Oracle Architecture - Oracle Essentials - page 48

Datafiles
Datafiles contain the actual data stored in the database, the tables and indexes that store
data, the data dictionary that maintains information about these data structures, and
the rollback segments used to implement multiuser concurrency.
A datafile is composed of Oracle Database blocks that, in turn, are composed of oper‐
ating system blocks on a disk. Oracle block sizes range from 2 KB to 32 KB. Prior to
Oracle9 i , only a single block size could be present in the entire database. In versions of
the database since the introduction of Oracle9 i , you still set a default block size for the
database, but you can also have up to five other block sizes in a database (though only
a single block size for each tablespace). Figure 2-4 illustrates the relationship of Oracle
blocks to operating system blocks.
Figure 2-4. 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 by
users.
Datafiles are the lowest level of granularity between an Oracle Database and the oper‐
ating system. When you lay out a database on the I/O subsystem, the smallest piece you
place in any location is a datafile. Tuning the I/O subsystem to improve Oracle perfor‐
mance typically involves moving datafiles from one set of disks to another. Automatic
Storage Management, included in Oracle Databases since Oracle Database 10 g , provides
automatic striping and eliminates manual effort for this tuning task.