcount = 0;
-
-		/* See if page already has a buffer assigned */
-		for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
-		{
-			if (ClogCtl->page_number[slotno] == pageno &&
-				ClogCtl->page_status[slotno] != CLOG_PAGE_EMPTY)
-				return slotno;
-		}
-
-		/*
-		 * If we find any EMPTY slot, just select that one. Else locate
-		 * the least-recently-used slot that isn't the latest CLOG page.
-		 */
-		for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
-		{
-			if (ClogCtl->page_status[slotno] == CLOG_PAGE_EMPTY)
-				return slotno;
-			if (ClogCtl->page_lru_count[slotno] > bestcount &&
-			 ClogCtl->page_number[slotno] != ClogCtl->latest_page_number)
-			{
-				bestslot = slotno;
-				bestcount = ClogCtl->page_lru_count[slotno];
-			}
-		}
-
-		/*
-		 * If the selected page is clean, we're set.
-		 */
-		if (ClogCtl->page_status[bestslot] == CLOG_PAGE_CLEAN)
-			return bestslot;
-
-		/*
-		 * We need to do I/O.  Normal case is that we have to write it
-		 * out, but it's possible in the worst case to have selected a
-		 * read-busy page.	In that case we use ReadCLOGPage to wait for
-		 * the read to complete.
-		 */
-		if (ClogCtl->page_status[bestslot] == CLOG_PAGE_READ_IN_PROGRESS)
-			(void) ReadCLOGPage(ClogCtl->page_number[bestslot],
-								InvalidTransactionId);
-		else
-			WriteCLOGPage(bestslot);
-
-		/*
-		 * Now loop back and try again.  This is the easiest way of
-		 * dealing with corner cases such as the victim page being
-		 * re-dirtied while we wrote it.
-		 */
-	}
-}
-
-/*
  * This must be called ONCE during postmaster or standalone-backend startup,
  * after StartupXLOG has initialized ShmemVariableCache->nextXid.
  */
@@ -831,7 +211,7 @@ StartupCLOG(void)
 	/*
 	 * Initialize our idea of the latest page number.
 	 */
-	ClogCtl->latest_page_number = TransactionIdToPage(ShmemVariableCache->nextXid);
+	SimpleLruSetLatestPage(ClogCtl, TransactionIdToPage(ShmemVariableCache->nextXid));
 }
 
 /*
@@ -840,18 +220,7 @@ StartupCLOG(void)
 void
 ShutdownCLOG(void)
 {
-	int			slotno;
-
-	LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
-
-	for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
-	{
-		WriteCLOGPage(slotno);
-		Assert(ClogCtl->page_status[slotno] == CLOG_PAGE_EMPTY ||
-			   ClogCtl->page_status[slotno] == CLOG_PAGE_CLEAN);
-	}
-
-	LWLockRelease(CLogControlLock);
+	SimpleLruFlush(ClogCtl, false);
 }
 
 /*
@@ -860,21 +229,7 @@ ShutdownCLOG(void)
 void
 CheckPointCLOG(void)
 {
-	int			slotno;
-
-	LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
-
-	for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
-	{
-		WriteCLOGPage(slotno);
-
-		/*
-		 * We cannot assert that the slot is clean now, since another
-		 * process might have re-dirtied it already.  That's okay.
-		 */
-	}
-
-	LWLockRelease(CLogControlLock);
+	SimpleLruFlush(ClogCtl, true);
 }
 
 
@@ -901,12 +256,12 @@ ExtendCLOG(TransactionId newestXact)
 
 	pageno = TransactionIdToPage(newestXact);
 
-	LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
+	LWLockAcquire(ClogCtl->locks->ControlLock, LW_EXCLUSIVE);
 
 	/* Zero the page and make an XLOG entry about it */
 	ZeroCLOGPage(pageno, true);
 
-	LWLockRelease(CLogControlLock);
+	LWLockRelease(ClogCtl->locks->ControlLock);
 }
 
 
@@ -928,126 +283,15 @@ void
 TruncateCLOG(TransactionId oldestXact)
 {
 	int			cutoffPage;
-	int			slotno;
 
 	/*
 	 * The cutoff point is the start of the segment containing oldestXact.
+	 * We pass the *page* containing oldestXact to SimpleLruTruncate.
 	 */
-	oldestXact -= oldestXact % CLOG_XACTS_PER_SEGMENT;
 	cutoffPage = TransactionIdToPage(oldestXact);
-
-	if (!ScanCLOGDirectory(cutoffPage, false))
-		return;					/* nothing to remove */
-
-	/* Perform a forced CHECKPOINT */
-	CreateCheckPoint(false, true);
-
-	/*
-	 * Scan CLOG shared memory and remove any pages preceding the cutoff
-	 * page, to ensure we won't rewrite them later.  (Any dirty pages
-	 * should have been flushed already during the checkpoint, we're just
-	 * being extra careful here.)
-	 */
-	LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
-
-restart:;
-
-	/*
-	 * While we are holding the lock, make an important safety check: the
-	 * planned cutoff point must be <= the current CLOG endpoint page.
-	 * Otherwise we have already wrapped around, and proceeding with the
-	 * truncation would risk removing the current CLOG segment.
-	 */
-	if (CLOGPagePrecedes(ClogCtl->latest_page_number, cutoffPage))
-	{
-		LWLockRelease(CLogControlLock);
-		elog(LOG, "unable to truncate commit log: apparent wraparound");
-		return;
-	}
-
-	for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
-	{
-		if (ClogCtl->page_status[slotno] == CLOG_PAGE_EMPTY)
-			continue;
-		if (!CLOGPagePrecedes(ClogCtl->page_number[slotno], cutoffPage))
-			continue;
-
-		/*
-		 * If page is CLEAN, just change state to EMPTY (expected case).
-		 */
-		if (ClogCtl->page_status[slotno] == CLOG_PAGE_CLEAN)
-		{
-			ClogCtl->page_status[slotno] = CLOG_PAGE_EMPTY;
-			continue;
-		}
-
-		/*
-		 * Hmm, we have (or may have) I/O operations acting on the page,
-		 * so we've got to wait for them to finish and then start again.
-		 * This is the same logic as in SelectLRUCLOGPage.
-		 */
-		if (ClogCtl->page_status[slotno] == CLOG_PAGE_READ_IN_PROGRESS)
-			(void) ReadCLOGPage(ClogCtl->page_number[slotno],
-								InvalidTransactionId);
-		else
-			WriteCLOGPage(slotno);
-		goto restart;
-	}
-
-	LWLockRelease(CLogControlLock);
-
-	/* Now we can remove the old CLOG segment(s) */
-	(void) ScanCLOGDirectory(cutoffPage, true);
+	SimpleLruTruncate(ClogCtl, cutoffPage);
 }
 
-/*
- * TruncateCLOG subroutine: scan CLOG directory for removable segments.
- * Actually remove them iff doDeletions is true.  Return TRUE iff any
- * removable segments were found.  Note: no locking is needed.
- */
-static bool
-ScanCLOGDirectory(int cutoffPage, bool doDeletions)
-{
-	bool		found = false;
-	DIR		   *cldir;
-	struct dirent *clde;
-	int			segno;
-	int			segpage;
-	char		path[MAXPGPATH];
-
-	cldir = opendir(ClogDir);
-	if (cldir == NULL)
-		elog(ERROR, "could not open transaction-commit log directory (%s): %m",
-			 ClogDir);
-
-	errno = 0;
-	while ((clde = readdir(cldir)) != NULL)
-	{
-		if (strlen(clde->d_name) == 4 &&
-			strspn(clde->d_name, "0123456789ABCDEF") == 4)
-		{
-			segno = (int) strtol(clde->d_name, NULL, 16);
-			segpage = segno * CLOG_PAGES_PER_SEGMENT;
-			if (CLOGPagePrecedes(segpage, cutoffPage))
-			{
-				found = true;
-				if (doDeletions)
-				{
-					elog(LOG, "removing commit log file %s", clde->d_name);
-					snprintf(path, MAXPGPATH, "%s/%s", ClogDir, clde->d_name);
-					unlink(path);
-				}
-			}
-		}
-		errno = 0;
-	}
-	if (errno)
-		elog(ERROR, "could not read transaction-commit log directory (%s): %m",
-			 ClogDir);
-	closedir(cldir);
-
-	return found;
-}
 
 /*
  * Decide which of two CLOG page numbers is "older" for truncation purposes.
@@ -1107,13 +351,13 @@ clog_redo(XLogRecPtr lsn, XLogRecord *record)
 
 		memcpy(&pageno, XLogRecGetData(record), sizeof(int));
 
-		LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
+		LWLockAcquire(ClogCtl->locks->ControlLock, LW_EXCLUSIVE);
 
 		slotno = ZeroCLOGPage(pageno, false);
-		WriteCLOGPage(slotno);
-		Assert(ClogCtl->page_status[slotno] == CLOG_PAGE_CLEAN);
+		SimpleLruWritePage(ClogCtl, slotno);
+		/* Assert(ClogCtl->page_status[slotno] == SLRU_PAGE_CLEAN); */
 
-		LWLockRelease(CLogControlLock);
+		LWLockRelease(ClogCtl->locks->ControlLock);
 	}
 }