How To Prevent Block Corruption In Oracle?

By Author: Manya Gupta
Total Articles: 5
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When corruption occurs in Oracle database or DBF file, it typically hits Oracle database block on hard drive or in memory. The memory damages are handled with operating system and Oracle kernel detection mechanism. It isn't a perfect solution in all cases; sometimes, even the good memory blocks are set as corrupt. Disk corruptions are most frequent types of corruptions. The blocks are not checked after they are written to disk, until a check is requested. Oracle follows block delivery till the block has reached the operating system, which then takes over block handling responsibility. However, since the operating system does not have knowledge about Oracle block structures, it may not check the blocks intelligently. In such cases, database becomes inaccessible and you encounter data loss. In order to access your valuable data at this point, oracle database recovery is required.

Oracle database offers various possibilities for defending databases against data block corruptions, which include:

DB_BLOCK_CHECKSUM=TRUE- The main method of Oracle to detect block corruption ...
... is through block checksum. After block modification, database writer calculates header bytes checksum. On next reread of the same block, it is validated. Along with regular rollback and data blocks, database also organizes redo log blocks. Redo log blocks contain undo information that is required for transaction recovery.

Checksum information is a representation of the structure of logical block. A block is good if it has standard Oracle block format. Checksum validation process doesn't read actual content of the block, thus, it is possible that the data inside a hard drive block is damaged. Setting this argument prevents those blocks from being saved, which do not confirm to standard block format of Oracle. If block corruption is caused by format modification, it is discovered as media error.

DB_BLOCK_CHECKING=TRUE- Setting up this instance argument instructs Oracle for making detailed logical block checking, before making block changes. This self-consistent block checking prevents disk and memory corruptions. Detailed logical block checking is always executed by Oracle kernel, which belongs to system tablespace.

The above methods are helpful in a number of cases in preventing database block corruption. However, if you do not use them or they fail to prevent corruption, Oracle block corruption may occur. It causes data loss and needs DBF Recovery to be fixed.

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Stellar Phoenix Oracle Recovery software repairs and restores all Oracle database objects, such as tables, indexes, views, roles, schemas, cluster tables, and synonyms. It restores damaged database of Oracle 9i. The software is compatible with Microsoft Windows 2003 and Microsoft Windows XP.