标签：each val 还需 drive nbsp some mic 改进 read
http://blog.chinaunix.net/uid-20737871-id-1881191.html
Let the Static Wear Leveler (SW Leveler) be associated with a Block Erasing Table (BET) to remember which block has been erased in a selected period of time. The SW Leveler is activated by some system parameters for the needs of static wear levelling. When the SW Leveler is running, it either resets the BET or picks up a block that has not been erased so far (based on the BET information) and triggers the Cleaner to do garbage collection on the block. The selection procedure of a block must be done in an efficient way within a bounded amount of time. Note that the BET must be updated whenever a block is erased. It could be done by a triggering action to the SW Leveler. The design of the BET must be scalable because of the rapid increasing of flash-memory capacity and the limited RAM space on a controller. Whenever a block is recycled by garbage collection, any modification to the address translation is done as the original design of a Flash Translation Layer driver. The implementation of the SW Leveler could be a thread or a procedure triggered by a timer or the Allocator/Cleaner based on some preset conditions.
The SW Leveler is invoked by the Cleaner to update the BET whenever any block is erased by the Cleaner in garbage collection when static wear levelling is needed. We can use two variables to keep track of the total number of block erases done since the BET is reset and the number of 1’s in the BET. If the ratio of the two tracked number is too high, the SW Leveler is triggered to move cold data from their original place by requesting the Cleaner to reclaim those blocks whose corresponding bit in the Block Erasing Table is 0.
The purpose of the Block Erasing Table (BET) is to remember which block has been erased in a pre-determined time frame, referred to as the resetting interval, so as to locate blocks of cold data. A BET is a bit array, in which each bit corresponds to a set of 2k contiguous blocks where k is an integer that is larger or equal to 0. Whenever a block is erased by the Cleaner, the SW Leveler is triggered to set the corresponding bit as 1.
动态平均读写解决了块的 Erase 周期的次数限制。动态平均读写算法并不是随机使用可用的块，而是平均使用块，因此，每个块都获得了相同的使用机会。静态平均读写算法解决了一个更有趣的问 题。除了最大化 Erase 周期的次数外，某些 flash 设备在两个 Erase 周期之间还受到最大化 Read 周期的影响。这意味着如果数据在块中存储的时间太长并且被读了很多次，数据会逐渐消耗直至丢失。静态平均读写算法解决了这一问题，因为它可以定期将数据移 动到新块。标签：each val 还需 drive nbsp some mic 改进 read
原文地址：http://www.cnblogs.com/constanto/p/7843586.html
