Oracle8 has introduced a change with the data block cache header format.  The
basic idea is that incarnation and sequence numbers stored in the cache header
have been replaced with an SCN number and sequence number.  The size of the
cache header has remained 20 bytes.  The size of the block trailer is still 4
bytes.  Only the format of the cache header and the trailer has changed.

Oracle7 Implementation
~~~~~~~~~~~~~~~~~~~~~~

The current Oracle7 implementation stores the incarnation and sequence number
in the cache header of each data block to determine the current version of the
block.  A compressed version of the incarnation/sequence is maintained at the
end of the data block.  The incarnation and sequence numbers are each 4 byte
values.  The low-order 2 bytes of each value are stored as the last 4 bytes of
the data block.  This information is used to detect media corruption when
reading the block for normal operations or during recovery, or when validating
the block.  It allows for a consistency check to ensure the top of the block
is in sync with the bottom of the block.

See [NOTE:33242.1] for more details on the Oracle7 format.

Oracle8 Implementation
~~~~~~~~~~~~~~~~~~~~~~

The basic idea is to store the current redo generating SCN in the cache header
when making a change to a block.  Since multiple changes to a block can be
made at the same SCN, a sequence number is also stored in the cache header to
differentiate between different changes at the same SCN.  The sequence number
is increased each time a change is made at the same SCN.  The sequence number
is reset to 1 when making a change at a higher SCN than the SCN currently in
the block.

Oracle8 data block layout
-------------------------

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

        +-------+-------+---------------+---------------+---------------+
        |Typ|Fmt|Filler |     RDBA      |    SCNBase    |SCNWrap|Seq|Flg|
        |-------+-------+---------------+---------------+---------------|
        |ChkVal |Filler | <------                                       |
        |-------+-------+                                               |
        :                       Body of                                 :
        :                        Data Block                             :
        :                                                               :
        |                                               +---------------|
        |                                    ---------> |     Tail      |
        +---------------------------------------------------------------+

        | 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).

        Fmt     Block format.  In Oracle8 this is 2, converted on-the-fly.

        Filler  Not used currently.

        RDBA    Relative database address of the block.

        SCNBase SCN Base

        SCNWrap SCN Wrap

        Seq     Sequence number.  Incremented for every change made to the
                block at the same SCN.

        Flg     Flag (defined in kcbh.h)

        ChkVal  Optional check value for the block.  Set so that an xor of all
                the ub2 values in the block equals zero.  This may require the
                value to be zero (unlike Oracle7).  ChkVal will be verified
                only if KCBHFCKV is set in the Flg field.
                   - always for file header blocks
                   - if the init.ora <parameter:DB_BLOCK_CHECKSUM> is set.

        Tail    Consistency information used to verify the beginning and the
                end of the block are of the same version.  Lower order 2 bytes
                of SCNBase, plus block Type, plus SCN Seq number.

Migration to the New Format
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Data block cache headers will be upgraded to the new Oracle8 format on-the-fly
when the block is read into the cache.  Oracle8 will do this automatically for
every block that is read into the cache during normal operation. The on-disk
copy is not updated unless the block becomes dirty.

Soft Corrupt in Oracle8
~~~~~~~~~~~~~~~~~~~~~~~

  Soft corrupt blocks in Oracle8 have:
     Seq# is 0xff
     flg  is 0x00