RAM 6.0 BUX II Series Guía de usuario descargar pdf (Pagina 28)

VxWorks

Hardware Considerations Guide, 6.0

Block Allocation and Data Clusters

As is required of a block device driver, TrueFFS maps flash memory into what

appears to be (from the users perspective) a contiguous array of storage blocks,

upon which a file system can read and write data. These blocks are numbered from

zero to one less than the total number of blocks. Currently, the only supported file

system is dosFs (for more information, see the appropriate VxWorks

programmer’s guide for your release).

Block Allocation Algorithm

To promote more efficient data retrieval, TrueFFS uses a flexible allocation strategy,

which clusters related data into a contiguous area in a single erase unit. These

clusters might be, for example, the blocks that comprise the sectors of a file.

TrueFFS follows a prioritized procedure for attempting to cluster related data in

the following order:

a. First, it tries to maintain a pool of physically consecutive free blocks that

are resident in the same erase unit.

b. If that fails, it then tries to assure that all the blocks in the pool reside in the

same erase unit.

c. If that fails, it finally tries to allocate a pool of blocks in the erase unit that

has the most space available.

Benefits of Clustering

Clustering related data in this manner has several benefits, listed and described

below.

■

Allows Faster Retrieval Times

For situations that require TrueFFS to access flash through a small memory

window, clustering related data minimizes the number of calls needed to map

physical blocks into the window. This allow faster retrieval times for files accessed

sequentially.

■

Minimizes Fragmentation.

Clustering related data cuts down on fragmentation because deleting a file tends

to free up complete blocks that can be easily reclaimed.

■

Speeds Garbage Collection

Minimizing fragmentation leads to faster garbage collection.

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RAM 6.0 BUX II Series Guía de usuario Pagina 28