Fujitsu MHA2021AT Product Manual page 154

(2) Alternate cylinder assignment
A defective sector is assigned to the spare sector in the alternate cylinder.
This processing is performed when the alternate assignment is specified in the
FORMAT TRACK command or when the automatic alternate processing is
performed at read error occurrence.
Figure 6.8 shows an example where (physical) sector 5 is detective on head 0 in
cylinder 0.
2 alternate cylinders are provided for each head in zone 12 (inner side).
When an access request to physical sector 5 is specified, the device accesses the
alternated sector in the alternate cylinder instead of sector 5. When an access
request to sectors next to sector 5 is specified, the device seeks to cylinder 0, head
0, and continues the processing.
(3) Automatic alternate assignment
The device performs the automatic alternate assignment when ECC correction
performance is increased during read error retry, a read error is recovered. Before
automatic alternate assignment, the device performs rewriting the corrected data
to the erred sector and rereading. If no error occurs at rereading, the automatic
alternate assignment is not performed.
An unrecoverable write error occurs during write error retry, automatic alternate
assignment is performed.
C141-E042-01EN
Figure 6.8 Alternate cylinder assignment
6.4 Defect Management
6-13