Many small or few large extents for RBS & Ora-1555

Greetings fellow-DBAs,

I just recently perused a paper by Tim Gorman entitled "Cats, Dogs, and ORA-01555" which used some analogies to give a very good explanation of the
"Snapshot Too Old" problem that can be an annoyance from time to time. I
believe the paper was recommended by someone on this list, but I forget the particulars of who actually recommended it and where I downloaded it.

However, the moral of the story of "Ora-1555" errors (as I see it) has to do with the possibility of these occurring when (unpredictably) a query transaction (T2) starts up prior to an update transaction (T1) doing a
"Commit". From the moment the "Commit" for T1 is issued until the time the
new query transaction (T2) ends, the undo blocks for the Rollback Segment used by T1 are liable to be overwritten. If this does happen and Transaction T2 needs some of those blocks for read consistency, the
"Ora-1555 Snapshot too old" can result. One of the recommendations was:

"Increase the total amount of space in rollback segments to the point where
overwrite does not occur until long after any queries that might need the before-image data have completed."

Suppose you decided to do so by creating a rollback segment of say 100M in size. Do you think that doing so in either of the following 2 ways would be more advantageous than the other, or would it not make any difference:

    (1) Creating 10 - 10M extents for the RBS     (2) Creating 100 - 1M extents for the RBS

As always, TIA for the historically great insights...

Jim Damiano

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Author: James Damiano
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