Topic: User Administration >> Undo retention
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| sai |
Posted: Sep 02, 2007 05:31:22 AM |
Total Post: 14
Joined: May, 2007
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| How do i set the optimal value for undo retention parameter? If i have higher value for undo retention what are the problems do we get? |
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 Mohammed Taj |
| Posted: Sep 02, 2007 11:03:57 AM |  |
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Total Post: 634
Joined: Jul, 2007
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http://download.oracle.com/docs/cd/B19306_01/server.102/b14196/storage003.htm#BABDJDBF
default is 900 sec.(15 mins) you can use undo advisor for optimil size.
Regards
Taj
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| dbavinod |
| Posted: Feb 15, 2008 02:17:49 AM | |
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Total Post: 81
Joined: Sep, 2006
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UNDO_RETENTION
A dynamic parameter specifying the length of time to retain undo. Default is 900 seconds.
The effect of the UNDO_RETENTION parameter is immediate, but it can only be honored if the current undo tablespace has enough space for the active transactions. If an active transaction requires undo space and the undo tablespace does not have available space, the system starts reusing unexpired undo space. Such action can potentially cause some queries to fail with the "snapshot too old" error.
Given a specific UNDO_RETENTION parameter setting and some system statistics, the amount of undo space required to satisfy the undo retention requirement can be estimated using the following formula:
UndoSpace = UR * UPS + overhead
where:
UndoSpace is the number of undo blocks
UR is UNDO_RETENTION in seconds
UPS is undo blocks for each second
overhead is the small overhead for metadata (transaction tables, bitmaps, and so forth)
Vinod Udapudi
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