In SQL Server, a latch is a short-term lightweight synchronization object.
Buffer latches including the PAGEIOLATCH_EX wait type are used to synchronize access to BUF structures and associated pages in the SQL Server database. The most frequently occurring buffer latching situation is when serialization is required on a buffer page. These buffer latches are not held for the complete period of the transaction. The PAGEIO latches are a subset of BUF latches used when the buffer and associated data page or the index page is in the middle of an IO operation. PAGEIOLATCH wait types are used for disk-to-memory transfers.
PAGEIOLATCH_EX (exclusive mode page IO latch request)
When a SQL Server user needs a page that is not in buffer cache, the database must first allocate a buffer page, and then puts an exclusive PAGEIOLATCH_EX latch on the buffer while the page is transferred from disk to cache. During this operation SQL Server puts a PAGEIOLATCH_EX request on the buffer on behalf of the user. After the write to cache is complete, the PAGEIOLATCH_EX latch is released.
Excessive PAGEIOLATCH_EX waits occur when data is transfered from disk to memory for update operations and these transfers take time to complete.
This may be an indication of disk contention or other disk subsystem bottlenecks.