Re: odd number of rollback segments

> 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.

Generation of archived redo log files has nothing to do with rollback segments. Rather, the frequency of when an archived redo log is generated has to do with how often the online redo log files switch to the next group. Increase the size of your online redo log members by at least a factor of ten, if not larger.

The ORA-1555 error is directly related to your rollback segment sizes. Increase the size of your rollback segments.

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

In Oracle 8, the database does not automatically create rollback segments for you (with the exception of the system rollback segment which is created on CREATE DATABASE. If you have rollback segments with the name RBS0 to RBS6, then someone explicitly created them.

> 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?

Oracle 8 does not create the rollback segments for you. According to the docs, "The minimum number of rollback segments acquired at startup is TRANSACTIONS divided by the value for this parameter." where *this* parameter refers to TRANSACTIONS_PER_ROLLBACK_SEGMENT. Note the keyword *acquired*, not created.

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

To change the value for PROCESSES, modify your INIT.ORA and bounce the instance. But why do you want to do this? It has nothing to do with your problem, and 50 processes is not that "big".

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

Because someone created the rollback segments different. Either that, or they have allocated additional extents with no OPTIMAL setting causing the rollback segment to shrink back to 2M.

At this point, I have to ask what specific problem you are trying to solve? You have talked about archived redo logs being created too frequently (10/min) and this sounds like the problem you are trying to address. But then the post talks about rollback segments and processes which have nothing to do with the above problem!

Cheers,
Brian

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Brian Peasland
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http://www.peasland.net

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