Re: snapshot to old again

Doug Cowles <dcowles_at_bigfoot.com> wrote in message news:374C21EE.C547B83C_at_bigfoot.com...
> A couple of things -

Sure. At the risk of being criticized again for sniping a post, I think it's probably in everybody's interest that we get to the subject quickly... (<jab> at another thread)

> sniperoo
> >
> > TRB starts after TRA, gets a SCN of ZZZZ = XXXX+20) (just an example!),
> > reads and updates a block ahead of TRA. It sets the block
> > number to ZZZZ in the table. A copy of the block
> > now lives in rollback segment with whatever number it last
> > had (< ZZZZ and < XXXX). Let's call it SCN AAAA.
> >
> > TRB commits and goes to heaven. ORACLE does not wipe the
> > block that was written to rlbseg just yet, because there
> > is another transaction busy on this table.
> >
>
> This SCN AAAA cannot be sitting in the rollback segment really can it?

Yes it is. The SCN is inside the block, in the header, and the whole block is copied to the RLBSEG.

>If is
> therebecause another transaction is on this table than isn't there a risk that
a
> rollback to
> block SCN AAAA wipes out the committed transaction from TRB? which now lives
in
> the datafile as ZZZZ?

Not really. Note that TRB has committed. So the block in the DB (not the RLBSEG) is now SCN ZZZZ (original and fixed SCn of TRB). So if TRA aborts and rollback happens, only blocks in RLBSEG that have SCN of XXXX (original and fixed SCN of TRA) will be rolled back.

>
>
> > TRC (or even TRA if it is updating/committing as it reads)
> > comes in and does a gazillion updates that use up all the space
> > on rollback segment and wipe the copy of SCN AAAA from there).
> >
>
> The thing is, in my situation, the rollback segment never came near it's
> maximumsize, it extended about 100MB, but there was much more room for it to
> extend,
> which makes me wonder by the SCN AAAA's would be wiped out.

My guess here as I have no proof of this other than prior experience. In all likelyhood, your rollback segment may either compete for space on the tablespace with other updaters, or it has also grown into additional rollback segments because you are modifying indexes as well as data blocks. You will get one rollback segment for your changed data, plus another for each index that gets changed as a result of your data change. Remember, in ORACLE indexes are automatically updated and they are also transactionally recoverable by rollback.

HTH
--
Cheers
Nuno Souto
nsouto_at_nsw.bigpond.net.au.nospam
Is there a nospam domain?
http://www.users.bigpond.net.au/the_Den Received on Wed May 26 1999 - 13:11:21 CDT