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One method of skipping bad sectors on an iPod's drive
Sorry, but that's not quite right. The drive does maintain a set of "spare" sectors as you said, but the remapping process is more complicated. If a sector fails to read, it does not immediately get remapped. Rather, it is added to the Current_Pending_Sector list, and the read fails. The drive leaves the sector as-is in the hope that some future attempt to read it will succeed, thus avoiding data loss. If the sector was remapped on a failed read, it would mean definitely losing data. If a later read attempt succeeds, the sector is generally removed from the Current_Pending_Sector list and is considered "good" again. Only if a write fails to a sector in the Current_Pending_Sector list does a sector get remapped (most drives don't verify writes to "good" sectors by default for performance reasons, so the only chance to catch a failed write is when the sector is in the Current_Pending_Sector list). In this case, the drive can try again, this time writing to a spare sector, without losing the data to be written. Now the bad sector gets remapped. The drive's Current_Pending_Sector count will decrease, and the Reallocated_Event_Count will increase by a corresponding amount. In fact, things can be even more complicated than this, as some drives don't like to remap individual sectors (for performance reasons), and will try to remap larger chunks or do other fancy things. Anyway, the point is that forcing the OS to write some junk data to sectors in the Current_Pending_Sector list will usually force them to be remapped. Even if they're not remapped, they at least won't accidentally get used for your important data*. * Again, there are subtleties. The OS sometimes moves active files to different parts of the disk (see Apple dev docs on "hot files"), and this could in principle re-expose the bad sectors if they weren't actually remapped. In practice, this is very unlikely, as there shouldn't be many requests for this junk data (and so it will never be "hot"). You can use smartmontools to see all sorts of really detailed information on your drive, including the Current_Pending_Sector count and the Reallocated_Event_Count. If a drive has dozens of Current_Pending_Sectors and Reallocated_Event_Counts, I usually take it as a sign that the drive is getting older, and should be backed up frequently if it contains important data. For a music-only iPod, however, I wouldn't worry about it. |
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