MySQL：Innodb表 Data free 的计算概要

简单记录一下，因为看了一下Data free的计算还算准确。不是统计值 大概是空闲extent的大小。

ST_FIELD_INFO tables_fields_info[]=
{...
  {"DATA_FREE", MY_INT64_NUM_DECIMAL_DIGITS, MYSQL_TYPE_LONGLONG, 0,
   (MY_I_S_MAYBE_NULL | MY_I_S_UNSIGNED), "Data_free", OPEN_FULL_TABLE},
...
}
tables_fields_info[12]就是Data_free

ST_SCHEMA_TABLE schema_tables[]=
{...
 {"TABLES", tables_fields_info, create_schema_table, 
   get_all_tables, make_old_format, get_schema_tables_record, 1, 2, 0,
   OPTIMIZE_I_S_TABLE},
...
}

填充方式get_schema_tables_record函数

其中有一句
table->field[12]->store((longlong) file->stats.delete_length, TRUE);
实际就是 file->stats.delete_length

其中 file->stats.delete_length调用进行取值

ha_innobase::info_low

其中包含
avail_space = fsp_get_available_space_in_free_extents(ib_table->space);
stats.delete_length = avail_space * 1024;

uintmax_t
fsp_get_available_space_in_free_extents(
	const fil_space_t*	space)
{
	ut_ad(space->n_pending_ops > 0);

	ulint	size_in_header = space->size_in_header;//总的页面大小
	if (size_in_header < FSP_EXTENT_SIZE) { //碎片页 直接返回0
		return(0);		/* TODO: count free frag pages and
					return a value based on that */
	}

	/* Below we play safe when counting free extents above the free limit:
	some of them will contain extent descriptor pages, and therefore
	will not be free extents */
	ut_ad(size_in_header >= space->free_limit);
	ulint	n_free_up =
		(size_in_header - space->free_limit) / FSP_EXTENT_SIZE; //(总的页面大小-已经初始化大小)/EXTENT SIZE

	page_size_t	page_size(space->flags);
	if (n_free_up > 0) {
		n_free_up--;
		n_free_up -= n_free_up / (page_size.physical()
					  / FSP_EXTENT_SIZE); //减去extent管理块?
	}

	/* We reserve 1 extent + 0.5 % of the space size to undo logs
	and 1 extent + 0.5 % to cleaning operations; NOTE: this source
	code is duplicated in the function above! */

	ulint	reserve = 2 + ((size_in_header / FSP_EXTENT_SIZE) * 2) / 200;
	ulint	n_free = space->free_len + n_free_up;

	if (reserve > n_free) {
		return(0);
	}

	return(static_cast<uintmax_t>(n_free - reserve)
	       * FSP_EXTENT_SIZE * (page_size.physical() / 1024));
}