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Difference between DBFile Sequential and Scattered Reads

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.

db file scattered read:

Similar to db file sequential reads, except that the session is reading multiple data blocks and scatters them into different discontinuous buffers in the SGA. This statistic is NORMALLY indicating disk contention on full table scans. Rarely, data from full table scans could be fitted into a contiguous buffer area, these waits would then show up as sequential reads instead of scattered reads. However, scattered read will never read just one block (p3 is always >= 2).

The following query shows average wait time for sequential versus
scattered reads:

SQL> tti "AVERAGE WAIT TIME FOR READ REQUESTS"
SQL>
SQL> SELECT a.average_wait "SEQ READ", b.average_wait "SCAT READ"
  2    FROM sys.v_$system_event a, sys.v_$system_event b
  3   WHERE a.event = 'db file sequential read'
  4     AND b.event = 'db file scattered read';

  SEQ READ  SCAT READ
---------- ----------
       .74        1.6