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Saturday, 2 August 2014

How to Setup Optimal Undo Parameter in Oracle Database

Automatic undo management allows the DBA to specify how long undo information should be retained after commit, which prevent "snapshot too old" errors on long running queries.
This is done by setting the UNDO_RETENTION parameter.  The default is 900 seconds (5 minutes), and you can set this parameter to guarantee that Oracle keeps undo logs for extended periods of time.
Rather than having to define and manage rollback segments, you can simply define an Undo tablespace and let Oracle take care of the rest.
Turning on automatic undo management is easy.  All you need to do is create an undo tablespace and set UNDO_MANAGEMENT = AUTO.
However it is worth to tune the following important parameters
1.  The size of the UNDO tablespace
2.  The UNDO_RETENTION parameter
Calculate UNDO_RETENTION  for given UNDO Tablespace:
You can choose to allocate a specific size for the UNDO tablespace and then set the UNDO_RETENTION parameter to an optimal value according to the UNDO size and the database activity. If your disk space is limited and you do not want to allocate more space than necessary to the UNDO tablespace, this is the way to proceed. The following query will help you to optimize the UNDO_RETENTION parameter:
Because these following queries use the V$UNDOSTAT statistics, run the queries only after the database has been running with UNDO for a significant and representative time!
To find Actual Undo Size:
SELECT SUM(a.bytes) "UNDO_SIZE"
FROM v$datafile a, v$tablespace b, dba_tablespaces c
Where c.contents = 'UNDO' AND c.status = 'ONLINE'
AND b.name = c.tablespace_name AND a.ts# = b.ts#
UNDO_SIZE
----------
209715200
Undo Blocks per Second:
SELECT MAX(undoblks/((end_time-begin_time)*3600*24)) "UNDO_BLOCK_PER_SEC"
FROM v$undostat;
UNDO_BLOCK_PER_SEC
------------------
3.12166667
Database Block Size:
SELECT TO_NUMBER(value) "DB_BLOCK_SIZE [KByte]"
FROM v$parameter
WHERE name = 'db_block_size';
DB_BLOCK_SIZE [Byte]
--------------------
4096
Optimal Undo Retention:
209'715'200 / (3.12166667 * 4'096) = 16'401 [Sec]
Using Inline Views, you can do all in one query:
SELECT d.undo_size/(1024*1024) "ACTUAL UNDO SIZE [MByte]",
SUBSTR(e.value,1,25) "UNDO RETENTION [Sec]", ROUND((d.undo_size / (to_number(f.value) *
g.undo_block_per_sec))) "OPTIMAL UNDO RETENTION [Sec]"
FROM (SELECT SUM(a.bytes) undo_size
FROM v$datafile a, v$tablespace b, dba_tablespaces c
WHERE c.contents = 'UNDO' AND c.status = 'ONLINE'
AND b.name = c.tablespace_name AND a.ts# = b.ts#) d, v$parameter e, v$parameter f,
(SELECT MAX(undoblks/((end_time-begin_time)*3600*24)) undo_block_per_sec
FROM v$undostat) g
WHERE e.name = 'undo_retention' AND f.name = 'db_block_size';
ACTUAL UNDO SIZE [MByte]
------------------------
200
UNDO RETENTION [Sec]
--------------------
10800
OPTIMAL UNDO RETENTION [Sec]
----------------------------
16401
Calculate Needed UNDO Size for given Database Activity:
If you are not limited by disk space, then it would be better to choose the UNDO_RETENTION time that is best for you (for FLASHBACK, etc.).
Allocate the appropriate size to the UNDO tablespace according to the database activity:
 Again, all in one query:
SELECT d.undo_size/(1024*1024) "ACTUAL UNDO SIZE [MByte]",
SUBSTR(e.value,1,25) "UNDO RETENTION [Sec]",
(TO_NUMBER(e.value) * TO_NUMBER(f.value) *
  g.undo_block_per_sec) / (1024*1024)  "NEEDED UNDO SIZE [MByte]"
  FROM (SELECT SUM(a.bytes) undo_size
  FROM v$datafile a,  v$tablespace b,  dba_tablespaces c
  WHERE c.contents = 'UNDO' AND c.status = 'ONLINE'
  AND b.name = c.tablespace_name AND a.ts# = b.ts#
  ) d, v$parameter e, v$parameter f,
 ( SELECT MAX(undoblks/((end_time-begin_time)*3600*24)) undo_block_per_sec
     FROM v$undostat ) g
 WHERE e.name = 'undo_retention'
  AND f.name = 'db_block_size'
ACTUAL UNDO SIZE [MByte]
------------------------
200
UNDO RETENTION [Sec]
--------------------
10800
NEEDED UNDO SIZE [MByte]
------------------------
131.695313
The previous query may return a "NEEDED UNDO SIZE" that is less than the "ACTUAL UNDO SIZE". If this is the case, you may be wasting space.
You can choose to resize your UNDO tablespace to a lesser value or increase your UNDO_RETENTION parameter to use the additional space.

1 comments:

  1. Your all posts are awesome and very helpful thanks

    ReplyDelete