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Disk Media Management
6 - 2
C141-E167
(2) Alternate block allocation
Alternate data blocks are allocated in defective sector units for defective data blocks (= sectors) on
the disk media by the IDD's internal defect management methods.
a. Spare sector area
Spare sectors for allocation as alternate blocks are secured in both of the following areas or
alternate cell. When the INIT initializes the disk media, it can specify the spare sector area to be
prepared on the disk drive and the size of that area. For details, see the description in Section
3.1.2 "Spare Area for Alternates" in the "Product Manual."
• Spare sectors within a cell
Spare sectors are secured in the final track of each cylinder. They are used as alternate
blocks for defective sectors within the same cylinder.
• Alternate cells
This is an exclusive cell for allocation of alternate blocks. Alternate cells are secured only in the
specified quantity in order from the topmost cylinder. Spare sectors in an alternate cell are used
only in the case that the spare sectors within each cell are used up and alternate block allocation is
not possible within the same cell.
b. Alternate block allocation methods
Alternate block allocation processing is performed using one of the following methods.
• Sector slip processing
This is a method of skipping defective sectors and writing the logical data blocks
corresponding to those sectors in order in physically consecutive sectors. Sector slip
processing is performed only within the same cell as the defective sectors and until all the
spare sectors within that cell are used up.
• Alternate sector processing
This is a method of allocating logical data blocks corresponding to defective sectors to unused
spare sectors within the same cell or to unused spare sectors in an alternate cell.
c. Alternate block allocation processing
Alternate block allocation processing is implemented by the FORMAT UNIT command,
REASSIGN BLOCKS command or "Auto Alternate Block Allocation Processing." In sector slip
processing, the defective sectors re excluded and logical data blocks are located in physically
consecutive sectors, but when alternate block processing is performed, the affected logical data
blocks are located in spare sectors which are not physically consecutive with the previous and
following logical data blocks which are physically located on the disk. For details, see the
description in Section 3.3.2 "Alternate Block Allocation" in the "Product Manual."