Re: understanding rollback segments

From: Dave Wotton <Dave.Wotton_at_it.camcnty.gov.uk>
Date: 1997/04/29
Message-ID: <5k4gtv$9r6@dns.camcnty.gov.uk>#1/1

Geert De Paep <gdp_at_cimad.be> wrote:
>Dave,
>
>A new transaction will always start to write its rollback information in
>an new oracle block, not necessarily in a new (rbs-)extent.
>For this type of activity you should take at least two rollback
>segments. Oracle will always assign you the rbs with the least nr of
>active transactions. This means that if the other user is 'blocking' the
>first rbs, you will be assigned the second (non-active) rbs which will
>wrap around and not grow.
>
>Geert.

Hi Geert,

thanks for the clarification. My experiment deliberately only used one RBS to simplify the situation - but you're right, if there's more than one RBS, they won't get blocked (so easily).

My confusion about allocating transactions to extents comes in part from a paragraph in the otherwise excellent "Oracle: Advanced Tuning and Administration" by Eyal Aronoff, Kevin Loney and Noorali Sonawalla and published by Oracle Press. In Chapter 5, page 104 it says:

     "Ideally, each transaction would be stored in its own rollback
      segment. Since that is not likely to be the case, each 
      transaction should be able to be stored in its own extent.
      Create extents that are large enough to support most of the
      transactions without wrapping."

Unless I've misunderstood, this seems pure gobbledygook: I've always understood that transactions MUST be stored in their own rollback segments - so the first sentence seems unnecessary,and the first part of the second sentence is simply wrong. The second part of the second sentence seems like nonsense if you assume the preceding sentence is true. Also if, as you say, and which I accept, transactions share rollback extents, how on earth can you size them to fit most transactions? There will always be cases where transactions are assigned to extents which are already nearly full. It was in trying to rationalise this paragraph that I started to believe the each transaction used its own extent of the RBS.

Dave. Received on Tue Apr 29 1997 - 00:00:00 CDT