ibdata中system segment header的初始化

/* Create the trx sys file block in a new allocated file segment */
block = fseg_create(TRX_SYS_SPACE, 0, TRX_SYS + TRX_SYS_FSEG_HEADER,
mtr); //建立segment
buf_block_dbg_add_level(block, SYNC_TRX_SYS_HEADER);

ut_a(block->page.id.page_no() == TRX_SYS_PAGE_NO);

page = buf_block_get_frame(block); //获取内存位置

mlog_write_ulint(page + FIL_PAGE_TYPE, FIL_PAGE_TYPE_TRX_SYS, //写入block 的类型
MLOG_2BYTES, mtr);

...
/* Start counting transaction ids from number 1 up */
mach_write_to_8(sys_header + TRX_SYS_TRX_ID_STORE, 1); // 初始化TRX_SYS_TRX_ID_STORE

/* Reset the rollback segment slots. Old versions of InnoDB
define TRX_SYS_N_RSEGS as 256 (TRX_SYS_OLD_N_RSEGS) and expect
that the whole array is initialized. */
ptr = TRX_SYS_RSEGS + sys_header;
len = ut_max(TRX_SYS_OLD_N_RSEGS, TRX_SYS_N_RSEGS)
* TRX_SYS_RSEG_SLOT_SIZE;//TRX_SYS_OLD_N_RSEGS 为256个
memset(ptr, 0xff, len); //将slot的信息的全部初始化为ff
ptr += len;
ut_a(ptr <= page + (UNIV_PAGE_SIZE - FIL_PAGE_DATA_END));

/* Initialize all of the page. This part used to be uninitialized. */
memset(ptr, 0, UNIV_PAGE_SIZE - FIL_PAGE_DATA_END + page - ptr); //将剩下的空间设置为0x00

mlog_log_string(sys_header, UNIV_PAGE_SIZE - FIL_PAGE_DATA_END
+ page - sys_header, mtr);

/* Create the first rollback segment in the SYSTEM tablespace */
slot_no = trx_sysf_rseg_find_free(mtr, false, 0);
page_no = trx_rseg_header_create(TRX_SYS_SPACE, univ_page_size,
ULINT_MAX, slot_no, mtr); //将第一个slot固定在ibdata中
完成了这一步过后ibdata的 block 5 就初始化完了，而且我们看到所有的rollback segment slots 都初始化完成(源码所示有256个，实际上最多只会有128个，其中0号solt固定在ibdata中)，注意这里的槽大小是TRX_SYS_RSEG_SLOT_SIZE设置的大小为8字节，4字节space id ,4字节 page no，它们会指向 rollback segment header所在的位置。

下面是system segment header的定位：

/** Transaction system header */
/*------------------------------------------------------------- @{ */
#define TRX_SYS_TRX_ID_STORE 0 /*!< the maximum trx id or trx
number modulo
TRX_SYS_TRX_ID_UPDATE_MARGIN
written to a file page by any
transaction; the assignment of
transaction ids continues from
this number rounded up by
TRX_SYS_TRX_ID_UPDATE_MARGIN
plus
TRX_SYS_TRX_ID_UPDATE_MARGIN
when the database is(http://www.amjmh.com/v/)
started */ //最大的事物ID，下次实例启动会加上TRX_SYS_TRX_ID_UPDATE_MARGIN启动
#define TRX_SYS_FSEG_HEADER 8 /*!< segment header for the
tablespace segment the trx
system is created into */
#define TRX_SYS_RSEGS (8 + FSEG_HEADER_SIZE)
/*!< the start of the array of
rollback segment specification
slots *///指向rollback segment header的槽