Rollback to Undo in a RAC env. 2004-08-13 - By Daniel W. Fink
It is not the number of concurrent users, but the number of concurrent
transactions. The goal of AUM is to assign each transaction to its own
undo segment if there is sufficient space in the undo tablespace. SMON
is responsible for mantaining the undo retention table in each undo
segment header. This maintainence does not necessarily coincide with the
end of a transaction using the particular undo segment. That much we
know for certain.
The theory (formulated by several of us outside of Oracle, so this is
not official) is that SMON becomes overworked when a large number of
undo segments are created by a large number of concurrent transactions.
As new transactions look to bind to an undo segment, they are unable to
find an 'available ' undo segment (as indicated by the segment header),
so they create a new segment since there is sufficient space. This only
creates more work for an already overworked SMON process, so the problem
snowballs. In extreme cases, we have seen the number of undo segments to
be 20x the maximum number of concurrent transactions. The final result
in some cases is that the database begins to throw 600 and 7445 errors
and finally takes a hard crash.
I have also heard the recommendation against AUM in high volume OLTP
systems from a very credible Oracle source.
Regards,
Daniel Fink
ryan.gaffuri@(protected) wrote:
>daniel,
>I Have not seen this anywhere. Why avoid undo tablespaces in high transaction oltps? Do you mean just with RAC? My last project had 30,000 users and a 4 node rac cluster.
>This one is up to about 400,000 without RAC and will go to 3,000,000. Why shouldn 't we use AUM?
>
>
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