Re: Rollback Segments

I think what the note in technet is trying to say is that Oracle will try to use the assigned rollback segment to the users transaction for the internal space management transactions (which are actually separate individual transactions). If space cannot be found in the assigned rollback segment (slots not available in the transaction table), the SYSTEM rbs will be used for these transactions.

For example, you insert data into a table, and it causes a new extent to be allocated. The insert part will use whatever transaction has been assigned by Oracle (say RBS5). When we see that we are going to cross the extent boundary, a new extent is allocated, and all the dictionary updates that result, (space management) create new, entirely separate transactions. We assume space is not available in RBS5, so use the SYSTEM rbs for the space management DML.

As Howard said, we would be deep do-do if we had no rbs space to track the internal DML operations. Having a SYSTEM rollback segment dedicated to these transactions ONLY, means Oracle will always have enough space to allow such operations to be carried out. Remember, we try to use the user rbs first, and only when we can't, we go to the SYSTEM rbs.

One thing to note is that if the original insert fails for some reason, and Oracle has already implicitly commited the space management transactions, the space will remain allocated to the object. Reason being is that Oracle assumes you will re-run the command, and will not have to do all that extra effort a second time.

Regards,
Stephan

"Howard J. Rogers" wrote:

> "Will C." <whc2u_at_hotmail.com> wrote in message
> news:84c3d20c.0106101213.2b7f2c45_at_posting.google.com...
> > Hi,
> >
> > I have a couple of questions about rollback segments. Any help is
> > appricated. So, my System: Oracle 8.1.6 on Linux
> >
> > Question 1:
> > According to Technet, if I don't specify rollback segments
> > in my init.ora file, the Oracle instance will automatically aquire the
> > rollback segments it needs (from those that are avaiable). At the
> > moment, I'm refering to PUBLIC rollback segments. Technet also
> > states that the SYSTEM rollback segment is aquired automaticaly
> > as well and that if there are too many transactions for the
> > non-SYSTEM rollback segments, Oracle will use the SYSTEM
> > segment. So, if this occurs, and I create and bring online additional
> > rollback segments, will they automatically be utilized by the
> > instance, or will I have to shutdown and restart the instance?
> >
>
> First, the SYSTEM rollback segment can not be used for transactions in
> non-SYSTEM tablespace, so I have no idea what that technet reference is
> trying to say. If it says that the SYSTEM rollback segment can ever hold
> rollback for DML in a USERS or DATA tablespace, it's just wrong.
>
> Otherwise, if you create a new rollback segment, then you must 'alter
> rollback segment newone online'. At that point, it becomes available for
> use by any NEW transactions. Since a transaction must finish in the same
> segment as it starts in, no existing segments can suddenly start using
> blocks from any new ones you create.
>
> > Question 2:
> > If I specify the rollback segments I want the instance
> > to use in init.ora, do I need to specify system segments as well,
> > or only just the public/private rollback segments? (ie, are
> > SYSTEM rollback segments always automatically aquired?)
>
> There is one and only one SYSTEM rollback segment, and yes, it is always
> acquired by the Instance at startup. Be clear what it is for, however. It
> is to hold the rollback of any DML performed on the data dictionary tables.
> Hence, if you create a new table, create an index, or do an insert that
> requires a segment to acquire a new extent -all those things require inserts
> and deletes on the data dictionary, and *that's* what the SYSTEM rollback
> segment is for. Hence, were an Instance *ever* to fail to acquire such a
> segment automatically, you would be in deep, deep trouble.
>
> >
> > Question 3:
> > If I added an additional SYSTEM rollback segment and bring it
> > online, its it utilized automatically?
> >
>
> You cannot, ever, create another SYSTEM rollback segment. The fact that a
> rollback segment happens to be housed in the SYSTEM tablespace does NOT make
> it the SYSTEM rollback segment. Hence, ALL other rollback segments,
> whatever tablespace they are created in, are non-SYSTEM rollback segments,
> and as such must be brought online either manually with the 'alter rollback
> segment ... online' command, or by editing the init.ora file.
>
> There is one and only one SYSTEM rollback segment, and it is the one created
> for you automatically as part of the 'CREATE DATABASE' command. It has the
> name 'SYSTEM', and it's housed in the 'SYSTEM' tablespace. No other
> rollback segment can ever satisfy those three requirements, and hence no
> other segment can ever be the SYSTEM rollback segment.
>
> And despite some comments posted by [an]other poster(s) here, it is
> point-blank utterly impossible ever to create a database without that SYSTEM
> rollback segment.
>
> Regards
> HJR
>
> > Much Thanks,
> >
> > Will C.
Received on Mon Jun 11 2001 - 14:19:39 CDT