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));
}

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