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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Geometry and Low-Level Data Structures | Hard Disk Formatting and Capacity ]
Defect Mapping and Spare Sectoring
Despite the precision manufacturing processes used to create hard disks, it is virtually impossible to create a disk with several million sectors and not have some errors show up. Imperfections in the media coating on the platter or other problems can make a sector inoperable. This usually shows up as errors attempting to read the sector, but there are other error types as well.
Modern disks use ECC to help identify when errors occur and in some cases correct them, however, there will still be physical flaws that prevent parts of a disk from being used. Usually these are individual sectors that don't work, and they are appropriately enough called bad sectors. If you've ever used a disk information utility on an older hard disk, or on a floppy disk, you've likely seen a report showing a few kilobytes worth of bad sectors. However, if you run such a utility on a modern hard disk, you will basically never see any reports of bad sectors on the disk. Why is this?
When the disk drive is manufactured, it is thoroughly tested for any areas that might have errors. All the sectors that have problems or are thought to be unreliable, are recorded in a special table. This is called defect mapping. On older hard disks, these were actually recorded--usually in hand-writing by the technician testing the drive--right on the top cover of the disk! This information was necessary because low-level formatting was often done by the end-user and this information was used to tell the controller which areas of the disk to avoid when formatting the disk. In addition to marking them in a physical table on the outside of the disk, each defect is marked inside the drive as well, to tell any high-level format program not to try to use that part of the disk. These markings are what cause the "bad sectors" to show up when examining the disk.
While early PC users accepted that a few bad sectors on a drive was normal, there was something distasteful about plopping down $1,000 for a new hard disk and having it report "bad sectors" as soon as you turned it on. There is no way to produce 100% perfect hard disks without them costing a small fortune, so hard disk manufacturers devised an interesting compromise.
On modern hard disks, a small number of sectors are reserved as substitutes for any bad sectors discovered in the main data storage area. During testing, any bad sectors that are found on the disk are programmed into the controller. When the controller receives a read or write for one of these sectors, it uses its designated substitute instead, taken from the pool of extra reserves. This is called spare sectoring. In fact, some drives have entire spare tracks available, if they are needed. This is all done completely transparently to the user, and the net effect is that all of the drives of a given model have the exact same capacity and there are no visible errors.
Really, when you think about it, the hard disk companies are sacrificing a small amount of storage for "good looks". It would be more efficient to use all of the sectors on the disk and just map out the few bad ones. However, sometimes marketing wins out over engineering, and it seems that more people want the warm feeling of thinking they have a perfect drive, even if it costs them theoretical storage in the process.
Because of spare sectoring, a brand new disk should not have any bad sectors. It is possible for a modern IDE/ATA or SCSI hard disk to develop new bad sectors over time. This is usually an indication of a greater problem with the disk. The vast majority of time, a single bad sector showing up will soon be accompanied by more of its friends. Make sure to contact the vendor if you see bad sectors appearing during scans, and make sure the data is backed up as well. See here for more.
Next:
Unformatted
and Formatted Capacity
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