Re: Differences between Undo and Rollback Segments?

In article <DtvM9.6748$jM5.18993_at_newsfeeds.bigpond.com>, "Howard says...
>

snip
>The only discernible differences in architecture are that (1) with automatic
>undo, a rollback segment can pinch unused extents from another, idle,
>rollback segment rather than declare an out-of-space error. Marketing
>decided to call this 'extent stealing', which is fine if you like that sort
>of thing.

Actually, the marketing term for this is dynamic extents transfer. I prefer extent stealing myself! :)

Pete

>And (2) the transaction allocation policy changed slightly. With
>ye olde methods, Oracle assigned your transaction to the rollback segment
>with the fewest number of active transactions. With sexie newe methods,
>Oracle prefers to create new rollback segments for each transactions, until
>it reaches a number of rollback segments where it feels happy to let
>transactions start sharing them... at which point it reverts to the 'fewest
>number of transactions' algorithm.
>
>Someone is bound to mention 'undo retention'. That's not an architectural
>change per se. It simply means that the old rule whereby the rollback
>segment's tail pointer was updated at the moment of commit is modified so
>that it is only updated when the timeout specified by undo_retention is
>reached. That's an artifice of kernel code, not an architectural change to
>the rollback segments themselves, however.
>
>And all of the above explains why there is (generally) bugger-all
>performance worries in going to automatic undo in 9i over traditional
>manually-managed rollback segments: because, underneath the glitz and
>glamour of the marketing-induced name-change, there's sod-all difference in
>the architecture either way.
>
>Regards
>HJR
>
>
>
>"dunesky" <dunesky_at_somewhere.com> wrote in message
>news:3e02617b$1_at_news.userve.net...
>> What's the best way to describe the differences between Undo and Rollback
>> segments?
>>
>> Why did Oracle replace Rollback Segments in version 9. I know it's still
>> there but they don't recommend using them.
>>
>> Thx
>>
>>
>
>

HTH. Additions and corrections welcome.

Pete

SELECT standard_disclaimer, witty_remark FROM company_requirements; Received on Fri Dec 20 2002 - 12:35:42 CST