Re: odd number of rollback segments


Nick Keighley wrote:
Nick Keighley wrote:
Hi,

I gather questions about Rollback Segments are quite common...

I'm running Oracle 8, and no I'm not going to be able to upgrade soon.

My system seems to be using a lot of disc space with ARC files. It's
generating
about 10 ARC files /min. I understand this is Bad and may indicates
too few
(or too small) rollback segments. Today I got an ORA-01555 for slowish
transaction.
Again this is probably related to RBSs.

Only two public rbs appear to be specifically created by the startup
SQL.
So presumably Oracle created the rest. There are 7 (RBS0-RBS6).

I've dug back though various configuration parameters

processes=50 (sounds big for my environment)
sessions=60
transactions=66
transactions_per_rollback_buffer=5 (TPR)

hence Oracle should allocate transactions/TPR=13.2 public RBS
and not a measly 7. Why?

What do I have to do to change the number of processes? Update the
init.ora
and restart the database?

My rollback segments are all 2M except for number 6 which is 4M. Why?

eek! I have now found the file that is really building the database. It
creates
7 RBS...


--
Nick Keighley

Nick, arc files are from the archived redo logs and have no direct
relationship with rollback segments.

If you are genenerating multiple archived redo logs files per minute it
means that your online redo log files are allocated too small. You can
find instructions in the DBA Administration manual on how to add and
drop redo logs files while the system is in use.
Basically add a new larger one, then drop any non-currently in use
archived log.

It is very possible that 7 RBS segments are all that your database
needs. How many concurrent transactions, not user sessions, do you see
in v$transaction. Is your database an OLTP or DSS/OLAP type system?
How large (amount of data) is the average transaction on your system.
Often a lesser number of larger rollback segments worked better than a
larger number of smaller segments.

HTH -- Mark D Powell --

.

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