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
- backup block data in XLogRecordAssemble() /
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: