xlogrecord.h

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/*
 * xlogrecord.h
 *
 * Definitions for the WAL record format.
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/access/xlogrecord.h
 */
#ifndef XLOGRECORD_H
#define XLOGRECORD_H
 
#include "access/rmgr.h"
#include "access/xlogdefs.h"
#include "port/pg_crc32c.h"
#include "storage/block.h"
#include "storage/relfilelocator.h"
 
/*
 * The overall layout of an XLOG record is:
 *      Fixed-size header (XLogRecord struct)
 *      XLogRecordBlockHeader struct
 *      XLogRecordBlockHeader struct
 *      ...
 *      XLogRecordDataHeader[Short|Long] struct
 *      block data
 *      block data
 *      ...
 *      main data
 *
 * There can be zero or more XLogRecordBlockHeaders, and 0 or more bytes of
 * rmgr-specific data not associated with a block.  XLogRecord structs
 * always start on MAXALIGN boundaries in the WAL files, but the rest of
 * the fields are not aligned.
 *
 * The XLogRecordBlockHeader, XLogRecordDataHeaderShort and
 * XLogRecordDataHeaderLong structs all begin with a single 'id' byte. It's
 * used to distinguish between block references, and the main data structs.
 */
typedef struct XLogRecord
{
    uint32      xl_tot_len;     /* total len of entire record */
    TransactionId xl_xid;       /* xact id */
    XLogRecPtr  xl_prev;        /* ptr to previous record in log */
    uint8       xl_info;        /* flag bits, see below */
    RmgrId      xl_rmid;        /* resource manager for this record */
    /* 2 bytes of padding here, initialize to zero */
    pg_crc32c   xl_crc;         /* CRC for this record */
 
    /* XLogRecordBlockHeaders and XLogRecordDataHeader follow, no padding */
 
} XLogRecord;
 
#define SizeOfXLogRecord    (offsetof(XLogRecord, xl_crc) + sizeof(pg_crc32c))
 
/*
 * The high 4 bits in xl_info may be used freely by rmgr. The
 * XLR_SPECIAL_REL_UPDATE and XLR_CHECK_CONSISTENCY bits can be passed by
 * XLogInsert caller. The rest are set internally by XLogInsert.
 */
#define XLR_INFO_MASK           0x0F
#define XLR_RMGR_INFO_MASK      0xF0
 
/*
 * XLogReader needs to allocate all the data of a WAL record in a single
 * chunk.  This means that a single XLogRecord cannot exceed MaxAllocSize
 * in length if we ignore any allocation overhead of the XLogReader.
 *
 * To accommodate some overhead, this value allows for 4M of allocation
 * overhead, that should be plenty enough for what the XLogReader
 * infrastructure expects as extra.
 */
#define XLogRecordMaxSize   (1020 * 1024 * 1024)
 
/*
 * If a WAL record modifies any relation files, in ways not covered by the
 * usual block references, this flag is set. This is not used for anything
 * by PostgreSQL itself, but it allows external tools that read WAL and keep
 * track of modified blocks to recognize such special record types.
 */
#define XLR_SPECIAL_REL_UPDATE  0x01
 
/*
 * Enforces consistency checks of replayed WAL at recovery. If enabled,
 * each record will log a full-page write for each block modified by the
 * record and will reuse it afterwards for consistency checks. The caller
 * of XLogInsert can use this value if necessary, but if
 * wal_consistency_checking is enabled for a rmgr this is set unconditionally.
 */
#define XLR_CHECK_CONSISTENCY   0x02
 
/*
 * Header info for block data appended to an XLOG record.
 *
 * 'data_length' is the length of the rmgr-specific payload data associated
 * with this block. It does not include the possible full page image, nor
 * XLogRecordBlockHeader struct itself.
 *
 * Note that we don't attempt to align the XLogRecordBlockHeader struct!
 * So, the struct must be copied to aligned local storage before use.
 */
typedef struct XLogRecordBlockHeader
{
    uint8       id;             /* block reference ID */
    uint8       fork_flags;     /* fork within the relation, and flags */
    uint16      data_length;    /* number of payload bytes (not including page
                                 * image) */
 
    /* If BKPBLOCK_HAS_IMAGE, an XLogRecordBlockImageHeader struct follows */
    /* If BKPBLOCK_SAME_REL is not set, a RelFileLocator follows */
    /* BlockNumber follows */
} XLogRecordBlockHeader;
 
#define SizeOfXLogRecordBlockHeader (offsetof(XLogRecordBlockHeader, data_length) + sizeof(uint16))
 
/*
 * Additional header information when a full-page image is included
 * (i.e. when BKPBLOCK_HAS_IMAGE is set).
 *
 * The XLOG code is aware that PG data pages usually contain an unused "hole"
 * in the middle, which contains only zero bytes.  Since we know that the
 * "hole" is all zeros, we remove it from the stored data (and it's not counted
 * in the XLOG record's CRC, either).  Hence, the amount of block data actually
 * present is (BLCKSZ - <length of "hole" bytes>).
 *
 * Additionally, when wal_compression is enabled, we will try to compress full
 * page images using one of the supported algorithms, after removing the
 * "hole". This can reduce the WAL volume, but at some extra cost of CPU spent
 * on the compression during WAL logging. In this case, since the "hole"
 * length cannot be calculated by subtracting the number of page image bytes
 * from BLCKSZ, basically it needs to be stored as an extra information.
 * But when no "hole" exists, we can assume that the "hole" length is zero
 * and no such an extra information needs to be stored. Note that
 * the original version of page image is stored in WAL instead of the
 * compressed one if the number of bytes saved by compression is less than
 * the length of extra information. Hence, when a page image is successfully
 * compressed, the amount of block data actually present is less than
 * BLCKSZ - the length of "hole" bytes - the length of extra information.
 */
typedef struct XLogRecordBlockImageHeader
{
    uint16      length;         /* number of page image bytes */
    uint16      hole_offset;    /* number of bytes before "hole" */
    uint8       bimg_info;      /* flag bits, see below */
 
    /*
     * If BKPIMAGE_HAS_HOLE and BKPIMAGE_COMPRESSED(), an
     * XLogRecordBlockCompressHeader struct follows.
     */
} XLogRecordBlockImageHeader;
 
#define SizeOfXLogRecordBlockImageHeader    \
    (offsetof(XLogRecordBlockImageHeader, bimg_info) + sizeof(uint8))
 
/* Information stored in bimg_info */
#define BKPIMAGE_HAS_HOLE       0x01    /* page image has "hole" */
#define BKPIMAGE_APPLY          0x02    /* page image should be restored
                                         * during replay */
/* compression methods supported */
#define BKPIMAGE_COMPRESS_PGLZ  0x04
#define BKPIMAGE_COMPRESS_LZ4   0x08
#define BKPIMAGE_COMPRESS_ZSTD  0x10
 
#define BKPIMAGE_COMPRESSED(info) \
    ((info & (BKPIMAGE_COMPRESS_PGLZ | BKPIMAGE_COMPRESS_LZ4 | \
              BKPIMAGE_COMPRESS_ZSTD)) != 0)
 
/*
 * Extra header information used when page image has "hole" and
 * is compressed.
 */
typedef struct XLogRecordBlockCompressHeader
{
    uint16      hole_length;    /* number of bytes in "hole" */
} XLogRecordBlockCompressHeader;
 
#define SizeOfXLogRecordBlockCompressHeader \
    sizeof(XLogRecordBlockCompressHeader)
 
/*
 * Maximum size of the header for a block reference. This is used to size a
 * temporary buffer for constructing the header.
 */
#define MaxSizeOfXLogRecordBlockHeader \
    (SizeOfXLogRecordBlockHeader + \
     SizeOfXLogRecordBlockImageHeader + \
     SizeOfXLogRecordBlockCompressHeader + \
     sizeof(RelFileLocator) + \
     sizeof(BlockNumber))
 
/*
 * The fork number fits in the lower 4 bits in the fork_flags field. The upper
 * bits are used for flags.
 */
#define BKPBLOCK_FORK_MASK  0x0F
#define BKPBLOCK_FLAG_MASK  0xF0
#define BKPBLOCK_HAS_IMAGE  0x10    /* block data is an XLogRecordBlockImage */
#define BKPBLOCK_HAS_DATA   0x20
#define BKPBLOCK_WILL_INIT  0x40    /* redo will re-init the page */
#define BKPBLOCK_SAME_REL   0x80    /* RelFileLocator omitted, same as
                                     * previous */
 
/*
 * XLogRecordDataHeaderShort/Long are used for the "main data" portion of
 * the record. If the length of the data is less than 256 bytes, the short
 * form is used, with a single byte to hold the length. Otherwise the long
 * form is used.
 *
 * (These structs are currently not used in the code, they are here just for
 * documentation purposes).
 */
typedef struct XLogRecordDataHeaderShort
{
    uint8       id;             /* XLR_BLOCK_ID_DATA_SHORT */
    uint8       data_length;    /* number of payload bytes */
}           XLogRecordDataHeaderShort;
 
#define SizeOfXLogRecordDataHeaderShort (sizeof(uint8) * 2)
 
typedef struct XLogRecordDataHeaderLong
{
    uint8       id;             /* XLR_BLOCK_ID_DATA_LONG */
    /* followed by uint32 data_length, unaligned */
}           XLogRecordDataHeaderLong;
 
#define SizeOfXLogRecordDataHeaderLong (sizeof(uint8) + sizeof(uint32))
 
/*
 * Block IDs used to distinguish different kinds of record fragments. Block
 * references are numbered from 0 to XLR_MAX_BLOCK_ID. A rmgr is free to use
 * any ID number in that range (although you should stick to small numbers,
 * because the WAL machinery is optimized for that case). A few ID
 * numbers are reserved to denote the "main" data portion of the record,
 * as well as replication-supporting transaction metadata.
 *
 * The maximum is currently set at 32, quite arbitrarily. Most records only
 * need a handful of block references, but there are a few exceptions that
 * need more.
 */
#define XLR_MAX_BLOCK_ID            32
 
#define XLR_BLOCK_ID_DATA_SHORT     255
#define XLR_BLOCK_ID_DATA_LONG      254
#define XLR_BLOCK_ID_ORIGIN         253
#define XLR_BLOCK_ID_TOPLEVEL_XID   252
 
#endif                          /* XLOGRECORD_H */