Oracle-L: Re: How are rollback segments assigned?

From: <Jared.Still_at_radisys.com>
Date: Tue, 26 Nov 2002 10:25:27 -0800
Message-ID: <F001.0050C504.20021126102527@fatcity.com>

Raj,

You're assuming that the transactions are lasting long enough for you to catch them in the rollback segments.

Jared

Rajesh.Rao_at_jpmchase.com
Sent by: root_at_fatcity.com
11/26/2002 09:42 AM
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        To:     Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
        cc: 
        Subject:        Re: How are rollback segments assigned?

Thanks Jared, thats not what I see. I have 20 rollback segments, and at no point in time, there were more than
4 transactions in the rollback segments. In fact, I happened to see it a couple of minutes ago.

select usn, xacts from v$rollstat showed me 0 in 19 rollback segments, and 2 in one of them. And I have got alerts in place to alert me when there are more than 4 transactions in all of the rollback segments. And yeah, the rollback segments are online.

Remains a mystery to me.

Raj

                    Jared.Still_at_r  
                    adisys.com           To:     Multiple recipients of 
list ORACLE-L <ORACLE-L_at_fatcity.com> 
                    Sent by:             cc:   
                    root_at_fatcity.        Subject:     Re: How are rollback 
segments assigned? 
                    com  
  
  
                    November 25,  
                    2002 08:59 PM  
                    Please  
                    respond to  
                    ORACLE-L

Here's one scenario:

4 transactions, 2 rollback segs

tx 1 - rbs1
tx 2 - rbs 2
tx 3 - rbs 1
tx 4 - rbs 2

tx1 and tx3 finish.

tx2 and tx4 have not yet committed.

Jared

Rajesh.Rao_at_jpmchase.com
Sent by: root_at_fatcity.com
11/25/2002 07:39 AM
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To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>

        cc:
        Subject:        How are rollback segments assigned?

Hello Friends,

Oracle Parallel Server 8.0.6.2.0 on Solaris 2.6 Nodes

We were having some locking and rollback issues, and I set up some scripts to alert me in case there are more than 4 transactions in the rollback segments, and more than 1 in any of the rollback segments.

Select sum(xacts) from v$rollstat --- Alert if more than 4, condition set to exclude system rollback
where usn != 0

Select count(*) from v$rollstat --- Alert if more than 0, where condition to exclude system rollback
where xacts >=2
and usn != 0

We have 20 rollback segments, and there is very minimal DML activity on this database. I happened to see a scenario where all rollback segments had
0 transactions, except for one which had 2 in them. Under what scenario, can this happen?

I thought the criteria for assigning rollback segments to transactions was

1. If object in system tablespace, use system rolback segment
2. Use the one with the least number of active transactions
3. If 2 or more rollback segments fit the second criteria, use the LRU

algorithm.

Thanks
Raj

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Received on Tue Nov 26 2002 - 12:25:27 CST