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rows will normally never obtain an XID at all. We already did things this way for subtransactions, but this patch extends the concept to top-level transactions. In applications where there are lots of short read-only transactions, this should improve performance noticeably; not so much from removal of the actual XID-assignments, as from reduction of overhead that's driven by the rate of XID consumption. We add a concept of a "virtual transaction ID" so that active transactions can be uniquely identified even if they don't have a regular XID. This is a much lighter-weight concept: uniqueness of VXIDs is only guaranteed over the short term, and no on-disk record is made about them. Florian Pflug, with some editorialization by Tom.
213 lines
7.9 KiB
C
213 lines
7.9 KiB
C
/*
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* xlog.h
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*
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* PostgreSQL transaction log manager
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*
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* Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.83 2007/09/05 18:10:48 tgl Exp $
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*/
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#ifndef XLOG_H
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#define XLOG_H
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#include "access/rmgr.h"
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#include "access/xlogdefs.h"
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#include "lib/stringinfo.h"
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#include "storage/buf.h"
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#include "utils/pg_crc.h"
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#include "utils/timestamp.h"
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/*
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* The overall layout of an XLOG record is:
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* Fixed-size header (XLogRecord struct)
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* rmgr-specific data
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* BkpBlock
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* backup block data
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* BkpBlock
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* backup block data
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* ...
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*
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* where there can be zero to three backup blocks (as signaled by xl_info flag
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* bits). XLogRecord structs always start on MAXALIGN boundaries in the WAL
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* files, and we round up SizeOfXLogRecord so that the rmgr data is also
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* guaranteed to begin on a MAXALIGN boundary. However, no padding is added
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* to align BkpBlock structs or backup block data.
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*
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* NOTE: xl_len counts only the rmgr data, not the XLogRecord header,
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* and also not any backup blocks. xl_tot_len counts everything. Neither
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* length field is rounded up to an alignment boundary.
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*/
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typedef struct XLogRecord
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{
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pg_crc32 xl_crc; /* CRC for this record */
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XLogRecPtr xl_prev; /* ptr to previous record in log */
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TransactionId xl_xid; /* xact id */
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uint32 xl_tot_len; /* total len of entire record */
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uint32 xl_len; /* total len of rmgr data */
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uint8 xl_info; /* flag bits, see below */
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RmgrId xl_rmid; /* resource manager for this record */
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/* Depending on MAXALIGN, there are either 2 or 6 wasted bytes here */
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/* ACTUAL LOG DATA FOLLOWS AT END OF STRUCT */
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} XLogRecord;
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#define SizeOfXLogRecord MAXALIGN(sizeof(XLogRecord))
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#define XLogRecGetData(record) ((char*) (record) + SizeOfXLogRecord)
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/*
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* XLOG uses only low 4 bits of xl_info. High 4 bits may be used by rmgr.
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*/
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#define XLR_INFO_MASK 0x0F
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/*
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* If we backed up any disk blocks with the XLOG record, we use flag bits in
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* xl_info to signal it. We support backup of up to 3 disk blocks per XLOG
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* record.
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*/
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#define XLR_BKP_BLOCK_MASK 0x0E /* all info bits used for bkp blocks */
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#define XLR_MAX_BKP_BLOCKS 3
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#define XLR_SET_BKP_BLOCK(iblk) (0x08 >> (iblk))
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#define XLR_BKP_BLOCK_1 XLR_SET_BKP_BLOCK(0) /* 0x08 */
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#define XLR_BKP_BLOCK_2 XLR_SET_BKP_BLOCK(1) /* 0x04 */
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#define XLR_BKP_BLOCK_3 XLR_SET_BKP_BLOCK(2) /* 0x02 */
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/*
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* Bit 0 of xl_info is set if the backed-up blocks could safely be removed
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* from a compressed version of XLOG (that is, they are backed up only to
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* prevent partial-page-write problems, and not to ensure consistency of PITR
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* recovery). The compression algorithm would need to extract data from the
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* blocks to create an equivalent non-full-page XLOG record.
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*/
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#define XLR_BKP_REMOVABLE 0x01
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/* Sync methods */
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#define SYNC_METHOD_FSYNC 0
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#define SYNC_METHOD_FDATASYNC 1
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#define SYNC_METHOD_OPEN 2 /* for O_SYNC and O_DSYNC */
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#define SYNC_METHOD_FSYNC_WRITETHROUGH 3
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extern int sync_method;
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/*
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* The rmgr data to be written by XLogInsert() is defined by a chain of
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* one or more XLogRecData structs. (Multiple structs would be used when
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* parts of the source data aren't physically adjacent in memory, or when
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* multiple associated buffers need to be specified.)
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*
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* If buffer is valid then XLOG will check if buffer must be backed up
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* (ie, whether this is first change of that page since last checkpoint).
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* If so, the whole page contents are attached to the XLOG record, and XLOG
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* sets XLR_BKP_BLOCK_X bit in xl_info. Note that the buffer must be pinned
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* and exclusive-locked by the caller, so that it won't change under us.
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* NB: when the buffer is backed up, we DO NOT insert the data pointed to by
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* this XLogRecData struct into the XLOG record, since we assume it's present
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* in the buffer. Therefore, rmgr redo routines MUST pay attention to
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* XLR_BKP_BLOCK_X to know what is actually stored in the XLOG record.
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* The i'th XLR_BKP_BLOCK bit corresponds to the i'th distinct buffer
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* value (ignoring InvalidBuffer) appearing in the rdata chain.
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*
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* When buffer is valid, caller must set buffer_std to indicate whether the
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* page uses standard pd_lower/pd_upper header fields. If this is true, then
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* XLOG is allowed to omit the free space between pd_lower and pd_upper from
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* the backed-up page image. Note that even when buffer_std is false, the
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* page MUST have an LSN field as its first eight bytes!
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*
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* Note: data can be NULL to indicate no rmgr data associated with this chain
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* entry. This can be sensible (ie, not a wasted entry) if buffer is valid.
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* The implication is that the buffer has been changed by the operation being
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* logged, and so may need to be backed up, but the change can be redone using
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* only information already present elsewhere in the XLOG entry.
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*/
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typedef struct XLogRecData
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{
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char *data; /* start of rmgr data to include */
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uint32 len; /* length of rmgr data to include */
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Buffer buffer; /* buffer associated with data, if any */
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bool buffer_std; /* buffer has standard pd_lower/pd_upper */
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struct XLogRecData *next; /* next struct in chain, or NULL */
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} XLogRecData;
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extern TimeLineID ThisTimeLineID; /* current TLI */
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extern bool InRecovery;
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extern XLogRecPtr XactLastRecEnd;
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/* these variables are GUC parameters related to XLOG */
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extern int CheckPointSegments;
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extern int XLOGbuffers;
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extern char *XLogArchiveCommand;
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extern int XLogArchiveTimeout;
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extern char *XLOG_sync_method;
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extern const char XLOG_sync_method_default[];
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extern bool log_checkpoints;
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#define XLogArchivingActive() (XLogArchiveCommand[0] != '\0')
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#ifdef WAL_DEBUG
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extern bool XLOG_DEBUG;
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#endif
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/*
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* OR-able request flag bits for checkpoints. The "cause" bits are used only
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* for logging purposes. Note: the flags must be defined so that it's
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* sensible to OR together request flags arising from different requestors.
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*/
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/* These directly affect the behavior of CreateCheckPoint and subsidiaries */
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#define CHECKPOINT_IS_SHUTDOWN 0x0001 /* Checkpoint is for shutdown */
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#define CHECKPOINT_IMMEDIATE 0x0002 /* Do it without delays */
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#define CHECKPOINT_FORCE 0x0004 /* Force even if no activity */
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/* These are important to RequestCheckpoint */
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#define CHECKPOINT_WAIT 0x0008 /* Wait for completion */
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/* These indicate the cause of a checkpoint request */
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#define CHECKPOINT_CAUSE_XLOG 0x0010 /* XLOG consumption */
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#define CHECKPOINT_CAUSE_TIME 0x0020 /* Elapsed time */
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/* Checkpoint statistics */
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typedef struct CheckpointStatsData
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{
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TimestampTz ckpt_start_t; /* start of checkpoint */
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TimestampTz ckpt_write_t; /* start of flushing buffers */
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TimestampTz ckpt_sync_t; /* start of fsyncs */
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TimestampTz ckpt_sync_end_t; /* end of fsyncs */
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TimestampTz ckpt_end_t; /* end of checkpoint */
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int ckpt_bufs_written; /* # of buffers written */
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int ckpt_segs_added; /* # of new xlog segments created */
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int ckpt_segs_removed; /* # of xlog segments deleted */
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int ckpt_segs_recycled; /* # of xlog segments recycled */
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} CheckpointStatsData;
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extern CheckpointStatsData CheckpointStats;
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extern XLogRecPtr XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata);
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extern void XLogFlush(XLogRecPtr RecPtr);
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extern void XLogBackgroundFlush(void);
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extern void XLogAsyncCommitFlush(void);
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extern bool XLogNeedsFlush(XLogRecPtr RecPtr);
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extern void XLogSetAsyncCommitLSN(XLogRecPtr record);
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extern void xlog_redo(XLogRecPtr lsn, XLogRecord *record);
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extern void xlog_desc(StringInfo buf, uint8 xl_info, char *rec);
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extern void UpdateControlFile(void);
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extern Size XLOGShmemSize(void);
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extern void XLOGShmemInit(void);
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extern void BootStrapXLOG(void);
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extern void StartupXLOG(void);
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extern void ShutdownXLOG(int code, Datum arg);
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extern void InitXLOGAccess(void);
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extern void CreateCheckPoint(int flags);
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extern void XLogPutNextOid(Oid nextOid);
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extern XLogRecPtr GetRedoRecPtr(void);
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extern XLogRecPtr GetInsertRecPtr(void);
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extern void GetNextXidAndEpoch(TransactionId *xid, uint32 *epoch);
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#endif /* XLOG_H */
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