It contains critical information
used to maintain the overall integrity of the database. One of the most critical
pieces of information in this header is the checkpoint structure. This is a logical timestamp
that indicates the last point at which changes were written to the datafile. This
timestamp is critical during an Oracle recovery process as the timestamp in the
header determines which redo logs to apply in bringing the datafile to the current
point in time.
Extents and segments
From a physical point of view, a datafile is stored as operating system blocks. From a
logical point of view, datafiles have three intermediate organizational levels: data
blocks, extents, and segments. An extent is a set of data blocks that are contiguous
within an Oracle datafile. A segment is an object that takes up space in an Oracle
database, such as a table or an index that is composed of one or more extents.
Setting the Database Block Size
Prior to Oracle9i, youset the database block size for an Oracle database at the time you
created the database, and you couldn??™t change it without re-creating the database.
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