Tablespaces can be either dictionary managed or locally managed. In a dictionary-managed
tablespace, extent management is recorded in data dictionary tables. Therefore, even if all
application tables are in the USERS tablespace, the SYSTEM tablespace will still be accessed for
managing DML on application tables. Because all users and applications must use the SYSTEM
tablespace for extent management, this creates a potential bottleneck for write-intensive applications.
In a locally managed tablespace, Oracle maintains a bitmap in each datafile of the tablespace to
track space availability. Only quotas are managed in the data dictionary, dramatically reducing
the contention for data dictionary tables.
As of Oracle9i, if the SYSTEM tablespace is locally managed, then all other tablespaces must
be locally managed if both read and write operations are to be performed on them. Dictionarymanaged
tablespaces must be read-only in databases with a locally managed SYSTEM tablespace.
Blocks
A database block is the smallest unit of storage in the Oracle database. The size of a block is a
specific number of bytes of storage within a given tablespace within the database.
A block is usually a multiple of the operating system block size to facilitate efficient disk I/O.
The default block size is specified by the Oracle initialization parameter DB_BLOCK_SIZE. As many
as four other block sizes may be defined for other tablespaces in the database, although the blocks
in the SYSTEM, SYSAUX, and any temporary tablespaces must be of the size DB_BLOCK_SIZE.
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