This document discusses creating rollback segments in locally managed tablespaces in Oracle databases. It explains that while rollback segments can be created in locally managed tablespaces, the first one must be created differently to avoid an error. It recommends creating a temporary rollback segment in a dictionary-managed tablespace first to store the rollback information needed to create the initial rollback segment in the locally managed tablespace. Once that first segment is created, additional segments can be added to the locally managed tablespace.

1. Rollback Segments in Locally Managed Tablespace Administration Tips
Creating Rollback Segments in Locally Managed Tablespace
There is no fundamental problem in creating rollback segments in locally managed
tablespace (LMTs). In fact, they lend themselves to being created in such tablespaces,
because LMTs are designed to minimise the performance impact associated with constant
acquisition and dropping of extents -and if there is one thing rollback segments are likely
to find themselves doing over time, it is frequently acquiring new extents (growing) and
releasing old ones (shrinking).
But it is impossible to create the first rollback segment in locally managed tablespace
without generating an ORA-1522: cannot use system rollback segment for non-system
tablespace 'BLAH' error message'.
There are two underlying reasons for this.
First, the only rollback segment created inherently with the database is one called
SYSTEM, stored in the SYSTEM tablespace, and known as the "system rollback segment".
All other rollback segments have to be created by yourself, manually, after the database
has been created and opened. The system rollback segment has always been reserved for
handling the DML on the data dictionary tables (which is generated when you perform DDL
elsewhere in the database). It cannot hold the rollback for any DML statements affecting
data stored anywhere other than the SYSTEM tablespace. For all updates to data in a non-
system tablespace, you must have a non-system rollback segment available to house the
rollback associated with that update.
Second, Locally Managed Tablespaces record the acquisition of extents by segments in a
bitmap, stored at the beginning of the datafile with which the tablespace was first
created. The bitmap is initially 64K in size, and when a segment acquires an extent
(either because it is being created, and thus acquiring its initial extent, or because data is
being loaded into it and thus causing it to extend and acquire a next extent), the bitmap
records the fact by flipping a "0" to a "1".
Put these two points together, and the problem becomes clear: creating your very first
non-system rollback segment in locally managed tablespace is a clear case of Catch-22. To
create such a segment, you need to update the bitmap of the LMT involved to show that
extents have been acquired by a new segment. But that requires updating data in a non-
system tablespace. And any updates to data in a non-system tablespace have to have their
rollback stored in a non-system rollback segment -which doesn't yet exist, because that's
precisely what you're trying to create in the first place!!
Fortunately, the cure is easy. Just create a temporary non-system rollback segment
somewhere (it will have to be a dictionary-managed tablespace, obviously), and allow that
to store the rollback generated by the bitmap update when you create the first rollback
segment in a locally managed tablespace. Once the new LMT rollback segment is then
Copyright © Howard Rogers 2001 10/18/2001 Page 1 of 2

2. Rollback Segments in Locally Managed Tablespace Administration Tips
created and brought online, it can become the segment that houses all rollback generated
by the creation of all other LMT rollback segments, and there is thus no need to retain the
original, temporary, dictionary-managed rollback segment at all.
Putting that into a sequence of simple steps, then, you might try something like this:
CREATE ROLLBACK SEGMENT TEMPRBS TABLESPACE SYSTEM;
ALTER ROLLBACK SEGMENT TEMPRBS ONLINE;
CREATE ROLLBACK SEGMENT RBS1 TABLESPACE RBSLMT;
ALTER ROLLBACK SEGMENT RBS1 ONLINE;
ALTER ROLLBACK SEGMENT TEMPRBS OFFLINE
DROP ROLLBACK SEGMENT TEMPRBS;
CREATE ROLLBACK SEGMENT RBS2 TABLESPACE RBSLMT;
CREATE ROLLBACK SEGMENT RBS3 TABLESPACE RBSLMT;
...and so on.
The key thing to note here is that the temporary first rollback segment is being created
within the SYSTEM tablespace. This does not make this a 'system rollback segment'. That
title is reserved for the segment created at the same time as the database was created
with the actual name 'SYSTEM'. So, even though housed within the system tablespace, this
new rollback segment is allowed to house rollback generated by updates to data taking
place in non-system tablespaces -including our subsequent update to the bitmap at the
start of the "rbslmt" tablespace.
Now you don't have to create the temporary rollback segment in the system tablespace -
any old dictionary-managed tablespace will do. Indeed, it is usually frowned upon to
create any segments of your own within the system tablespace. So if you can create it
elsewhere, do so. The only reason I show the system tablespace being used here is
because (1) it's guaranteed to exist; (2) it's guaranteed to be dictionary-managed and (3)
the segment being created is so small that it probably won't do any damage to house it
there.
Copyright © Howard Rogers 2001 10/18/2001 Page 2 of 2