Comment:
The fact that a PUBLIC rollback segment comes online at startup
without altering the init.ora is a very useful feature.
I am at this moment creating a 40 gig RBS tablespace in an SAP
database, to be used by a single PUBLIC rollback segment.
There are certain utility operations ( such as client delete )
that delete huge amounts of data.
We don't have control of the code, and are never quite sure how
much rollback is enough. There's also the problem of multiple
rollback segments being used for several large transactions, and
using up so much space that other segments run out of space on
subsequent transactions.
By making one large PUBLIC RBS and taking the others offline, the
SAP Admin cat bounce the instance to her heart's delight, and the
large RBS will continue to be online, *without* touching the init.ora.
The single RBS can extend out to the full size of the tablespace
if necessary, and then I can just drop it and the TBS when done.
I like not having to change the init.ora for this. One less step to
deal with when the temporary large segment gets dropped.
And yes, SAP requires some tricks at times to compensate for
less than optimal programming of the app.
Jared
"Tim Gorman" <[EMAIL PROTECTED]>
Sent by: [EMAIL PROTECTED]
10/17/2002 10:59 AM
Please respond to ORACLE-L
To: Multiple recipients of list ORACLE-L <[EMAIL PROTECTED]>
cc:
Subject: Re: [Q] Public and private rollback segment?
This is my understanding of "private" versus "public" rollback segments:
Private RBS can be brought online only via explicit ALTER ROLLBACK SEGMENT
xxx ONLINE command.
The "init.ora" parameter ROLLBACK_SEGMENTS is only a list for the
automatic execution of ALTER ... ONLINE command at database instance
startup, so it is essentially the same as an explicit ALTER ... ONLINE
command...
Public RBS can also be brought online via explicit ALTER ... ONLINE or
"init.ora" parameter ROLLBACK_SEGMENTS, if you wish...
Public RBS can also be brought online at instance startup according to the
formula using the parameters (TRANSACTIONS /
TRANSACTIONS_PER_ROLLBACK_SEGMENT). In an OPS/RAC environment, this
allows each instance to select as many public RBS from a "pool" of public
RBS. The mechanism still works even in non-OPS/RAC environments too...
The use of public RBS is not restricted to OPS/RAC, but it doesn't make
much sense without it...
it doesn't make much sense *with* OPS/RAC either, but that's just my
opinion...
The upshot is that the only difference between public and private RBS is
this additional ability to forego the onlining at instance startup via the
ROLLBACK_SEGMENTS parameter or subsequently via ALTER ... ONLINE
statements. Instead, at instance startup the simple formula using
TRANSACTIONS and TRANSACTIONS_PER_ROLLBACK_SEGMENT is used. Since these
are all static parameters and the difference in functionality occurs at
only one point-in-time (i.e. instance startup), it really is purely a
matter of style. I see no advantage to using public RBS at all...
Corrections? Comments? Questions?
----- Original Message -----
To: "Multiple recipients of list ORACLE-L" <[EMAIL PROTECTED]>
Sent: Thursday, October 17, 2002 10:09 AM
>
>
> Can't remember where I copied this from, but I believe it came from
> metalink. I don't have a version associated with it, so things might
have
> changed in v9.
>
> HTH
> Barb
>
>
> Public vs. Private Rollback Segments
> ------------------------------------
> A common misconception about `Private' rollback segments is that they
are
> segments reserved for a particular use or a particular transaction. The
only
> difference between Public and Private rollback segments is in relation
to
> the Parallel Server Option. A public rollback segment can be acquired
> implicitly by any instance in a parallel server environment. A private
> rollback segment must be explicitly acquired by a particular instance
using
> the rollback_segments parameter. If not using OPS, the difference
between
> the two is insignificant.
>
>
> > ----------
> > From: dist cash[SMTP:[EMAIL PROTECTED]]
> > Reply To: [EMAIL PROTECTED]
> > Sent: Thursday, October 17, 2002 9:18 AM
> > To: Multiple recipients of list ORACLE-L
> > Subject: [Q] Public and private rollback segment?
> >
> >
> >
> >
> > We have ORACLE 8.1.7.4 running on NT. My questions are:
> >
> > 1. What difference between public rollback segment and private
segement?
> >
> > 2. what is benefit on public segment than private segment?
> >
> > 3. does public segment only use on "paraller server (RAC)"?
> >
> > Thanks.
> >
> >
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