In Oracle 8i and below, Rollback Segments provide read consistency and the ability to rollback transactions. In Oracle 9i, Undo segments can be used to provide this functionality. The advantage of using Automatic Undo Management is that it relieves the DBA of manually creating, sizing and monitoring the rollback segments in the database.
Both "db file sequential read" and "db file scattered read" events signify time waited for I/O read requests to complete. Time is reported in 100's of a second for Oracle 8i releases and below, and 1000's of a second for Oracle 9i and above. Most people confuse these events with each other as they think of how data is read from disk. Instead they should think of how data is read into the SGA buffer cache or user PGA memory.
db file sequential read:
A sequential read operation reads data into contiguous memory (usually a single-block read with p3=1, but can be multiple blocks). Single block I/Os are usually the result of using indexes. This event is also used for rebuilding the controlfile and reading datafile headers (P2=1). In general, this event is indicative of disk contention on index reads.
Database listeners can be stopped remotely from any operating system account (not just oracle). To prevent this, all listeners on production machines MUST be password protected.
by Nidhi Jain
Prior to Oracle 8i, a listener was statically configured (listener.ora) to service a given set of SIDs. From 8i, PMON dynamically registers a database service with the listener.