了解物理和逻辑坏块

Physical and Logical Block Corruptions. All you wanted to know about it. [ID 840978.1]

Oracle classifies the Data File Block corruptions as Physical and Logical.  This is also referred as intra block corruptions. 

Physical Block Corruptions

This kind of block corruptions are normally reported by Oracle with error ORA-1578 and the detailed corruption description is printed in the alert log. 

Corruption Examples are:

• Bad header - the beginning of the block (cache header) is corrupt with invalid values
• The block is Fractured/Incomplete - header and footer of the block do not match
• The block checksum is invalid
• The block is misplaced
• Zeroed out blocks / ORA-8103

Detailed Corruption Description:

Fractured Block

A Fractured block means that the block is incomplete. Information from the block header does not match the block tail.

Corrupt block relative dba: 0x0380e573 (file 14, block 58739)
Fractured block found during buffer read
Data in bad block -
type: 6 format: 2 rdba: 0x0380e573
last change scn: 0x0288.8e5a2f78 seq: 0x1 flg: 0x04
consistency value in tail: 0x00780601
check value in block header: 0x8739, computed block checksum: 0x2f00
spare1: 0x0, spare2: 0x0, spare3: 0x0
***
Reread of rdba: 0x0380e573 (file 14, block 58739) found same corrupted data

Bad Checksum

Block Checksums are used to identify if the block was changed by something external to Oracle and after the block was last written by Oracle.
Checksum is calculated by DBWR or direct loader before writing the block to disk and stored in the block header. Every time that the block is read and if db_block_checksum is different than false, Oracle calculates a checksum and compares it to the one stored in the block header. Reference [Note 30706.1]

Example of a corrupt block due to invalid checksum:

Corrupt block relative dba: 0x0380a58f (file 14, block 42383)
Bad check value found during buffer read
Data in bad block -
type: 6 format: 2 rdba: 0x0380a58f
last change scn: 0x0288.7784c5ee seq: 0x1 flg: 0x06
consistency value in tail: 0xc5ee0601
check value in block header: 0x68a7, computed block checksum: 0x2f00
spare1: 0x0, spare2: 0x0, spare3: 0x0
***
Reread of rdba: 0x0380a58f (file 14, block 42383) found same corrupted data

A value different than zero (0x0) in "computed block checksum" means that the checksum
differs and the result of this comparison is printed. 

Block Misplaced

This is when Oracle detected that the content of the block being read belongs to a different block and the checksum is valid:

Corrupt block relative dba: 0x0d805a89 (file 54, block 23177)
Bad header found during buffer read
Data in bad block -
type: 6 format: 2 rdba: 0x0d805b08 ----> Block is different than expected 0x0d805a89
last change scn: 0x0692.86dc08e3 seq: 0x1 flg: 0x04
consistency value in tail: 0x08e30601
check value in block header: 0x2a6e, computed block checksum: 0x0
spare1: 0x0, spare2: 0x0, spare3: 0x0
***

 

Logical Block Corruptions

This is when block contains a valid checksum and the structure below the beginning of the block is corrupt (Block content is corrupt). It may cause different ORA-600 errors. 

The detailed corruption description for Logical Corruptions are not normally printed in the alert.log. DBVerify will report what is logically corrupted in the block.

Corruption Examples are:

• row locked by non-existent transaction - ORA-600 [4512],etc
•  the amount of space used is not equal to block size
• avsp bad
• etc.

When db_block_checking is enabled, it may produce the internal errors ORA-600 [kddummy_blkchk] or ORA-600 [kdBlkCheckError].

If db_block_checking is enabled and the block is already logically corrupt on disk, the next block update will mark the block as Soft Corrupt and future reads of this block will produce the error ORA-1578. In that case DBVerify reports this corruption with error "DBV-200: Block, dba , already marked corrupted".