SQL Server Locks: PAGEIOLATCH_EX

PAGEIOLATCH_EX is a wait event often seen in SQL Server related to I/O, It actually corresponds to an "Exclusive I/O page latch".
This wait event occurs when a user needs a page that is not in the buffer cache, SQL Server has to first allocate a buffer page, and then puts a exclusive PAGEIOLATCH_EX latch on the buffer while the page is transferred from disk to cache. After the write to cache finishes, the PAGEIOLATCH_EX latch is released.
N.B. Waits of this type are to be expected whenever SQL Server carries out I/O operations, but if you see excessive waits of this type then it may indicate problems with the disk subsystem.