Rockwell Automation Allen-Bradley 6181P-12NPXPH, Руководство пользователя

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66 Publication 6181P-UM001G-EN-P - July 2008
Appendix D Solid State Drive
Wear leveling algorithms in flash memory drives offer an advantage to
the overall throughput of the memory and the life expectancy of the
drive. Wear leveling evenly distributes data that is written to memory
across all free space on the drive. As one block of memory is written
and filled, another is made available for the next data transfer. This
block mapping occurs across all the free space and then starts again at
the initial block. At the same time, the drive’s controller erases blocks
previously used allowing memory to be available for updates without
delaying the write requests from the host CPU.
When a memory cell fails within a specific sector, the entire block
where the sector resides is marked as bad and removed from use. A
new block from the pool of spares replaces the bad block. The
amount of spare blocks within the pool is determined by the size of
the drive memory. The size of the pool typically falls in the range of 1
to 1.5% of the total drive memory space and is set when initially
formatted.
Typical Solid State Drive
Application Example One
An application updates a 100K data file every second on the 4 GB
SimpleTech drive that is approximately half full with the OS image
and application software.
Use this formula to calculate expected life.
(Flashblocks x Write Cycle limit x Drive Freespace) / [(1 block/sec) x (31,536,000sec/yr)]
(32,768 x 2,000,000 x .5) / [ (1 block/sec) x (31,536,000 sec/yr)] = 1039 years
Drive SimpleTech 4 GB
Sector 512 bytes
Sectors 8,388,608
Blocks 32,768 (1 block = 131,072 bytes)
Zone 512 blocks (wear level boundary)
Write Cycle Limit 2 million
Free Space 4 GB (OS image + installed applications)
1 Year 8,760 hours
Seconds/Year 31,536,000
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