Sector Sparing

According to my solution manual, "Sector sparing can cause an extra track switch and rotational latency, causing an unlucky request to require an extra 8 ms of time."

What do they mean by this and why is it true?
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If it means that it is not using a particular sector, the disk would have to spin past that sector before it can read data again.  So it would increase the delay in finding the data of interest.
Sector sparing is used in defect management, typically a sparing table and "spare sectors" are set  aside for when the system encounters "bad tracks" (bad blocks, bad sectors)   The defect management tries its' level best to read the data from the bad spot one last time and then that "chunk" of information is copied (moved) to a "spare" and the bad sector is "bad tracked", marked bad not to be used.  The file chain sequence has to "detour" to the spare sector.  Often this bad sector handling is handled directly by the logic circuit on the drive itself, "under the blankets" if you will.  So, if from the operating system's perspective you were to "perfectly" defragment that file, on the drive itself physically the read-write heads would still have to make a "detour" to the spare sector likely at a different area of the disk thus adding a slight delay to the "unlucky" request as stated.

Sector slipping is a way of handling physical bad sectors at the time the logical layout of the drive is done, typically during "low level" formatting.  When a bad sector is identified the disk tables are set to simply ignore it, passes right over it, and the next link in the file chain "slips" to the next good sector after it, so no "detour" jumping back and forth physically need ocurr.

Good luck with your exams

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JCW2Author Commented:
Thank you for your help.
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System Utilities

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