Description
During a low-level format, defect lists are populated which store which sectors are bad. The bad sectors are then mapped and a sector slipping algorithm is utilized. When using sector slipping for bad sectors, disk access time is not largely affected. The drive will just skip over the sectors and takes the same time as it would have to read the sector. Spare sectors are located on the disk to aid in having sectors to “slip” other sectors down to, allowing for the preservation of sequential ordering of the data. Accuracy of programs reliant on static knowledge of cylinders and block positions will be compromised though.
Bad sectors that are found during normal usage of the disk are not capable of having the sector slipping algorithm applied. Instead a linear reallocation, or sector forwarding, is used where a bad sector is replaced with a sector from a spare area. Doing this does affect access times as the disk will need to seek to the spare sector since all further lookups of the bad sector will redirect to the new sector.
Read more about this topic: Sector Slipping
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