Re: rollback after crash

"Pradeep" <agarwalp_at_eeism.com> wrote in message news:1134577082.919500.136810_at_g43g2000cwa.googlegroups.com...
> Lewis,
>
> thanks for you reply, but i a m not able to understand what you are
> trying to say.
>
>
> Jonathan Lewis wrote:
>> "Pradeep" <agarwalp_at_eeism.com> wrote in message
>> news:1134574000.974721.270670_at_g43g2000cwa.googlegroups.com...
>> > Thought of this situation while going through one of the threads on
>> > asktom on checkpoint.
>> > Suppose
>> > - uncommitted data is flushed to the disk and
>> > - corresponding rollback is also flushed
>> > - corresponding redo is later overwritten
>> > - instance crash.
>> >
>> > How does it recover now ?? or how does is rollbacks the uncommitted
>> > data that is on the disk.
>> >
>>
>>
>> If the transaction is uncommitted, then the
>> rollback will be held open, and cannot be
>> overwritten (undo segment header will have
>> a transaction table slot in state 10).
>>
>> In the event of a crash, any redo that has
>> to be re-applied will still leave the transaction
>> visible as an uncommitted transaction in the
>> rollback segment header.
>>
>> So the transaction will be rolled back.
>>
>>
>>
>> --
>> Regards
>>
>> Jonathan Lewis
>>
>> http://www.jlcomp.demon.co.uk/faq/ind_faq.html
>> The Co-operative Oracle Users' FAQ
>>
>> http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html
>> Cost Based Oracle: Fundamentals
>>
>> http://www.jlcomp.demon.co.uk/appearances.html
>> Public Appearances - schedule updated 29th Nov 2005
>

You start a transaction

    Mark slot 1 of undo segment header 1 as active call this block S1     Update block T1 of table1
    which results in using undo block B1     all three changes result in redo being generated

    T1, U1, and B1 are written to disc.
    You do NOT commit the transaction.

    Redo log is reused several minutes later     You do NOT commit the transaction

    System crashes

    Recovery initiated
    Rollforward completes.

    T1, U1, and B1 may not have been touched     from the moment they were written to     disc up to this point.

    smon (or is it pmon) starts to read the undo segment     header blocks to see if there are any uncommitted     transactions after recovery.

        Finds U1 with slot 1 marked as active
        rolls back the transaction, which means
            reading undo block B1
            applying rollback to table block T1
            clearing slot 1 of U1

-- 
Regards

Jonathan Lewis

http://www.jlcomp.demon.co.uk/faq/ind_faq.html
The Co-operative Oracle Users' FAQ

http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html
Cost Based Oracle: Fundamentals

http://www.jlcomp.demon.co.uk/appearances.html
Public Appearances - schedule updated 29th Nov 2005