7.3 Alternate Block Allocation Processing
C141-C013 395
7.3 Alternate Block Allocation Processing If errors on the disk media which cannot be recovered from are detected, or if a
recoverable error occurs repeatedly in the same data block, it is necessary for the
INIT to allocate an alternate block for the data block where the error occurred
using the REASSIGN BLOCKS command. The procedure for alternate block
processing is shown below.
An attempt will be made to copy the contents of the data area of the
logical data block specified in the "Defect Data" list to the alternate
block allocated by this command, but in some cases, the data cannot
be copied. Confirmation of the contents of data in the allocated
alternate block, saving of data before issuing this command and
restoring of data after executing this command are the responsibility
of the INIT.
1) Verify data blocks in accordance with the procedure in Section 7.3 for data
blocks where error occurs. When such a data block is judged to be normal,
alternate block allocation need not be implemented. In this case, rewrite the
original data to that data block if necessary. When the data block is judged to
be defective, implement the processing in item 2) and after.
2) Specify a logical block address for a data block which is judged to be
defective and issue the REASSIGN BLOCKS command.
3) If the REASSIGN BLOCKS command is terminated normally, verify that
logical data block according to the procedure in Section 7.3. If that data
block is judged to be correct, allocate an alternate block and terminate
processing. When the data block is judged to be defective, return to 2) and
issue the REASSIGN BLOCKS command again, then try reallocating another
alternate block for that logical data block.
The alternate block allocation history (defect position information) is registered as
an addition to the defect list (G List). Therefore, when reinitializing the hard disk
with the FORMAT UNIT command (see Section 7.2.2), if use of the G List is
specified, defect processing can be performed which reflects the operation results
up to that time. Also, since "Alternate sector processing" is applied in alternate
block allocation in the REASSIGN BLOCKS command, the physical connectivity
of the logical data block positions on the disk media is destroyed, but by
reinitializing the disk, as long as the number of defective sectors in the cylinder
does not exceed the number of spare sectors per cylinder, the logical data blocks
are relocated by sector slip processing so that their physical connectivity is
maintained.