However, in the better flash devices the FTL will remap even the unchanging sectors over time such that the wear-leveling is spread across all the device's erase sectors. These devices approach the ideal erase/rewrite limit (number of sectors * rewrites per sector).
For example, assume we have a flash device with five sectors: four visible plus one spare. After writing to the full device there are four active sectors (phys. 1-4) and one on the clean list (5). Rewriting logical sector 3 changes the active mapping to (1->1, 2->2, 3->5, 4->4), empties the clean list, and adds phys. sector 3 to the trash list. Rewriting sector three again with a simple FTL just causes phys. sector three to be erased and swapped with sector 5 on the trash list; none of the other sectors are wear-leveled. With a better FTL, however, the device may decide to place the new data in phys. sector 1, moving the original data for the first logical sector over to phys. sector 5 instead (active: 1->5, 2->2, 3->1, 4->4; trash: 3). This introduces an extra erase operation (on *some* writes), but now the changes are spread across three sectors rather than the original two, and additional writes would be further spread to sectors 2 and 4 as well. The end result is that all the sectors end up with similar numbers of rewrites.
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