4 * Definitions for the WAL record format.
6 * Portions Copyright (c) 1996-2014, PostgreSQL Global Development Group
7 * Portions Copyright (c) 1994, Regents of the University of California
9 * src/include/access/xlogrecord.h
14 #include "access/rmgr.h"
15 #include "access/xlogdefs.h"
16 #include "storage/block.h"
17 #include "storage/relfilenode.h"
18 #include "utils/pg_crc.h"
21 * The overall layout of an XLOG record is:
22 * Fixed-size header (XLogRecord struct)
23 * XLogRecordBlockHeader struct
24 * XLogRecordBlockHeader struct
26 * XLogRecordDataHeader[Short|Long] struct
32 * There can be zero or more XLogRecordBlockHeaders, and 0 or more bytes of
33 * rmgr-specific data not associated with a block. XLogRecord structs
34 * always start on MAXALIGN boundaries in the WAL files, but the rest of
35 * the fields are not aligned.
37 * The XLogRecordBlockHeader, XLogRecordDataHeaderShort and
38 * XLogRecordDataHeaderLong structs all begin with a single 'id' byte. It's
39 * used to distinguish between block references, and the main data structs.
41 typedef struct XLogRecord
43 uint32 xl_tot_len; /* total len of entire record */
44 TransactionId xl_xid; /* xact id */
45 XLogRecPtr xl_prev; /* ptr to previous record in log */
46 uint8 xl_info; /* flag bits, see below */
47 RmgrId xl_rmid; /* resource manager for this record */
48 /* 2 bytes of padding here, initialize to zero */
49 pg_crc32 xl_crc; /* CRC for this record */
51 /* XLogRecordBlockHeaders and XLogRecordDataHeader follow, no padding */
55 #define SizeOfXLogRecord (offsetof(XLogRecord, xl_crc) + sizeof(pg_crc32))
58 * The high 4 bits in xl_info may be used freely by rmgr. The
59 * XLR_SPECIAL_REL_UPDATE bit can be passed by XLogInsert caller. The rest
60 * are set internally by XLogInsert.
62 #define XLR_INFO_MASK 0x0F
63 #define XLR_RMGR_INFO_MASK 0xF0
66 * If a WAL record modifies any relation files, in ways not covered by the
67 * usual block references, this flag is set. This is not used for anything
68 * by PostgreSQL itself, but it allows external tools that read WAL and keep
69 * track of modified blocks to recognize such special record types.
71 #define XLR_SPECIAL_REL_UPDATE 0x01
74 * Header info for block data appended to an XLOG record.
76 * Note that we don't attempt to align the XLogRecordBlockHeader struct!
77 * So, the struct must be copied to aligned local storage before use.
78 * 'data_length' is the length of the payload data associated with this,
79 * and includes the possible full-page image, and rmgr-specific data. It
80 * does not include the XLogRecordBlockHeader struct itself.
82 typedef struct XLogRecordBlockHeader
84 uint8 id; /* block reference ID */
85 uint8 fork_flags; /* fork within the relation, and flags */
86 uint16 data_length; /* number of payload bytes (not including page
89 /* If BKPBLOCK_HAS_IMAGE, an XLogRecordBlockImageHeader struct follows */
90 /* If !BKPBLOCK_SAME_REL is not set, a RelFileNode follows */
91 /* BlockNumber follows */
92 } XLogRecordBlockHeader;
94 #define SizeOfXLogRecordBlockHeader (offsetof(XLogRecordBlockHeader, data_length) + sizeof(uint16))
97 * Additional header information when a full-page image is included
98 * (i.e. when BKPBLOCK_HAS_IMAGE is set).
100 * As a trivial form of data compression, the XLOG code is aware that
101 * PG data pages usually contain an unused "hole" in the middle, which
102 * contains only zero bytes. If hole_length > 0 then we have removed
103 * such a "hole" from the stored data (and it's not counted in the
104 * XLOG record's CRC, either). Hence, the amount of block data actually
105 * present is BLCKSZ - hole_length bytes.
107 typedef struct XLogRecordBlockImageHeader
109 uint16 hole_offset; /* number of bytes before "hole" */
110 uint16 hole_length; /* number of bytes in "hole" */
111 } XLogRecordBlockImageHeader;
113 #define SizeOfXLogRecordBlockImageHeader sizeof(XLogRecordBlockImageHeader)
116 * Maximum size of the header for a block reference. This is used to size a
117 * temporary buffer for constructing the header.
119 #define MaxSizeOfXLogRecordBlockHeader \
120 (SizeOfXLogRecordBlockHeader + \
121 SizeOfXLogRecordBlockImageHeader + \
122 sizeof(RelFileNode) + \
126 * The fork number fits in the lower 4 bits in the fork_flags field. The upper
127 * bits are used for flags.
129 #define BKPBLOCK_FORK_MASK 0x0F
130 #define BKPBLOCK_FLAG_MASK 0xF0
131 #define BKPBLOCK_HAS_IMAGE 0x10 /* block data is an XLogRecordBlockImage */
132 #define BKPBLOCK_HAS_DATA 0x20
133 #define BKPBLOCK_WILL_INIT 0x40 /* redo will re-init the page */
134 #define BKPBLOCK_SAME_REL 0x80 /* RelFileNode omitted, same as previous */
137 * XLogRecordDataHeaderShort/Long are used for the "main data" portion of
138 * the record. If the length of the data is less than 256 bytes, the short
139 * form is used, with a single byte to hold the length. Otherwise the long
142 * (These structs are currently not used in the code, they are here just for
143 * documentation purposes).
145 typedef struct XLogRecordDataHeaderShort
147 uint8 id; /* XLR_BLOCK_ID_DATA_SHORT */
148 uint8 data_length; /* number of payload bytes */
149 } XLogRecordDataHeaderShort;
151 #define SizeOfXLogRecordDataHeaderShort (sizeof(uint8) * 2)
153 typedef struct XLogRecordDataHeaderLong
155 uint8 id; /* XLR_BLOCK_ID_DATA_LONG */
156 /* followed by uint32 data_length, unaligned */
157 } XLogRecordDataHeaderLong;
159 #define SizeOfXLogRecordDataHeaderLong (sizeof(uint8) + sizeof(uint32))
162 * Block IDs used to distinguish different kinds of record fragments. Block
163 * references are numbered from 0 to XLR_MAX_BLOCK_ID. A rmgr is free to use
164 * any ID number in that range (although you should stick to small numbers,
165 * because the WAL machinery is optimized for that case). A couple of ID
166 * numbers are reserved to denote the "main" data portion of the record.
168 * The maximum is currently set at 32, quite arbitrarily. Most records only
169 * need a handful of block references, but there are a few exceptions that
172 #define XLR_MAX_BLOCK_ID 32
174 #define XLR_BLOCK_ID_DATA_SHORT 255
175 #define XLR_BLOCK_ID_DATA_LONG 254
177 #endif /* XLOGRECORD_H */