Notes on selection of the flash disk drives | September 11, 2018 by Zibby Rytelewski
We offer premium flash drives with MLC and SLC cells. In practice only nuclear and MIL type clients can justify the cost of SLC flash drives:
• SLC -- Single Level Cell: longest lived (high P/E), and generally fastest. Bits are stored only as 2 voltage levels, or a "1" or "0." In SLC less data is stored per cell, so the per unit storage cost is about 3 times higher than MLC flash.
• MLC -- Multi-Level Cell: is consumer grade type used in personal computers, phones, cameras, and USB sticks. The stored charge in MLC may be interpreted as a variety of values, 0 to 3, or 4 possible states, and may store 2 bits. With shorter lifetimes, usually 10x less than SLC, the advantage of this memory is that the cost is 2- 4x less than SLC, but with lower write speeds. MLC typically uses some form of error correction code per block.
Transduction was one of the first companies to offer solid state flash drives. We liked the idea of using flash drives in the industrial environment on the promise that they were more reliable and much faster than hard drives. Unfortunately, early offerings of the flash drives were unreliable and we ended up replacing many of them. They all looked the same in the sealed cases and we did not have diagnostic tools to check them well.
Fortunately, the S.M.A.R.T. technology came out, which gave us means of testing flash drives in detail, to see usage of the flash, error rate, read and write transfer speeds, bad blocks and many other parameters. Prior to that purchasing flash drive was like buying cat in a bag. Reliability of the flash drives is a big issue as much depends on the grade of the MLC flash. Some Asian suppliers use lowest grade flash purchased on the spot market and try to mask it with error correction logic. This is why flash drives need to be purchased from reputable sources that use high quality flash cells. Another problem we faced was that our industrial clients like logging (writing and erasing) a lot of event files. This was chewing the flash cells very fast and on the average industrial installation the drive would last only a year. Even worse, conditioned by the old habit of defragmentation of the hard drives, clients would “defrag” flash drives, which with the multiple read a and writes chews up a lot of flash space. There is no need to defragment flash drives as seek times (no moving head) are very fast. For these reasons we increased capacity of the flash drives from 32 to 128 GB.
But another problem popped out related to improper power shutdown of Windows O/S. It was related to the cache buffer inside the flash drive. Upon sudden power loss there was not time to write contents of the cache to the flash. This create bad blocks on the drive that in many instances prevented re-boot of the system. This problem was fixed by the C-CAP, also known as POWER-GUARD feature. This is a bank of extra capacitors and logic to provide enough power to write contents of the cache buffer in the event of the loss of power.
With the arrival of the fanless computers we learnt to offer flash drives only with the extended temperature range as only those would survive over long periods of time. Fanless computers generate a lot of internal heat that accelerates aging of the internal components. Hard drives should not be used in the high temperature environments.
In summary the good “industrial” type flash drives should be reliable and be readily accessible with S.M.A.R.T. diagnostic tools, have capacity of minimum 128GB with reasonable read/write speeds, operate over wide temperature range and have POWER-GUARD feature. And MTBF > 100,000 hours.
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