InnoDB has its own per-table cache, variously called a table definition cache or data dictionary, which you cannot configure. When InnoDB opens a table, it adds a corresponding object to the data dictionary. Each table can take up 4 KB or more of memory (although much less space is required in MySQL 5.1). Tables are not removed from the data dictionary when they are closed.----表的定义都缓存在数据字典中
The main performance issue—besides memory requirements—is opening and computing statistics for the tables, which is expensive because it requires a lot of I/O. In contrast to MyISAM, InnoDB doesn’t store statistics in the tables permanently; it recomputes them each time it starts. This operation is serialized by a global mutex in current versions of MySQL, so it can’t be done in parallel. ----当打开每个表的时候,innodb会重新计算表的相关统计数据,而且是串行的,所以比较慢. If you have a lot of tables, your server can take hours to start and fully warm up, during which time it might not be doing much other than waiting for one I/O operation after another. We mention this to make sure you know about it, even though there’s nothing you can do to change it. It’s normally a problem only when you have many (thousands or tens of thousands) large tables, which cause the process to be I/O-bound.
If you use InnoDB’s innodb_file_per_table option ,there’s also a separate limit on the number of .ibd files InnoDB can keep open at any time. This is handled by the InnoDB storage engine, not the MySQL server, and is controlled by innodb_open_files. InnoDB doesn’t open files the same way MyISAM does: whereas MyISAM uses the table cache to hold file descriptors for open tables, in InnoDB there is no direct relationship between open tables and open files. InnoDB uses a single, global file descriptor for each .ibd file. If you can afford it, it’s best to set innodb_open_files large enough that the server can keep all .ibd files open simultaneously.
