December 2002
Intermediate to advanced
928 pages
85h 29m
English
Content preview from Oracle in a Nutshell
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Datafiles
Datafiles contain the actual data stored in the database. This data includes the tables and indexes created by users of the database; the data dictionary, which keeps information about these data structures; and the rollback segments, which are used to implement Oracle’s consistency scheme.
A datafile is composed of Oracle database blocks that are, in turn, composed of operating system blocks on a disk. Oracle block sizes range from 2K to 32K. If you’re using Oracle with very large memory (VLM) support, you may use big Oracle blocks (BOBs), which can be as large as 64K in size.
Prior to Oracle9i, you could have only a single block size for an entire database. With Oracle9i, you still set a default block size for the database, but you can also have five different nonstandard block sizes in the database. Each datafile can support only one block size, but you can have mixed block sizes within the database.
Figure 1-4 illustrates the relationship of Oracle blocks to operating system blocks.
Figure 1-4. Oracle blocks and operating system blocks
Datafiles belong to only one database and to only one tablespace within that database. Data is read in units of Oracle blocks from the datafiles into memory as needed, based on the work users are doing. Blocks of data are written from memory to the datafiles stored on disk, as needed, to ensure that the database reliably records changes made ...
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