Rockwell Automation 6181X-12TPXPDC Industrial Integrated Computers for Hazardous Locations - Manual (Page 61)

Rockwell Automation Publication 6181X-UM001B-EN-P - May 2011 61
Solid-state Drive Appendix D
Life Expectancy The limitation of the solid-state drive is the finite amount of writes to a specific
memory location. The life expectancy of a solid-state drive varies depending on
the following:
• How often an application writes data to the drive
• The size of the file written to the drive
• The amount of available space on the drive that can be used by the drive’s
controller to write data
• The write/erase cycle limitation the manufacturer specifies
On the flash drive, the smallest space that can be written to is a sector, which is
512 B. However, the smallest space that can be erased is a block of memory.
Unlike magnetic media, an erase cycle is required for flash memory before a write
can be made. The actual block size varies depending on the overall capacity of the
flash drive, for example, 63 sectors, or 32 KB on a 4 GB 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…1.5% of the total drive memory space and is set when initially
formatted.
Table 7 - Typical Solid-state Drive
Attribute Value
Drive 4 GB
Sector 512 B
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)
1year 8,760 hr
Seconds/year 31,536,000