One issue I have observed with System-Managed Undo
is that Oracle seems to be very aggressive about keeping
the rollback segments the 'right' size for the keep-time.
In the one set of tests I ran, this resulted in frequent
extends and shrinks of the generated rollback segment,
with a resulting I/O load appearing on the datafile
holding the rollback segments. The work-around was
to increase the keep time to a higher value than
necessary - which still resulted in extra I/O, but
not such an excessive amount.
I still have to set up further test scenarios but, at first
usage, it seems that system managed rollback segments
MAY be a problem when used on a finely tuned system
with carefully selected rollback segment sizes.
Jonathan Lewis
http://www.jlcomp.demon.co.uk
Host to The Co-Operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html
Author of:
Practical Oracle 8i: Building Efficient Databases
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