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/* * xlogrecord.h * * Definitions for the WAL record format. * * Portions Copyright (c) 1996-2016, 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/relfilenode.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 bit can be passed by XLogInsert caller. The rest * are set internally by XLogInsert. */ #define XLR_INFO_MASK 0x0F #define XLR_RMGR_INFO_MASK 0xF0 /* * 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 /* * 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 RelFileNode 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). * * As a trivial form of data compression, the XLOG code is aware that * PG data pages usually contain an unused "hole" in the middle, which * contains only zero bytes. If the length of "hole" > 0 then we have removed * such a "hole" 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 - the length of "hole" bytes. * * When wal_compression is enabled, a full page image which "hole" was * removed is additionally compressed using PGLZ compression algorithm. * 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_IS_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_IS_COMPRESSED 0x02 /* page image is compressed */ /* * 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(RelFileNode) + \ 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 /* RelFileNode 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 couple of ID * numbers are reserved to denote the "main" data portion of the record. * * 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 #endif /* XLOGRECORD_H */