Re: Simple LRU for clog and (later) subtrans

Newest version applied --- new files added. Thanks.

---------------------------------------------------------------------------

Manfred Koizar wrote:
> 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.
>
> Servus
> Manfred

> diff -ruN ../base/src/backend/access/transam/Makefile src/backend/access/transam/Makefile
> --- ../base/src/backend/access/transam/Makefile 2001-08-25 20:52:41.000000000 +0200
> +++ src/backend/access/transam/Makefile 2003-05-01 20:11:44.000000000 +0200
> @@ -12,7 +12,7 @@
> top_builddir = ../../../..
> include $(top_builddir)/src/Makefile.global
>
> -OBJS = clog.o transam.o varsup.o xact.o xid.o xlog.o xlogutils.o rmgr.o
> +OBJS = clog.o transam.o varsup.o xact.o xid.o xlog.o xlogutils.o rmgr.o slru.o
>
> all: SUBSYS.o
>
> diff -ruN ../base/src/backend/access/transam/clog.c src/backend/access/transam/clog.c
> --- ../base/src/backend/access/transam/clog.c 2003-04-18 19:09:07.000000000 +0200
> +++ src/backend/access/transam/clog.c 2003-05-02 00:20:07.000000000 +0200
> @@ -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[NUM_CLOG_BUFFERS]; /* 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 @@
> 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->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 @@
>
> *byteptr |= (status << bshift);
>
> - ClogCtl->page_status[slotno] = CLOG_PAGE_DIRTY;
> + /* ...->page_status[slotno] = CLOG_PAGE_DIRTY; already done */
>
> - LWLockRelease(CLogControlLock);
> + LWLockRelease(ClogCtl->ControlLock);
> }
>
> /*
> @@ -251,18 +125,17 @@
> 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->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->ControlLock);
>
> return status;
> }
> @@ -275,40 +148,14 @@
> int
> CLOGShmemSize(void)
> {
> - return MAXALIGN(sizeof(ClogCtlData) + CLOG_BLCKSZ * NUM_CLOG_BUFFERS);
> + return SimpleLruShmemSize();
> }
>
> void
> CLOGShmemInit(void)
> {
> - bool found;
> - char *bufptr;
> - int slotno;
> -
> - /* this must agree with space requested by CLOGShmemSize() */
> - ClogCtl = (ClogCtlData *)
> - ShmemInitStruct("CLOG Ctl",
> - MAXALIGN(sizeof(ClogCtlData) +
> - CLOG_BLCKSZ * NUM_CLOG_BUFFERS),
> - &found);
> - 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;
> - ClogBufferLocks[slotno] = LWLockAssign();
> - bufptr += CLOG_BLCKSZ;
> - }
> -
> - /* ClogCtl->latest_page_number will be set later */
> -
> - /* Init CLOG directory path */
> - snprintf(ClogDir, MAXPGPATH, "%s/pg_clog", DataDir);
> + SimpleLruInit(ClogCtl, "CLOG Ctl", "pg_clog");
> + ClogCtl->PagePrecedes = CLOGPagePrecedes;
> }
>
> /*
> @@ -322,16 +169,16 @@
> {
> int slotno;
>
> - LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
> + LWLockAcquire(ClogCtl->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->ControlLock);
> }
>
> /*
> @@ -346,24 +193,7 @@
> 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);
> @@ -372,430 +202,6 @@
> }
>
> /*
> - * 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.
> */
> @@ -805,7 +211,7 @@
> /*
> * Initialize our idea of the latest page number.
> */
> - ClogCtl->latest_page_number = TransactionIdToPage(ShmemVariableCache->nextXid);
> + SimpleLruSetLatestPage(ClogCtl, TransactionIdToPage(ShmemVariableCache->nextXid));
> }
>
> /*
> @@ -814,18 +220,7 @@
> 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);
> }
>
> /*
> @@ -834,21 +229,7 @@
> 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);
> }
>
>
> @@ -875,12 +256,12 @@
>
> pageno = TransactionIdToPage(newestXact);
>
> - LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
> + LWLockAcquire(ClogCtl->ControlLock, LW_EXCLUSIVE);
>
> /* Zero the page and make an XLOG entry about it */
> ZeroCLOGPage(pageno, true);
>
> - LWLockRelease(CLogControlLock);
> + LWLockRelease(ClogCtl->ControlLock);
> }
>
>
> @@ -902,126 +283,15 @@
> 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.
> @@ -1081,13 +351,13 @@
>
> memcpy(&pageno, XLogRecGetData(record), sizeof(int));
>
> - LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
> + LWLockAcquire(ClogCtl->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->ControlLock);
> }
> }
>
> diff -ruN ../base/src/backend/access/transam/slru.c src/backend/access/transam/slru.c
> --- ../base/src/backend/access/transam/slru.c 1970-01-01 01:00:00.000000000 +0100
> +++ src/backend/access/transam/slru.c 2003-05-02 00:15:26.000000000 +0200
> @@ -0,0 +1,850 @@
> +/*-------------------------------------------------------------------------
> + *
> + * slru.c
> + * Simple LRU
> + *
> + * This module replaces the old "pg_log" access code, which treated pg_log
> + * essentially like a relation, in that it went through the regular buffer
> + * manager. The problem with that was that there wasn't any good way to
> + * recycle storage space for transactions so old that they'll never be
> + * looked up again. Now we use specialized access code so that the commit
> + * log can be broken into relatively small, independent segments.
> + *
> + * Portions Copyright (c) 2003, PostgreSQL Global Development Group
> + * Portions Copyright (c) 1994, Regents of the University of California
> + *
> + * $Header: $
> + *
> + *-------------------------------------------------------------------------
> + */
> +#include "postgres.h"
> +
> +#include <fcntl.h>
> +#include <dirent.h>
> +#include <errno.h>
> +#include <sys/stat.h>
> +#include <unistd.h>
> +
> +#include "access/slru.h"
> +#include "storage/lwlock.h"
> +#include "miscadmin.h"
> +
> +
> +/*
> + * Define segment size. A page is the same BLCKSZ as is used everywhere
> + * else in Postgres. The segment size can be chosen somewhat arbitrarily;
> + * we make it 32 pages by default, or 256Kb, i.e. 1M transactions for CLOG
> + * or 64K transactions for SUBTRANS.
> + *
> + * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
> + * page numbering also wraps around at 0xFFFFFFFF/xxxx_XACTS_PER_PAGE (where
> + * xxxx is CLOG or SUBTRANS, respectively), and segment numbering at
> + * 0xFFFFFFFF/xxxx_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT. We need
> + * take no explicit notice of that fact in this module, except when comparing
> + * segment and page numbers in SimpleLruTruncate (see PagePrecedes()).
> + */
> +
> +#define SLRU_PAGES_PER_SEGMENT 32
> +
> +
> +/*----------
> + * Shared-memory data structures for SLRU control
> + *
> + * We use a simple least-recently-used scheme to manage a pool of page
> + * buffers. Under ordinary circumstances we expect that write
> + * traffic will occur mostly to the latest 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 a control 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.
> + *----------
> + */
> +
> +typedef enum
> +{
> + SLRU_PAGE_EMPTY, /* buffer is not in use */
> + SLRU_PAGE_READ_IN_PROGRESS, /* page is being read in */
> + SLRU_PAGE_CLEAN, /* page is valid and not dirty */
> + SLRU_PAGE_DIRTY, /* page is valid but needs write */
> + SLRU_PAGE_WRITE_IN_PROGRESS /* page is being written out */
> +} SlruPageStatus;
> +
> +/*
> + * Shared-memory state
> + */
> +typedef struct SlruSharedData
> +{
> + /*
> + * Info for each buffer slot. Page number is undefined when status is
> + * EMPTY. lru_count is essentially the number of page switches since
> + * last use of this page; the page with highest lru_count is the best
> + * candidate to replace.
> + */
> + char *page_buffer[NUM_CLOG_BUFFERS];
> + SlruPageStatus 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;
> +} SlruSharedData;
> +typedef SlruSharedData *SlruShared;
> +
> +
> +#define SlruFileName(ctl, path, seg) \
> + snprintf(path, MAXPGPATH, "%s/%04X", (ctl)->Dir, seg)
> +
> +/*
> + * Macro to mark a buffer slot "most recently used".
> + */
> +#define SlruRecentlyUsed(shared, slotno) \
> + do { \
> + if ((shared)->page_lru_count[slotno] != 0) { \
> + int iilru; \
> + for (iilru = 0; iilru < NUM_CLOG_BUFFERS; iilru++) \
> + (shared)->page_lru_count[iilru]++; \
> + (shared)->page_lru_count[slotno] = 0; \
> + } \
> + } while (0)
> +
> +/* Saved info for SlruReportIOError */
> +typedef enum
> +{
> + SLRU_OPEN_FAILED,
> + SLRU_CREATE_FAILED,
> + SLRU_SEEK_FAILED,
> + SLRU_READ_FAILED,
> + SLRU_WRITE_FAILED
> +} SlruErrorCause;
> +static SlruErrorCause slru_errcause;
> +static int slru_errno;
> +
> +
> +static bool SlruPhysicalReadPage(SlruCtl ctl, int pageno, int slotno);
> +static bool SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno);
> +static void SlruReportIOError(SlruCtl ctl, int pageno, TransactionId xid);
> +static int SlruSelectLRUPage(SlruCtl ctl, int pageno);
> +static bool SlruScanDirectory(SlruCtl ctl, int cutoffPage, bool doDeletions);
> +
> +
> +/*
> + * Initialization of shared memory
> + */
> +
> +int
> +SimpleLruShmemSize(void)
> +{
> + return MAXALIGN(sizeof(SlruSharedData)) + BLCKSZ * NUM_CLOG_BUFFERS;
> +}
> +
> +void
> +SimpleLruInit(SlruCtl ctl, const char *name, const char *subdir)
> +{
> + bool found;
> + char *bufptr;
> + int slotno;
> + SlruShared shared;
> +
> + shared = (SlruShared) ShmemInitStruct(name, SimpleLruShmemSize(), &found);
> + Assert(!found);
> +
> + ctl->ControlLock = LWLockAssign();
> + ctl->shared = shared;
> + memset(shared, 0, sizeof(SlruSharedData));
> +
> + bufptr = ((char *) shared) + MAXALIGN(sizeof(SlruSharedData));
> +
> + for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
> + {
> + shared->page_buffer[slotno] = bufptr;
> + shared->page_status[slotno] = SLRU_PAGE_EMPTY;
> + shared->page_lru_count[slotno] = 1;
> + ctl->BufferLocks[slotno] = LWLockAssign();
> + bufptr += BLCKSZ;
> + }
> +
> + /* shared->latest_page_number will be set later */
> +
> + /* Init directory path */
> + snprintf(ctl->Dir, MAXPGPATH, "%s/%s", DataDir, subdir);
> +}
> +
> +/*
> + * Initialize (or reinitialize) a page to zeroes.
> + *
> + * The page is not actually written, just set up in shared memory.
> + * The slot number of the new page is returned.
> + *
> + * Control lock must be held at entry, and will be held at exit.
> + */
> +int
> +SimpleLruZeroPage(SlruCtl ctl, int pageno)
> +{
> + int slotno;
> + SlruShared shared = (SlruShared) ctl->shared;
> +
> + /* Find a suitable buffer slot for the page */
> + slotno = SlruSelectLRUPage(ctl, pageno);
> + Assert(shared->page_status[slotno] == SLRU_PAGE_EMPTY ||
> + shared->page_status[slotno] == SLRU_PAGE_CLEAN ||
> + shared->page_number[slotno] == pageno);
> +
> + /* Mark the slot as containing this page */
> + shared->page_number[slotno] = pageno;
> + shared->page_status[slotno] = SLRU_PAGE_DIRTY;
> + SlruRecentlyUsed(shared, slotno);
> +
> + /* Set the buffer to zeroes */
> + MemSet(shared->page_buffer[slotno], 0, BLCKSZ);
> +
> + /* Assume this page is now the latest active page */
> + shared->latest_page_number = pageno;
> +
> + return slotno;
> +}
> +
> +/*
> + * Find a 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 address of the page.
> + * The buffer's LRU access info is updated.
> + * If forwrite is true, the buffer is marked as dirty.
> + *
> + * Control lock must be held at entry, and will be held at exit.
> + */
> +char *
> +SimpleLruReadPage(SlruCtl ctl, int pageno, TransactionId xid, bool forwrite)
> +{
> + SlruShared shared = (SlruShared) ctl->shared;
> +
> + /* 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 = SlruSelectLRUPage(ctl, pageno);
> +
> + /* Did we find the page in memory? */
> + if (shared->page_number[slotno] == pageno &&
> + shared->page_status[slotno] != SLRU_PAGE_EMPTY)
> + {
> + /* If page is still being read in, we cannot use it yet */
> + if (shared->page_status[slotno] != SLRU_PAGE_READ_IN_PROGRESS)
> + {
> + /* otherwise, it's ready to use */
> + SlruRecentlyUsed(shared, slotno);
> + if (forwrite)
> + shared->page_status[slotno] = SLRU_PAGE_DIRTY;
> + return shared->page_buffer[slotno];
> + }
> + }
> + else
> + {
> + /* We found no match; assert we selected a freeable slot */
> + Assert(shared->page_status[slotno] == SLRU_PAGE_EMPTY ||
> + shared->page_status[slotno] == SLRU_PAGE_CLEAN);
> + }
> +
> + /* Mark the slot read-busy (no-op if it already was) */
> + shared->page_number[slotno] = pageno;
> + shared->page_status[slotno] = SLRU_PAGE_READ_IN_PROGRESS;
> +
> + /*
> + * Temporarily mark page as recently-used to discourage
> + * SlruSelectLRUPage from selecting it again for someone else.
> + */
> + SlruRecentlyUsed(shared, slotno);
> +
> + /* Release shared lock, grab per-buffer lock instead */
> + LWLockRelease(ctl->ControlLock);
> + LWLockAcquire(ctl->BufferLocks[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 (shared->page_number[slotno] != pageno ||
> + shared->page_status[slotno] != SLRU_PAGE_READ_IN_PROGRESS)
> + {
> + LWLockRelease(ctl->BufferLocks[slotno]);
> + LWLockAcquire(ctl->ControlLock, LW_EXCLUSIVE);
> + continue;
> + }
> +
> + /* Okay, do the read */
> + ok = SlruPhysicalReadPage(ctl, pageno, slotno);
> +
> + /* Re-acquire shared control lock and update page state */
> + LWLockAcquire(ctl->ControlLock, LW_EXCLUSIVE);
> +
> + Assert(shared->page_number[slotno] == pageno &&
> + shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS);
> +
> + shared->page_status[slotno] = ok ? SLRU_PAGE_CLEAN : SLRU_PAGE_EMPTY;
> +
> + LWLockRelease(ctl->BufferLocks[slotno]);
> +
> + /* Now it's okay to elog if we failed */
> + if (!ok)
> + SlruReportIOError(ctl, pageno, xid);
> +
> + SlruRecentlyUsed(shared, slotno);
> + if (forwrite)
> + shared->page_status[slotno] = SLRU_PAGE_DIRTY;
> + return shared->page_buffer[slotno];
> + }
> +}
> +
> +/*
> + * Write a 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.
> + */
> +void
> +SimpleLruWritePage(SlruCtl ctl, int slotno)
> +{
> + int pageno;
> + bool ok;
> + SlruShared shared = (SlruShared) ctl->shared;
> +
> + /* Do nothing if page does not need writing */
> + if (shared->page_status[slotno] != SLRU_PAGE_DIRTY &&
> + shared->page_status[slotno] != SLRU_PAGE_WRITE_IN_PROGRESS)
> + return;
> +
> + pageno = shared->page_number[slotno];
> +
> + /* Release shared lock, grab per-buffer lock instead */
> + LWLockRelease(ctl->ControlLock);
> + LWLockAcquire(ctl->BufferLocks[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 (shared->page_number[slotno] != pageno ||
> + (shared->page_status[slotno] != SLRU_PAGE_DIRTY &&
> + shared->page_status[slotno] != SLRU_PAGE_WRITE_IN_PROGRESS))
> + {
> + LWLockRelease(ctl->BufferLocks[slotno]);
> + LWLockAcquire(ctl->ControlLock, 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.)
> + */
> + shared->page_status[slotno] = SLRU_PAGE_WRITE_IN_PROGRESS;
> +
> + /* Okay, do the write */
> + ok = SlruPhysicalWritePage(ctl, pageno, slotno);
> +
> + /* Re-acquire shared control lock and update page state */
> + LWLockAcquire(ctl->ControlLock, LW_EXCLUSIVE);
> +
> + Assert(shared->page_number[slotno] == pageno &&
> + (shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS ||
> + shared->page_status[slotno] == SLRU_PAGE_DIRTY));
> +
> + /* Cannot set CLEAN if someone re-dirtied page since write started */
> + if (shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS)
> + shared->page_status[slotno] = ok ? SLRU_PAGE_CLEAN : SLRU_PAGE_DIRTY;
> +
> + LWLockRelease(ctl->BufferLocks[slotno]);
> +
> + /* Now it's okay to elog if we failed */
> + if (!ok)
> + SlruReportIOError(ctl, 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 SlruReportIOError 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
> +SlruPhysicalReadPage(SlruCtl ctl, int pageno, int slotno)
> +{
> + SlruShared shared = (SlruShared) ctl->shared;
> + int segno = pageno / SLRU_PAGES_PER_SEGMENT;
> + int rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
> + int offset = rpageno * BLCKSZ;
> + char path[MAXPGPATH];
> + int fd;
> +
> + SlruFileName(ctl, 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
> + * SlruPhysicalWritePage). 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)
> + {
> + slru_errcause = SLRU_OPEN_FAILED;
> + slru_errno = errno;
> + return false;
> + }
> +
> + elog(LOG, "file %s doesn't exist, reading as zeroes", path);
> + MemSet(shared->page_buffer[slotno], 0, BLCKSZ);
> + return true;
> + }
> +
> + if (lseek(fd, (off_t) offset, SEEK_SET) < 0)
> + {
> + slru_errcause = SLRU_SEEK_FAILED;
> + slru_errno = errno;
> + return false;
> + }
> +
> + errno = 0;
> + if (read(fd, shared->page_buffer[slotno], BLCKSZ) != BLCKSZ)
> + {
> + slru_errcause = SLRU_READ_FAILED;
> + slru_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 SlruReportIOError 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
> +SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno)
> +{
> + SlruShared shared = (SlruShared) ctl->shared;
> + int segno = pageno / SLRU_PAGES_PER_SEGMENT;
> + int rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
> + int offset = rpageno * BLCKSZ;
> + char path[MAXPGPATH];
> + int fd;
> +
> + SlruFileName(ctl, 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
> + * SimpleLruZeroPage 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 SlruPhysicalReadPage.)
> + */
> + fd = BasicOpenFile(path, O_RDWR | PG_BINARY, S_IRUSR | S_IWUSR);
> + if (fd < 0)
> + {
> + if (errno != ENOENT)
> + {
> + slru_errcause = SLRU_OPEN_FAILED;
> + slru_errno = errno;
> + return false;
> + }
> +
> + fd = BasicOpenFile(path, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
> + S_IRUSR | S_IWUSR);
> + if (fd < 0)
> + {
> + slru_errcause = SLRU_CREATE_FAILED;
> + slru_errno = errno;
> + return false;
> + }
> + }
> +
> + if (lseek(fd, (off_t) offset, SEEK_SET) < 0)
> + {
> + slru_errcause = SLRU_SEEK_FAILED;
> + slru_errno = errno;
> + return false;
> + }
> +
> + errno = 0;
> + if (write(fd, shared->page_buffer[slotno], BLCKSZ) != BLCKSZ)
> + {
> + /* if write didn't set errno, assume problem is no disk space */
> + if (errno == 0)
> + errno = ENOSPC;
> + slru_errcause = SLRU_WRITE_FAILED;
> + slru_errno = errno;
> + return false;
> + }
> +
> + close(fd);
> + return true;
> +}
> +
> +/*
> + * Issue the error message after failure of SlruPhysicalReadPage or
> + * SlruPhysicalWritePage. Call this after cleaning up shared-memory state.
> + */
> +static void
> +SlruReportIOError(SlruCtl ctl, int pageno, TransactionId xid)
> +{
> + int segno = pageno / SLRU_PAGES_PER_SEGMENT;
> + int rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
> + int offset = rpageno * BLCKSZ;
> + char path[MAXPGPATH];
> +
> + /* XXX TODO: provide xid as context in error messages */
> +
> + SlruFileName(ctl, path, segno);
> + errno = slru_errno;
> + switch (slru_errcause)
> + {
> + case SLRU_OPEN_FAILED:
> + elog(ERROR, "open of %s failed: %m", path);
> + break;
> + case SLRU_CREATE_FAILED:
> + elog(ERROR, "creation of file %s failed: %m", path);
> + break;
> + case SLRU_SEEK_FAILED:
> + elog(ERROR, "lseek of file %s, offset %u failed: %m",
> + path, offset);
> + break;
> + case SLRU_READ_FAILED:
> + elog(ERROR, "read of file %s, offset %u failed: %m",
> + path, offset);
> + break;
> + case SLRU_WRITE_FAILED:
> + elog(ERROR, "write of file %s, offset %u failed: %m",
> + path, offset);
> + break;
> + default:
> + /* can't get here, we trust */
> + elog(ERROR, "unknown SimpleLru 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
> +SlruSelectLRUPage(SlruCtl ctl, int pageno)
> +{
> + SlruShared shared = (SlruShared) ctl->shared;
> + /* 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 (shared->page_number[slotno] == pageno &&
> + shared->page_status[slotno] != SLRU_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 page.
> + */
> + for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
> + {
> + if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
> + return slotno;
> + if (shared->page_lru_count[slotno] > bestcount &&
> + shared->page_number[slotno] != shared->latest_page_number)
> + {
> + bestslot = slotno;
> + bestcount = shared->page_lru_count[slotno];
> + }
> + }
> +
> + /*
> + * If the selected page is clean, we're set.
> + */
> + if (shared->page_status[bestslot] == SLRU_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 SimpleLruReadPage to wait for
> + * the read to complete.
> + */
> + if (shared->page_status[bestslot] == SLRU_PAGE_READ_IN_PROGRESS)
> + (void) SimpleLruReadPage(ctl, shared->page_number[bestslot],
> + InvalidTransactionId, false);
> + else
> + SimpleLruWritePage(ctl, 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
> + */
> +void
> +SimpleLruSetLatestPage(SlruCtl ctl, int pageno)
> +{
> + SlruShared shared = (SlruShared) ctl->shared;
> +
> + shared->latest_page_number = pageno;
> +}
> +
> +/*
> + * This is called during checkpoint and postmaster/standalone-backend shutdown
> + */
> +void
> +SimpleLruFlush(SlruCtl ctl, bool checkpoint)
> +{
> + SlruShared shared = (SlruShared) ctl->shared;
> + int slotno;
> +
> + LWLockAcquire(ctl->ControlLock, LW_EXCLUSIVE);
> +
> + for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
> + {
> + SimpleLruWritePage(ctl, slotno);
> + /*
> + * When called during a checkpoint,
> + * we cannot assert that the slot is clean now, since another
> + * process might have re-dirtied it already. That's okay.
> + */
> + Assert(checkpoint ||
> + shared->page_status[slotno] == SLRU_PAGE_EMPTY ||
> + shared->page_status[slotno] == SLRU_PAGE_CLEAN);
> + }
> +
> + LWLockRelease(ctl->ControlLock);
> +}
> +
> +/*
> + * Remove all segments before the one holding the passed page number
> + *
> + * When this is called, we know that the database logically contains no
> + * reference to transaction IDs older than oldestXact. However, we must
> + * not remove any segment until we have performed a checkpoint, to ensure
> + * that no such references remain on disk either; else a crash just after
> + * the truncation might leave us with a problem. Since CLOG segments hold
> + * a large number of transactions, the opportunity to actually remove a
> + * segment is fairly rare, and so it seems best not to do the checkpoint
> + * unless we have confirmed that there is a removable segment. Therefore
> + * we issue the checkpoint command here, not in higher-level code as might
> + * seem cleaner.
> + */
> +void
> +SimpleLruTruncate(SlruCtl ctl, int cutoffPage)
> +{
> + int slotno;
> + SlruShared shared = (SlruShared) ctl->shared;
> +
> + /*
> + * The cutoff point is the start of the segment containing cutoffPage.
> + */
> + cutoffPage -= cutoffPage % SLRU_PAGES_PER_SEGMENT;
> +
> + if (!SlruScanDirectory(ctl, cutoffPage, false))
> + return; /* nothing to remove */
> +
> + /* Perform a forced CHECKPOINT */
> + CreateCheckPoint(false, true);
> +
> + /*
> + * Scan 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(ctl->ControlLock, LW_EXCLUSIVE);
> +
> +restart:;
> +
> + /*
> + * While we are holding the lock, make an important safety check: the
> + * planned cutoff point must be <= the current endpoint page.
> + * Otherwise we have already wrapped around, and proceeding with the
> + * truncation would risk removing the current segment.
> + */
> + if (ctl->PagePrecedes(shared->latest_page_number, cutoffPage))
> + {
> + LWLockRelease(ctl->ControlLock);
> + elog(LOG, "unable to truncate %s: apparent wraparound", ctl->Dir);
> + return;
> + }
> +
> + for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
> + {
> + if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
> + continue;
> + if (!ctl->PagePrecedes(shared->page_number[slotno], cutoffPage))
> + continue;
> +
> + /*
> + * If page is CLEAN, just change state to EMPTY (expected case).
> + */
> + if (shared->page_status[slotno] == SLRU_PAGE_CLEAN)
> + {
> + shared->page_status[slotno] = SLRU_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 SlruSelectLRUPage.
> + */
> + if (shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS)
> + (void) SimpleLruReadPage(ctl, shared->page_number[slotno],
> + InvalidTransactionId, false);
> + else
> + SimpleLruWritePage(ctl, slotno);
> + goto restart;
> + }
> +
> + LWLockRelease(ctl->ControlLock);
> +
> + /* Now we can remove the old segment(s) */
> + (void) SlruScanDirectory(ctl, cutoffPage, true);
> +}
> +
> +/*
> + * SlruTruncate subroutine: scan 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
> +SlruScanDirectory(SlruCtl ctl, int cutoffPage, bool doDeletions)
> +{
> + bool found = false;
> + DIR *cldir;
> + struct dirent *clde;
> + int segno;
> + int segpage;
> + char path[MAXPGPATH];
> +
> + cldir = opendir(ctl->Dir);
> + if (cldir == NULL)
> + elog(ERROR, "could not open directory (%s): %m", ctl->Dir);
> +
> + 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 * SLRU_PAGES_PER_SEGMENT;
> + if (ctl->PagePrecedes(segpage, cutoffPage))
> + {
> + found = true;
> + if (doDeletions)
> + {
> + elog(LOG, "removing file %s/%s", ctl->Dir, clde->d_name);
> + snprintf(path, MAXPGPATH, "%s/%s", ctl->Dir, clde->d_name);
> + unlink(path);
> + }
> + }
> + }
> + errno = 0;
> + }
> + if (errno)
> + elog(ERROR, "could not read directory (%s): %m", ctl->Dir);
> + closedir(cldir);
> +
> + return found;
> +}
> diff -ruN ../base/src/backend/storage/lmgr/lwlock.c src/backend/storage/lmgr/lwlock.c
> --- ../base/src/backend/storage/lmgr/lwlock.c 2002-10-10 19:07:27.000000000 +0200
> +++ src/backend/storage/lmgr/lwlock.c 2003-05-01 21:31:34.000000000 +0200
> @@ -108,8 +108,8 @@
> /* bufmgr.c needs two for each shared buffer */
> numLocks += 2 * NBuffers;
>
> - /* clog.c needs one per CLOG buffer */
> - numLocks += NUM_CLOG_BUFFERS;
> + /* clog.c needs one per CLOG buffer + one control lock */
> + numLocks += NUM_CLOG_BUFFERS + 1;
>
> /* Perhaps create a few more for use by user-defined modules? */
>
> diff -ruN ../base/src/include/access/slru.h src/include/access/slru.h
> --- ../base/src/include/access/slru.h 1970-01-01 01:00:00.000000000 +0100
> +++ src/include/access/slru.h 2003-05-01 20:50:14.000000000 +0200
> @@ -0,0 +1,57 @@
> +/*
> + * slru.h
> + *
> + * Simple LRU
> + *
> + * Portions Copyright (c) 2003, PostgreSQL Global Development Group
> + * Portions Copyright (c) 1994, Regents of the University of California
> + *
> + * $Id: $
> + */
> +#ifndef SLRU_H
> +#define SLRU_H
> +
> +#include "access/xlog.h"
> +
> +/* exported because lwlock.c needs it */
> +#define NUM_CLOG_BUFFERS 8
> +
> +
> +typedef struct SlruCtlData
> +{
> + void *shared; /* pointer to SlruSharedData */
> + LWLockId ControlLock; // = CLogControlLock
> +/*
> + * BufferLocks 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.
> + */
> + LWLockId BufferLocks[NUM_CLOG_BUFFERS]; /* Per-buffer I/O locks */
> +
> +/*
> + * Dir is set during SimpleLruShmemInit and does not change thereafter.
> + * The value is automatically inherited by backends via fork, and
> + * doesn't need to be in shared memory.
> + */
> + char Dir[MAXPGPATH];
> +
> +/*
> + * Decide which of two page numbers is "older" for truncation purposes.
> + * We need to use comparison of TransactionIds here in order to do the right
> + * thing with wraparound XID arithmetic.
> + */
> + bool (*PagePrecedes)(int, int);
> +
> +} SlruCtlData;
> +typedef SlruCtlData *SlruCtl;
> +
> +extern int SimpleLruShmemSize(void);
> +extern void SimpleLruInit(SlruCtl ctl, const char *name, const char *subdir);
> +extern int SimpleLruZeroPage(SlruCtl ctl, int pageno);
> +extern char *SimpleLruReadPage(SlruCtl ctl, int pageno, TransactionId xid, bool forwrite);
> +extern void SimpleLruWritePage(SlruCtl ctl, int slotno);
> +extern void SimpleLruSetLatestPage(SlruCtl ctl, int pageno);
> +extern void SimpleLruFlush(SlruCtl ctl, bool checkpoint);
> +extern void SimpleLruTruncate(SlruCtl ctl, int cutoffPage);
> +
> +#endif /* SLRU_H */
> diff -ruN ../base/src/include/storage/lwlock.h src/include/storage/lwlock.h
> --- ../base/src/include/storage/lwlock.h 2002-06-21 02:12:30.000000000 +0200
> +++ src/include/storage/lwlock.h 2003-05-01 21:32:21.000000000 +0200
> @@ -37,7 +37,6 @@
> WALWriteLock,
> ControlFileLock,
> CheckpointLock,
> - CLogControlLock,
> RelCacheInitLock,
>
> NumFixedLWLocks, /* must be last except for

>
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Don't 'kill -9' the postmaster

--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073