In addition, the performance of concurrent DML operations is improved because the segment??™s
bitmap allows concurrent access. In a freelist-managed segment, the data block in the segment
header that contains the freelist is locked out to all other writers of the block when a single writer
is looking for a free block in the segment. Although allocating multiple freelists for very active
segments does somewhat solve the problem, it is another structure that the DBA has to manage.
Chapter 6: Monitoring Space Usage 177
Another advantage of LMTs is that rollback information is reduced or eliminated when any
LMT space-related operation is performed. Because the update of a bitmap in a tablespace is not
recorded in a data dictionary table, no rollback information is generated for this transaction.
Other than third-party applications, such as older versions of SAP that require dictionarymanaged
tablespaces, there are no other reasons for creating new dictionary-managed tablespaces
in Oracle 10g. As mentioned earlier, compatibility is provided in part to allow dictionary-managed
tablespaces from previous versions of Oracle to be ???plugged into??? an Oracle 11g database, although
if the SYSTEM tablespace is locally managed, any dictionary-managed tablespaces must be opened
read-only. Later in this chapter, you??™ll see some examples where we can optimize space and
performance by moving a tablespace from one database to another and allocating additional
data buffers for tablespaces with different sizes.
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