PostgreSQL WAL解析:构建WAL记录准备

时间:2022-07-28
本文章向大家介绍PostgreSQL WAL解析:构建WAL记录准备,主要内容包括其使用实例、应用技巧、基本知识点总结和需要注意事项,具有一定的参考价值,需要的朋友可以参考一下。

以heap_insert为例,简述WAL的插入过程。

在构建WAL日志记录的过程中主要涉及2个数据变量:static XLogRecData *rdatas数组和static registered_buffer *registered_buffers数组。这两个数组分别用来保存WAL数据和管理rdatas链表。

主要涉及3个重要的函数:XLogRegisterData、XLogRegisterBuffer和XLogRegisterBufData。这三个函数的作用分别是将WAL记录的特殊结构体数据注册到WAL,比如heap_insert中的xl_heap_insert结构体;将涉及到的buf注册到wal记录,比如heap_insert中page页赋予regbuf->page;将元组内容注册到WAL记录,比如insert语句的元组数据等。

下面首先介绍相关数据结构。

1、数据结构

HeapTupleData

typedef struct HeapTupleData
{
    uint32 t_len; /* length of *t_data */
    ItemPointerData t_self; /* SelfItemPointer */
    Oid t_tableOid; /* table the tuple came from */
    HeapTupleHeader t_data; /* -> tuple header and data */
} HeapTupleData;

xl_heap_header

/*
 * We don't store the whole fixed part (HeapTupleHeaderData) of an inserted
 * or updated tuple in WAL; we can save a few bytes by reconstructing the
 * fields that are available elsewhere in the WAL record, or perhaps just
 * plain needn't be reconstructed.  These are the fields we must store.
 * NOTE: t_hoff could be recomputed, but we may as well store it because
 * it will come for free due to alignment considerations.
 */
typedef struct xl_heap_header
{
    uint16 t_infomask2;
    uint16 t_infomask;
    uint8 t_hoff;
} xl_heap_header;

xl_heap_insert

/* This is what we need to know about insert */
typedef struct xl_heap_insert
{
    OffsetNumber offnum; /* inserted tuple's offset */
    uint8 flags;
 
/* xl_heap_header & TUPLE DATA in backup block 0 */
} xl_heap_insert;

XLogRecData

/*
 * The functions in xloginsert.c construct a chain of XLogRecData structs
 * to represent the final WAL record.
 */
typedef struct XLogRecData
{
    struct XLogRecData *next; /* next struct in chain, or NULL */
    char    *data; /* start of rmgr data to include */
    uint32 len; /* length of rmgr data to include */
} XLogRecData;

registered_buffer

/*
 * For each block reference registered with XLogRegisterBuffer, we fill in
 * a registered_buffer struct.
 */
typedef struct
{
bool in_use; /* is this slot in use? */
uint8 flags; /* REGBUF_* flags */
RelFileNode rnode; /* identifies the relation and block */
ForkNumber forkno;
BlockNumber block;
Page page; /* page content */
uint32 rdata_len; /* total length of data in rdata chain */
XLogRecData *rdata_head; /* head of the chain of data registered with this block */
XLogRecData *rdata_tail; /* last entry in the chain, or &rdata_head if empty */
XLogRecData bkp_rdatas[2]; /* temporary rdatas used to hold references to
 * backup block data in XLogRecordAssemble() */
/* buffer to store a compressed version of backup block image */
char compressed_page[PGLZ_MAX_BLCKSZ];
} registered_buffer;

2、heap_insert涉及WAL的流程

第一步中,得到如下结果,mainrdata_last保存rdata[0],存储的是xl_heap_insert结构:

第二步,得到如下结果,取registered_buffer[0],其rdata_head->next指向rdata[1],存储tuple记录的头信息:

接着进入第三步,取rdata[2],将其放到rdata[1]->next中,即加入registered_buffers[0]的rdata_head链表中,存储TUPLE值:

以上是构建WAL记录的准备阶段,下一节介绍WAL的构建及其通用结构。