Subject:  QREF: Physical Oracle7 Data Block Layout 
Creation Date:  04-NOV-1996 

Physical Oracle Data Block Layout
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    This is a quick outline of the physical layout of an Oracle datafile block.
    It does not apply to online redo blocks or controlfiles.

    The physical data block size is determined by the init.ora 
    <parameter:DB_BLOCK_SIZE> set at database creation.

    Each formatted data block on disk has a block wrapper to identify it.
    This consists of a 20 byte header and a 4 byte trailer as below.
    Unformatted blocks are completely ZERO throughout.

	|   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
	| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 |11 |12 |13 |14 |15 |

	+-------+-------+---------------+---------------+---------------+
	|Typ|Ver|Filler |      DBA      |  Incarnation  |   Sequence    |
	|-------+-------+---------------+---------------+---------------|
	|ChkSum |Filler |<------ 					|
	|-------+-------+						|
	:		   Body of 					:
	:		    Data Block  				:
	:		    [NOTE:33188.1]				:
	|                                               +---------------|
	|                                     --------->|    IncSeq     |
	+---------------------------------------------------------------+

	| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | A | B | C | D | E | F |
	|   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |

    The bytes in the header are defined as:

	Typ	Block Type (defined in k.h). For a full list see [NOTE:33188.1]

	Ver	Block version. In Oracle6 and 7 this is 0x01.
		0x02 indicates and Oracle8 format block

	Filler	Not used at present

	DBA	Database address of the block. See [NOTE:33311.1]
		Use 'odba' to decode a DBA.    See [NOTE:30322.1]

	Inc	Incarnation of the block. Is incremented each time the 
		block is 'newed'.

	Seq	Sequence number of the block. A change version for the current
		incarnation.

	IncSeq	Lower order 2 bytes of Incarnation plus lower 2 order bytes
		of the Sequence number. This is used to help detect split
	 	blocks where the tail of the block may be from a different
		version of the block to the header.

	ChkSum	This is a simple checksum on the contents of the block.
		It uses the routine smschk() and is only present:
		    a) always for file header blocks
		or  b) if <Parameter:DB_BLOCK_CHECKSUM> is set.


What is a Soft Corrupt Block ?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	A soft corrupt block is defined as an Oracle data block which has the
	Seq set to ZERO. The 'IncSeq' at the tail of the block is set to 
	match the block header as normal. This special condition is used by
	special block-checking code to mark a block as corrupt from an Oracle
	viewpoint. Subsequent attempts to access the block will result in 
	ORA-01578 (unless <Event:10231> is used with a full table scan).

	This special block checking is performed:

		a) By PMON
		b) By any Oracle process if any of the events below are set:
			Data Blocks	<Event:10210>
			Index Blocks	<Event:10211>
			Cluster Blocks	<Event:10212>


 

Search Words
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ORA-1578