Sam-Alpati Notes

LOGICAL DATABASE STRUCTURES

DATA BLOCK

A data block is at the foundation of the database storage hierarchy and is the basis of all database storage in an Oracle database. A data block consists of a several bytes of disk space in the Operating System’s Storage System. All Oracle space allocation and usage is in terms of Oracle Data Blocks. Oracle data blocks are generally of sizes 2KB, 4KB, 8KB, 16KB, and 32KB. Oracle data block should be multiple of disk block size, if not; you would be wasting time reading and writing whole disk blocks. Size of Oracle data blocks is controlled by setting DB_BLOCK_SIZE parameter in Oracle initialization file.

Oracle offers following guidelines for choosing the database block size:

1. Choose a smaller if your rows are small access is predominantly random.

2. Choose a larger block size if rows are small and access is mostly sequential or if you have large rows.

MULTIPLE ORACLE DATA BLOCK SIZES

Oracle provides you the feature of having additional block sizes all within a single database. If you wish to configure Multiple Oracle Data Blocks, then you must also configure corresponding sub caches in buffer cache of System Global Area.

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