How To Size UNDO Tablespace For Automatic Undo Management

Sizing an UNDO tablespace requires three pieces of data.

1. (UR) UNDO_RETENTION in seconds
2. (UPS) Number of undo data blocks generated per second
3. (DBS) Overhead varies based on extent and file size (db_block_size)

The undo space needed is calculated as:

UndoSpace = UR * (UPS * DBS)

Two of the pieces of information can be obtained from the instance configuration: UNDO_RETENTION and DB_BLOCK_SIZE. The third piece of the formula requires a query being run against the database. The maximum number of undo blocks generated per second can be acquired from V$UNDOSTAT.

The following formula calculates the peak undo blocks generated per second:

SQL> SELECT undoblks/((end_time-begin_time)*86400) “Peak Undo Block Generation” FROM v$undostat WHERE undoblks=(SELECT MAX(undoblks) FROM v$undostat);
Column END_TIME and BEGIN_TIME are DATE data types. When DATE data types are subtracted, the resulting value is the # of days between both dates. To convert days to seconds, you multiply by 86400, the number of seconds in a day (24 hours * 60 minutes * 60 seconds).

The following query calculates the number of bytes needed to handle a peak undo activity:

SQL> SELECT (UR * (UPS * DBS)) AS "Bytes"
     FROM (SELECT value AS UR FROM v$parameter WHERE name = 'undo_retention'),
          (SELECT undoblks/((end_time-begin_time)*86400) AS UPS
           FROM v$undostat
           WHERE undoblks = (SELECT MAX(undoblks) FROM v$undostat)),
          (SELECT block_size AS DBS
           FROM dba_tablespaces
           WHERE tablespace_name = (SELECT UPPER(value) FROM v$parameter WHERE name = 'undo_tablespace'));

For 10g and Higher Versions where Tuned undo retention is being used,please use below query:

SQL>SELECT (UR * (UPS * DBS)) AS "Bytes"
FROM (select max(tuned_undoretention) AS UR from v$undostat),
(SELECT undoblks/((end_time-begin_time)*86400) AS UPS
FROM v$undostat
WHERE undoblks = (SELECT MAX(undoblks) FROM v$undostat)),
(SELECT block_size AS DBS
FROM dba_tablespaces
WHERE tablespace_name = (SELECT UPPER(value) FROM v$parameter WHERE name = 'undo_tablespace'));