Multiblock I/Os
For example, 32 database blocks, each consisting of 8 KB, for a total I/O size of
256 KB. Multiblock I/O is used for large-scale operations, such as full table
scans. The number of blocks in one multiblock I/O is determined by the initialization
parameter DB_FILE_MULTIBLOCK_READ_COUNT.
The Oracle database can read larger amounts of data with multiblock I/Os, so there
are times when a full table scan might actually retrieve data faster than an index-based
retrieval (e.g., if the selectivity of the index is low). Oracle can perform multiblock
operations faster than the corresponding collection of single-block operations.
I/O Planning Principles for an Oracle Database
When you??™re planning the disk layout and subsequent placement of the various files
that make up your database, you need to consider the different reasons Oracle performs
I/O and the potential performance impacts.
Oracle and Disk I/O Resources | 161
The main destinations of the I/O operations Oracle performs are the following:
??? Redo logs
??? Data contained in tables
??? Indexes on the tables
??? The data dictionary, which goes in the SYSTEM tablespace
??? Sort activity, which goes in the TEMP tablespace of the user performing the sort
??? Rollback information, which is spread across the datafiles of the tablespace
containing the database??™s rollback segments
??? Archived redo logs, which go to the archived log destination (assuming the database
is in ARCHIVELOG mode)
The following simple principles for managing these types of I/O can optimize
Oracle??™s use of the database server??™s disk subsystem:
Use disk-striping technologies to spread I/O evenly across multiple spindles
These technologies are covered in detail in the next section, ???Using RAID Disk
Array Technology.
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