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pg_include/access/slru.h

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+/*-------------------------------------------------------------------------
+ *
+ * slru.h
+ *		Simple LRU buffering for transaction status logfiles
+ *
+ * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/access/slru.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef SLRU_H
+#define SLRU_H
+
+#include "access/xlogdefs.h"
+#include "storage/lwlock.h"
+
+
+/*
+ * Define SLRU 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 slru.c, except when comparing
+ * segment and page numbers in SimpleLruTruncate (see PagePrecedes()).
+ *
+ * Note: slru.c currently assumes that segment file names will be four hex
+ * digits.	This sets a lower bound on the segment size (64K transactions
+ * for 32-bit TransactionIds).
+ */
+#define SLRU_PAGES_PER_SEGMENT	32
+
+/*
+ * Page status codes.  Note that these do not include the "dirty" bit.
+ * page_dirty can be TRUE only in the VALID or WRITE_IN_PROGRESS states;
+ * in the latter case it implies that the page has been re-dirtied since
+ * the write started.
+ */
+typedef enum
+{
+	SLRU_PAGE_EMPTY,			/* buffer is not in use */
+	SLRU_PAGE_READ_IN_PROGRESS, /* page is being read in */
+	SLRU_PAGE_VALID,			/* page is valid and not being written */
+	SLRU_PAGE_WRITE_IN_PROGRESS /* page is being written out */
+} SlruPageStatus;
+
+/*
+ * Shared-memory state
+ */
+typedef struct SlruSharedData
+{
+	LWLockId	ControlLock;
+
+	/* Number of buffers managed by this SLRU structure */
+	int			num_slots;
+
+	/*
+	 * Arrays holding info for each buffer slot.  Page number is undefined
+	 * when status is EMPTY, as is page_lru_count.
+	 */
+	char	  **page_buffer;
+	SlruPageStatus *page_status;
+	bool	   *page_dirty;
+	int		   *page_number;
+	int		   *page_lru_count;
+	LWLockId   *buffer_locks;
+
+	/*
+	 * Optional array of WAL flush LSNs associated with entries in the SLRU
+	 * pages.  If not zero/NULL, we must flush WAL before writing pages (true
+	 * for pg_clog, false for multixact, pg_subtrans, pg_notify).  group_lsn[]
+	 * has lsn_groups_per_page entries per buffer slot, each containing the
+	 * highest LSN known for a contiguous group of SLRU entries on that slot's
+	 * page.
+	 */
+	XLogRecPtr *group_lsn;
+	int			lsn_groups_per_page;
+
+	/*----------
+	 * We mark a page "most recently used" by setting
+	 *		page_lru_count[slotno] = ++cur_lru_count;
+	 * The oldest page is therefore the one with the highest value of
+	 *		cur_lru_count - page_lru_count[slotno]
+	 * The counts will eventually wrap around, but this calculation still
+	 * works as long as no page's age exceeds INT_MAX counts.
+	 *----------
+	 */
+	int			cur_lru_count;
+
+	/*
+	 * latest_page_number is the page number of the current end of the log;
+	 * 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;
+
+/*
+ * SlruCtlData is an unshared structure that points to the active information
+ * in shared memory.
+ */
+typedef struct SlruCtlData
+{
+	SlruShared	shared;
+
+	/*
+	 * This flag tells whether to fsync writes (true for pg_clog and multixact
+	 * stuff, false for pg_subtrans and pg_notify).
+	 */
+	bool		do_fsync;
+
+	/*
+	 * 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);
+
+	/*
+	 * Dir is set during SimpleLruInit and does not change thereafter. Since
+	 * it's always the same, it doesn't need to be in shared memory.
+	 */
+	char		Dir[64];
+} SlruCtlData;
+
+typedef SlruCtlData *SlruCtl;
+
+
+extern Size SimpleLruShmemSize(int nslots, int nlsns);
+extern void SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns,
+			  LWLockId ctllock, const char *subdir);
+extern int	SimpleLruZeroPage(SlruCtl ctl, int pageno);
+extern int SimpleLruReadPage(SlruCtl ctl, int pageno, bool write_ok,
+				  TransactionId xid);
+extern int SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno,
+						   TransactionId xid);
+extern void SimpleLruWritePage(SlruCtl ctl, int slotno);
+extern void SimpleLruFlush(SlruCtl ctl, bool checkpoint);
+extern void SimpleLruTruncate(SlruCtl ctl, int cutoffPage);
+
+typedef bool (*SlruScanCallback) (SlruCtl ctl, char *filename, int segpage,
+											  void *data);
+extern bool SlruScanDirectory(SlruCtl ctl, SlruScanCallback callback, void *data);
+
+/* SlruScanDirectory public callbacks */
+extern bool SlruScanDirCbReportPresence(SlruCtl ctl, char *filename,
+							int segpage, void *data);
+extern bool SlruScanDirCbDeleteAll(SlruCtl ctl, char *filename, int segpage,
+					   void *data);
+
+#endif   /* SLRU_H */