Fujitsu MAX3073NC - SCSI,80, Dell DC961,Hard Drive Scsi Logical Interface Specifications page 286

Disk Media Management
(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 cell. They are used as alternate blocks for
defective sectors within the same cell.
• 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 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."
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