Re: Poor Performance of undo space management in 10.2 - bug info

These are actually problems that date back to 9i. The first problem we encountered was the incorrect status of the extent. The second was the excessive growth of the undo tablespace as new undo segments were created in excess of actual transaction load (eventually crashing the instance).


We found (and could reproduce) a situation where extents containing just committed (less than 1 minute with a 60 minute undo_retention) transactions were marked as expired. Other extents containing data committed over 24 hours previous were still marked as unexpired. We never received an adequate explanation about the behavior.

The second issue was hard to reproduce as it only occurred on large oltp systems with very high transaction load (thousands of transactions per second, though only about 2 thousand concurrent transactions) and was very sporadic. Segments were created when other segments were actually usable (no active transactions in them). Eventually, thousands of undo segments were created and once created an undo segment is *never* dropped (a real weakness in automatic undo). This was the result of SMON managing the segments and how it tracked used/usable segments.

All in all, automatic undo (from 9.2 on) is pretty good and works well for 99% of the systems out there. Like anything, you do need to be careful when working with a high volume system. If you want to learn more about automatic undo (9i...I never updated the research nor paper for 10g), you can download my paper on Automatic Undo Internals at http://www.optimaldba.com/papers/AutomaticUndoInternals.pdf

Regards,
Daniel Fink

--
Daniel Fink

Oracle Performance, Diagnosis and Training

OptimalDBA    http://www.optimaldba.com
Oracle Blog   http://optimaldba.blogspot.com


John Hallas wrote:

This note is a heads-u on a bug which has caused me some problems over the last few days and yet is easily identifiable and resolvable (once you recognize the issue).

We have spent the last 2 days trying to run a benchmark to prove end to end performance of a new code set. We have been plagued by undo tablespace problems which had the following symptons :-

    * Rapid growth of used undo tablespace
    * Serious deterioration in performance as the undo tablespace got
      very full
    * Loss of application connectivity as responses were not received
      in time (trading system with about 7 servers being used to hold
      components of the system)

The complex set up of the test rig tended to mask the database aspect on each run and it was only this morning that we really focused on the undo tablespace.

We had AUM set and a 60 second retention period and various sized t/s using both Ramsan and local disk.

The problem was identified as undo extents remaining marked as unexpired well past the retention period, despite all connections being terminated and no active transactions running.

Searching on Metalink showed Note 5387030.1  which refers to a bug with the 
TUNED_UINDORETENTION setting. This can be seen in v$undostat and once we had run alter 
system set "_smu_debug_mode" = 33554432; the v$undostat.tuned_undoretention 
statistic dropped from 345600 to 2188 and performance improved with unexpired undo 
segments hardly rising despite heavy throughput.
This bug is common through 10.2.01 to 10.2.0.3 or is fixed in 10.2.0.4 or V11. John
+44 (0)113 223 2274 (direct)

+44 (0)113 297 9797

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