PostgreSQL
Wal
WAL日志的插入

WAL日志的插入

接口函数

一个WAL记录包含

  1. WAL记录类型。（TODO不同的修改有不同的记录方式？）
  2. 这个页的修改方式
  3. 被修改的页的信息。被修改的页通过一个唯一ID标识，也可以有更多的关联数据（“record-specific data associated with the block”）。如果要写full page，就没有关联数据

构建一个WAL记录包含5个核心函数

  • void XLogBeginInsert(void)
    • 初始化相关状态
    • 如果当前无法构建WAL日志（例如在recovery模式），则报错
  • void XLogRegisterBuffer(uint8 block_id, Buffer buf, uint8 flags);
    • 增加了数据块的信息；注册一个buffer的引用，相当于上述WAL日志的第三部分

    • block_id is an arbitrary number used to identify this page reference in the redo routine

    • 在redo阶段，可以根据这些信息找到需要redo的page
    regbuf = &registered_buffers[block_id];
    /*
     * Returns the relfilenode, fork number and block number associated with
     * a buffer
     */
    BufferGetTag(buffer, &regbuf->rnode, &regbuf->forkno, &regbuf->block);
    regbuf->page = BufferGetPage(buffer);
    regbuf->flags = flags;
    regbuf->rdata_tail = (XLogRecData *) &regbuf->rdata_head;
    regbuf->rdata_len = 0;

registered_buffer的结构

typedef struct
{
    /* xxx */

    /* info to re-find the page */
	ForkNumber	forkno;
	BlockNumber block;
	Page		page;

    /* a loop-linked structure to store the data change of each buffer */
    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 */

    /* xxx */

} registered_buffer;

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;
  • void XLogRegisterData(char *data, int len);
    • 向WAL日志中写入任意数据
    • 可多次调用，保证连续。这样在rodo时，就可以得到连续的数据
    rdata = &rdatas[num_rdatas++];
    rdata->data = data;
    rdata->len = len;
  • void XLogRegisterBufData(uint8 block_id, char *data, int len);
    rdata = &rdatas[num_rdatas++];
    rdata->data = data;
    rdata->len = len;

    regbuf = &registered_buffers[block_id];
    regbuf->rdata_tail->next = rdata;
    regbuf->rdata_tail = rdata;
    regbuf->rdata_len += len;

可见，XLogRegisterBufData 和 XLogRegisterData 的核心区别在，前者写入的数据会关联到具体的buffer，而后者没有

  • XLogInsert
    • Insert the record.
    do
    {
        GetFullPageWriteInfo(&RedoRecPtr, &doPageWrites);
        rdt = XLogRecordAssemble(rmid, info, RedoRecPtr, doPageWrites,
                                 &fpw_lsn, &num_fpi);

        EndPos = XLogInsertRecord(rdt, fpw_lsn, curinsert_flags, num_fpi);
	} while (EndPos == InvalidXLogRecPtr);

数据结构汇总

registered_buffers

每一个buffer对应registered_buffers中的一个元素（一个registered buffer）

void
XLogEnsureRecordSpace(int max_block_id, int ndatas)
{
    if (nbuffers > max_registered_buffers)
    {
        registered_buffers = (registered_buffer *)
            repalloc(registered_buffers, sizeof(registered_buffer) * nbuffers);
        max_registered_buffers = nbuffers;
    }
}

具体的插入方式

上述代码中的XLogRecordAssemble和XLogInsertRecord已经概括了具体的插入步骤

XLogRecordAssemble

Assemble a WAL record from the registered data and buffers into an XLogRecData chain

static XLogRecData *
XLogRecordAssemble(RmgrId rmid, uint8 info,
				   XLogRecPtr RedoRecPtr, bool doPageWrites,
				   XLogRecPtr *fpw_lsn, int *num_fpi)
{
    for (block_id = 0; block_id < max_registered_block_id; block_id++)
    {
        if (needs_data)
        {
            rdt_datas_last->next = regbuf->rdata_head;
        }
    }
}
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WAL基础