Sequential Read vs Scattered Read

DB file sequential read waits:

single block read into one SGA buffer

contention on index reads

V$SESSION_WAIT parameter P3 (The number of blocks) should be 1

DB file scattered read waits:

multiblock read into many discontinuous SGA buffers

contention on full table scans

V$SESSION_WAIT parameter P3 (The number of blocks) should be greater than 1

Direct read:

single or multiblock read into the PGA, bypassing the SGA

Parameter:
db_file_multiblock_read_count which might affect the chose of read method, however, starting at Oralce 10g, it is auto tuned parameter if it is not set explicitly.

Reference:
http://book.csdn.net/bookfiles/626/10062620033.shtml
http://www.dba-oracle.com/art_builder_io_speed.htm

http://www.orafaq.com/node/1

http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/instance_tune.htm#sthref720