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Disk Media Management
390 C141-C013
(2) Alternate block allocation
Alternate data blocks are allocated in defective sector units for defective data
blocks (= sectors) on the disk media by the HDD'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.
− 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 cells for allocation of alternate blocks. Alternate
cells are secured only in the specified quantity in order from the topmost
cells. 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.