This is an Exadata wait event that occurs when fast full index scans are offloaded to the storage cells. This event is related to cell smart table scan except that the object is an index. This event does not show up very frequently in Exadata probably because it is very good at performing full table scans and index reads may actually be less efficient. Also, only full fast scans (not range or full scans) of indexes are eligible for smart scans, which reduces the likelihood of seeing this event.
Solutions
Offloading the index scans to the storage cells within an Exadata machine is probably a good thing, so there may be no tuning required. However, if you have a query that waits extensively on this wait event, it may mean that query tuning is required so it utilizes indexes more effectively. Also review the Objects tab in Ignite to understand which is causing the most wait time.
本文转自maclean_007 51CTO博客，原文链接:http://blog.51cto.com/maclean/1277895
