RE: Users reading from rollback segments

People also obtain read consistency data from the rollback segments after transactions have committed. If a query began before someone else's transaction committed, but continues reading, then needs the reconstruct the data from before the commit, in needs rollback data that is both committed and impossible to predict.

I suppose if you could determine that the age of all undo entries in the portion of RBS that you will obliterate through shrinking are older than any query currently running in the database, then you could be sure that the shrink will not cause an ORA-01555.

But the flaw in your logic is believing that once committed, rollback entries will not be needed for read consistency. They very well may.

Because a query doesn't know what rollback entries it may need further down the road, you can't predict if your shrink will obliterate undo entries that a long-running query might need in the future. You keep asking if we can tell who is reading the rollback segments. The answer is that it doesn't matter. What you really need to ask is if we can tell who will need to read the rollback segments sometime soon. And you can't.

--
Jeremiah Wilton
http://www.speakeasy.net/~jwilton

On Tue, 26 Feb 2002, Glenn Travis wrote:



> Hmmm. I think I CAN predict FUTURE needs of the rollback segments.


> If there are transactions using the rollback space, users MAY need

> it.  If there are no transactions, then they won't.  If I were able

> to know who is reading from rollback, I would know if shrinking

> might cause ORA-01555.

> 

> Tell me if I'm off on this...

> 

> Users will not read from the rollback segment unless they need

> read-consistent data due to an open transaction against the data

> they are looking for (thus reading the redo or undo info from

> rollback).  Otherwise they read from the data segments (committed

> data).

> 

> Oracle will not shrink the rollback segment if it contains open

> transactions.

> 

> So, if there are no users reading from rollback and I issue a

> 'shrink' command, and it works, then the transactions are complete

> and any user coming in after that will read from the data segments.

> 

> If there are no users reading from rollback and I issue a 'shrink'

> command, and it DOES NOT work, then the transactions are NOT

> complete and any user coming in after that will read from the

> rollback segments (the data is still there).

> 

> If there ARE users reading from rollback and I issue a 'shrink'

> command, and it works, then users run the risk of getting ORA-01555

> (the data MAY be gone).  Which is exactly why I asked my original

> question ("How do I identify READERS of the rollback segments?") :)





> > -----Original Message-----

> > From: Jeremiah Wilton [mailto:jwilton_at_speakeasy.net]

> > 

> > Since you cannot predict who might need to generate consistent reads


> > from the RBS in the FUTURE, you cannot predict if you will cause

> > ORA-01555 or not by shrinking.

> > 

> > Your best bet is to get rid of people bloating up RBSs by limiting

> > their growth, and enforcing the use of smaller transactions.  That way

> > you won't have to shrink so much.

> > 

> > --

> > Jeremiah Wilton

> > http://www.speakeasy.net/~jwilton

> > 

> > On Tue, 26 Feb 2002, Glenn Travis wrote:

> > 

> > > Is there a way to tell if anyone is reading from the rollback

> > > segments?

> > > 

> > > I would like to manually issue 'alter rollback segment XXX shrink;',

> > > but do not want to do so if there are users reading read consistent

> > > data from the rollback space (thus giving them the ORA-01555 error).

> > > 

> > > Is there a way to check if the rollback segment is in use first?

> > > 

> > > Can I try to take it offline?  Will it fail if there is someone

> > > reading from it?


-- 
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-- 
Author: Jeremiah Wilton
  INET: jwilton_at_speakeasy.net

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