Re: Rollback segments

"Greg Stark" <greg-spare-1_at_mit.edu> wrote in message news:87k882xiea.fsf_at_stark.yi.org...
> "Howard J. Rogers" <howardjr_at_www.com> writes:
>
> > Incidentally, you are asking for trouble having rollback segments with
 only
> > two extents. The chances of them having to acquire more extents is very
> > high (assuming you eventually bring them online).
> >
> > Rollbacks should have around half a dozen extents to minimise the need
 to
> > extend dynamically
>
> I don't understand how you can say that without knowing the size of the
> extents and activity. Is there any major functional difference between a
> rollback segment with 6 1M extents and a rollback segment with 2 3M
 extents?
>
>
> --
> greg

Yes, a huge one. New transactions cannot move into an extent of a rollback segment which has an active transaction within it. With just two extents, the chances of filling up the second extent and thereby needing to move back into the first which is likely to still have one active transaction in it (however trivial that transaction is) is extremely high. The more extents you have, the lower the probability. When it can't move back into the first extent because of the presence of an active transaction, the segment will instead acquire (dynamically) an additional extent.

Since it is the presence of a single transaction that prevents extent reuse, it's the number of extents, not the size of them per se that is the trouble. 2 3M extents are, other things being equal, much more likely to have a single live transaction in them somewhere than 6 1M extents. Of course, it should take longer to fill up the second extent, and thus need (or, rather, want) to move back into the first... but we are talking probabilities here. You are more *likely* to need to acquire a third extent. Naturaly, activity levels determine the actual need, or lack thereof, for doing so.

Regards
HJR Received on Fri Jan 12 2001 - 07:56:51 CST