There are 2 main I/O related wait events in Oracle: “db file scattered read” and “db file sequential read”. In this post we will understand the “db file sequential read” wait event (to read about “db file scattered read” go to the db file scattered read post)

A server process is waiting on “db file sequential read” wait event after it performs a single block I/O operation and it is waiting for the operating system to complete it.

A single block I/O operation occurs when a server process request a single block for the operating system. This happens during operations such as index unique or range scans, table access by rowid, etc.

Seeing this wait event in the top wait events is expected and doesn’t mean that something is wrong. Remember, users are working and reading indexes or table blocks from the disk is common. However, if we see that this event is very high and consuming unreasonable time, we need to check our SQL statement, they might need tuning, or might have unnecessary hints such as the “index” hint.

To read about analyzing wait events in an AWR report, see my post analyzing oracle awr reports – top 5 events.