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Expanded Remapping and Spare Sectoring

When any hard disk detects a problem with a particular sector on the surface of the drive, it will remap that sector and mark it as "bad" so that it will not be written to again in the future. This ensures that repeat problems with this bad spot will not recur. This process is called remapping and spare sectoring and is described in this section on hard disk formatting.

Some drives go a step beyond ordinary remapping. Instead of just remapping the sector where an error is encountered, or where a number of retries were necessary to get the data from the disk, the controller remaps a zone of sectors around the defective location. The logic behind this feature is that if an area of the disk is damaged badly enough to create a bad sector, the problem might not be limited to just that sector, even if the errors are only showing up there right now. After all, a single sector on a hard disk is very small; it stands to reason that there is a much higher chance that a sector near the defective one will go bad than a random sector somewhere else on the disk.

The "buffer area" that is remapped can include sectors before and after the bad sector, as well as adjacent tracks, thus covering a two-dimensional space centered on the bad disk location. This process occurs transparently to the user. It makes use of spare sectors allocated on the drive for the purpose of remapping.

Western Digital calls this feature defect margining. As with the wear leveling feature, defect margining does not appear to have been universally adopted by other hard disk manufacturers. Unlike wear leveling, there is not to my knowledge any major drawback to this feature, and it seems in theory to be a very good idea. (I suppose it might require a few extra spare sectors to be placed on the drive, but capacity is very cheap these days.) I do not know why the other drive makers are not also doing this--maybe they are doing it but just not publicizing it.

Next: Redundant Arrays of Inexpensive Disks (RAID)

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