在之前版本，碎片回收操作是主线程的一部分，这经定期调度的方式运行，但会阻塞数据库的其他操作；到5.5以后，可以将这个线程独立出来 ；方法：innodb_purge_threads =1
这个应该能让碎片回收得更及时而且不影响其他线程的操作；
Improved Purge Scheduling
 - The InnoDB purge operation is a type of garbage collection that runs
periodically. In previous versions, the purge was part of the master thread, meaning that it could
block other database operations when running. In MySQL 5.5 this operation can run in its own
thread, allowing for more concurrency. Users can control whether the purge operation is split into its
own thread with the innodb_purge_threads configuration option, which can be set to 0 (the
default) or 1 (for a single separate purge thread).