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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 one or both of the following
areas. 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 3.1.2
"Spare Area for Alternates" in the "Product Manual."
• Spare Sectors Within a Cylinder
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 Cylinders
This is an exclusive cylinder for allocation of alternate blocks. Alternate cylinders are
secured only in the specified quantity in order from the topmost cylinder. Spare sectors in
an alternate cylinder are used only in the case that the spare sectors within each cylinder
are used up and alternate block allocation is not possible within the same cylinder.
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 cylinder as the defective sectors and until all
the spare sectors within that cylinder 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 cylinder or to unused spare sectors in an alternate
cylinder.
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 3.3.2 "Alternate Block Allocation"
in the "Product Manual."
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C141-E039-01EN
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