IO_COMPLETION Used to indicate a wait for I/O for operation (typically synchronous) like sorts and various situations where the engine needs to do a synchronous I/O
If wait times are high then you have a disk I/O bottleneck. The problem will be determining what type of operation and where the bottleneck exists. For sorts, it is on the storage system associated with tempdb. Note that database page I/O does not use this wait type. Instead look at PAGEIOLATCH waits.
Occurs while waiting for I/O operations to complete. This wait type generally represents non-data page I/Os.
Data page I/O completion waits appear as PAGEIOLATCH_* waits.
Waits on this wait type can result from inefficient SQL statements. Tuning the SQL statement will allow it to execute while reading less data, which will reduce wait times on this event.
参考文档：
http://blog.sqlauthority.com/2011/02/10/sql-server-io_completion-wait-type-day-10-of-28/
原文：http://www.cnblogs.com/ljhdo/p/4911878.html