预写式日志相关结构体

xlog文件内容结构

PageHeaderData
/*
 * disk page organization
 *
 * space management information generic to any page
 *
 * pd_lsn - identifies xlog record for last change to this page.
 * pd_checksum - page checksum, if set.
 * pd_flags - flag bits.
 * pd_lower - offset to start of free space.
 * pd_upper - offset to end of free space.
 * pd_special - offset to start of special space.
 * pd_pagesize_version - size in bytes and page layout version number.
 * pd_prune_xid - oldest XID among potentially prunable tuples on page.
 *
 * The LSN is used by the buffer manager to enforce the basic rule of WAL:
 * "thou shalt write xlog before data".  A dirty buffer cannot be dumped
 * to disk until xlog has been flushed at least as far as the page's LSN.
 *
 * pd_checksum stores the page checksum, if it has been set for this page;
 * zero is a valid value for a checksum. If a checksum is not in use then
 * we leave the field unset. This will typically mean the field is zero
 * though non-zero values may also be present if databases have been
 * pg_upgraded from releases prior to 9.3, when the same byte offset was
 * used to store the current timelineid when the page was last updated.
 * Note that there is no indication on a page as to whether the checksum
 * is valid or not, a deliberate design choice which avoids the problem
 * of relying on the page contents to decide whether to verify it. Hence
 * there are no flag bits relating to checksums.
 *
 * pd_prune_xid is a hint field that helps determine whether pruning will be
 * useful.  It is currently unused in index pages.
 *
 * The page version number and page size are packed together into a single
 * uint16 field.  This is for historical reasons: before PostgreSQL 7.3,
 * there was no concept of a page version number, and doing it this way
 * lets us pretend that pre-7.3 databases have page version number zero.
 * We constrain page sizes to be multiples of 256, leaving the low eight
 * bits available for a version number.
 *
 * Minimum possible page size is perhaps 64B to fit page header, opaque space
 * and a minimal tuple; of course, in reality you want it much bigger, so
 * the constraint on pagesize mod 256 is not an important restriction.
 * On the high end, we can only support pages up to 32KB because lp_off/lp_len
 * are 15 bits.
 */


typedef struct PageHeaderData
{
/* XXX LSN is member of *any* block, not only page-organized ones */
PageXLogRecPtr pd_lsn;/* LSN: next byte after last byte of xlog
* record for last change to this page */
uint16 pd_checksum; /* checksum */
uint16 pd_flags; /* flag bits, see below */
LocationIndex pd_lower;/* offset to start of free space */
LocationIndex pd_upper;/* offset to end of free space */
LocationIndex pd_special;/* offset to start of special space */
uint16 pd_pagesize_version;
TransactionId pd_prune_xid; /* oldest prunable XID, or zero if none */
ItemIdData pd_linp[1]; /* beginning of line pointer array */
} PageHeaderData;


typedef PageHeaderData *PageHeader;






XLogRecord
/*
 * The overall layout of an XLOG record is:
 * Fixed-size header (XLogRecord struct)
 * rmgr-specific data
 * BkpBlock
 * backup block data
 * BkpBlock
 * backup block data
 * ...
 *
 * where there can be zero to four backup blocks (as signaled by xl_info flag
 * bits).  XLogRecord structs always start on MAXALIGN boundaries in the WAL
 * files, and we round up SizeOfXLogRecord so that the rmgr data is also
 * guaranteed to begin on a MAXALIGN boundary.  However, no padding is added
 * to align BkpBlock structs or backup block data.
 *
 * NOTE: xl_len counts only the rmgr data, not the XLogRecord header,
 * and also not any backup blocks.  xl_tot_len counts everything.  Neither
 * length field is rounded up to an alignment boundary.
 */
typedef struct XLogRecord
{
uint32 xl_tot_len; /* total len of entire record */
TransactionId xl_xid;/* xact id */
uint32 xl_len; /* total len of rmgr data */
uint8 xl_info;/* flag bits, see below */
RmgrId xl_rmid; /* resource manager for this record */
/* 2 bytes of padding here, initialize to zero */
XLogRecPtr xl_prev; /* ptr to previous record in log */
pg_crc32 xl_crc; /* CRC for this record */


/* If MAXALIGN==8, there are 4 wasted bytes here */


/* ACTUAL LOG DATA FOLLOWS AT END OF STRUCT */


} XLogRecord;


PageXLogRecPtr
/*
 * For historical reasons, the 64-bit LSN value is stored as two 32-bit
 * values.
 */
typedef struct
{
uint32 xlogid; /* high bits */
uint32 xrecoff; /* low bits */
} PageXLogRecPtr;




XLogRecData
/*
 * The rmgr data to be written by XLogInsert() is defined by a chain of
 * one or more XLogRecData structs.  (Multiple structs would be used when
 * parts of the source data aren't physically adjacent in memory, or when
 * multiple associated buffers need to be specified.)
 *
 * If buffer is valid then XLOG will check if buffer must be backed up
 * (ie, whether this is first change of that page since last checkpoint).
 * If so, the whole page contents are attached to the XLOG record, and XLOG
 * sets XLR_BKP_BLOCK(N) bit in xl_info.  Note that the buffer must be pinned
 * and exclusive-locked by the caller, so that it won't change under us.
 * NB: when the buffer is backed up, we DO NOT insert the data pointed to by
 * this XLogRecData struct into the XLOG record, since we assume it's present
 * in the buffer.  Therefore, rmgr redo routines MUST pay attention to
 * XLR_BKP_BLOCK(N) to know what is actually stored in the XLOG record.
 * The N'th XLR_BKP_BLOCK bit corresponds to the N'th distinct buffer
 * value (ignoring InvalidBuffer) appearing in the rdata chain.
 *
 * When buffer is valid, caller must set buffer_std to indicate whether the
 * page uses standard pd_lower/pd_upper header fields.  If this is true, then
 * XLOG is allowed to omit the free space between pd_lower and pd_upper from
 * the backed-up page image.  Note that even when buffer_std is false, the
 * page MUST have an LSN field as its first eight bytes!
 *
 * Note: data can be NULL to indicate no rmgr data associated with this chain
 * entry.  This can be sensible (ie, not a wasted entry) if buffer is valid.
 * The implication is that the buffer has been changed by the operation being
 * logged, and so may need to be backed up, but the change can be redone using
 * only information already present elsewhere in the XLOG entry.
 */
typedef struct XLogRecData
{
char   *data;/* start of rmgr data to include */
uint32 len; /* length of rmgr data to include */
Buffer buffer; /* buffer associated with data, if any */
bool buffer_std;/* buffer has standard pd_lower/pd_upper */
struct XLogRecData *next;/* next struct in chain, or NULL */
} XLogRecData;


typedef struct xl_xact_abort
{
TimestampTz xact_time;/* time of abort */
int nrels;/* number of RelFileNodes */
int nsubxacts;/* number of subtransaction XIDs */
/* Array of RelFileNode(s) to drop at abort */
RelFileNode xnodes[1];/* VARIABLE LENGTH ARRAY */
/* ARRAY OF ABORTED SUBTRANSACTION XIDs FOLLOWS */
} xl_xact_abort;


RmgrData


/*
 * Method table for resource managers.
 *
 * This struct must be kept in sync with the PG_RMGR definition in
 * rmgr.c.
 *
 * RmgrTable[] is indexed by RmgrId values (see rmgrlist.h).
 */
typedef struct RmgrData
{
const char *rm_name;
void (*rm_redo) (XLogRecPtr lsn, struct XLogRecord *rptr);
void (*rm_desc) (StringInfo buf, uint8 xl_info, char *rec);
void (*rm_startup) (void);
void (*rm_cleanup) (void);
} RmgrData;




BkpBlock
/*
 * Header info for a backup block appended to an XLOG record.
 *
 * 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 hole_length > 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 following the BkpBlock struct is BLCKSZ - hole_length bytes.
 *
 * Note that we don't attempt to align either the BkpBlock struct or the
 * block's data.  So, the struct must be copied to aligned local storage
 * before use.
 */
typedef struct BkpBlock
{
RelFileNode node;/* relation containing block */
ForkNumber fork; /* fork within the relation */
BlockNumber block;/* block number */
uint16 hole_offset; /* number of bytes before "hole" */
uint16 hole_length; /* number of bytes in "hole" */


/* ACTUAL BLOCK DATA FOLLOWS AT END OF STRUCT */
} BkpBlock;




RelFileNode
/*
 * RelFileNode must provide all that we need to know to physically access
 * a relation, with the exception of the backend ID, which can be provided
 * separately. Note, however, that a "physical" relation is comprised of
 * multiple files on the filesystem, as each fork is stored as a separate
 * file, and each fork can be divided into multiple segments. See md.c.
 *
 * spcNode identifies the tablespace of the relation.  It corresponds to
 * pg_tablespace.oid.
 *
 * dbNode identifies the database of the relation.  It is zero for
 * "shared" relations (those common to all databases of a cluster).
 * Nonzero dbNode values correspond to pg_database.oid.
 *
 * relNode identifies the specific relation.  relNode corresponds to
 * pg_class.relfilenode (NOT pg_class.oid, because we need to be able
 * to assign new physical files to relations in some situations).
 * Notice that relNode is only unique within a database in a particular
 * tablespace.
 *
 * Note: spcNode must be GLOBALTABLESPACE_OID if and only if dbNode is
 * zero.  We support shared relations only in the "global" tablespace.
 *
 * Note: in pg_class we allow reltablespace == 0 to denote that the
 * relation is stored in its database's "default" tablespace (as
 * identified by pg_database.dattablespace).  However this shorthand
 * is NOT allowed in RelFileNode structs --- the real tablespace ID
 * must be supplied when setting spcNode.
 *
 * Note: in pg_class, relfilenode can be zero to denote that the relation
 * is a "mapped" relation, whose current true filenode number is available
 * from relmapper.c.  Again, this case is NOT allowed in RelFileNodes.
 *
 * Note: various places use RelFileNode in hashtable keys.  Therefore,
 * there *must not* be any unused padding bytes in this struct.  That
 * should be safe as long as all the fields are of type Oid.
 */
typedef struct RelFileNode
{
Oid spcNode;/* tablespace */
Oid dbNode;/* database */
Oid relNode;/* relation */
} RelFileNode;