RAM 6.0 BUX II Series Guía de usuario descargar pdf (Pagina 32)

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Hardware Considerations Guide, 6.0

degradation of performance. Given the large number of allowed erase cycles, this

less-than-absolute approach to wear-leveling is sufficient.

Dead Locks

Finally, the TrueFFS wear-leveling algorithm is further enhanced to break a failure

mode known as dead locks. Some simple wear-leveling algorithms have been

shown to “flip-flop” transfers between only two or more units for very long

periods, neglecting all other units. The wear-leveling mechanism used by TrueFFS

ensures that such loops are kept to a minimum. For optimal wear-leveling

performance, TrueFFS requires at least 12 erase units.

Garbage Collection

Garbage collection, described in Reclaiming Erased Blocks, p.26, is used by the erase

cycle to reclaim erased blocks. However, if garbage collection is done too

frequently, it defeats the wear-leveling algorithm and degrades the overall

performance of the flash disk. Thus, garbage collection uses an algorithm that

relies on the block allocation algorithm (Block Allocation Algorithm, p.24), and is

triggered only as needed.

The block allocation algorithm maintains a pool of free consecutive blocks that are

resident in the same erase unit. When this pool becomes too small, the block

allocation algorithm launches the garbage collection algorithm, which then finds

and reclaims an erase unit that best matches the following criteria:

■

the largest number of garbage blocks

■

the least number of erase cycles

■

the most static areas

In addition to these measurable criteria, the garbage collection algorithm also

factors in a random selection process. This helps guarantee that the reclamation

process covers the entire medium evenly and is not biased due to the way

applications use data.

Fault Recovery in TrueFFS

A fault can occur whenever data is written to flash. For example, a fault can occur:

■

in response to a write request from the file system

■

during garbage collection

■

during erase operations

■

during formatting

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RAM 6.0 BUX II Series Guía de usuario Pagina 32