Technical Success Stories: SLC SD Card Solution- Terabytes Written & Drive Writes
“We were told that Terabytes Written (TBW) and Drive Writes Per Day (DWPD) are the best way to compare NAND storage.”
The technology surrounding SLC SD cards, and Industrial Grade Flash storage in general is making it harder and harder to compare one solution, or one supplier to another. It used to be as easy as comparing SLC flash and MLC flash, but eventually this progressed into TLC, eMLC (Enterprise MLC) and now 3D NAND. For some time, the preferred method was to analyze the P/E cycles (Program/Erase) of the flash. This allowed some math to be done to guess, based on your file size and total data written, how long the flash would last for the host. This was overtaken by TBW, or the ability to determine the Total Terabytes Written over time, and then rate flash this way. The truth is that the behavior of flash is actually determined by a number of factors. You must take into account performance, data retention and endurance, density, and the design of the flash and CPU in order to truly make a more accurate determination of the life of the storage you are considering to buy.
If you dig a little deeper into NAND flash, one concept most users are not aware of is the Write Amplification (WA) associated with flash. In simple terms, WA is when an entire block needs to be programmed, when a relatively small amount of data is written. This means the entire block is programmed or worn when only a small piece of new or changed information was introduced. This negates the value of TBW or DWPD based flash life calculators, as in this case the quantity of data written has been amplified dramatically as the design of the flash requires an entire block to be programmed. This is why a flash life calculator, in order to have any accuracy at all, needs to take into account more than just the amount of data written over time, but the design and density of the flash. Note that generally most card-based NAND storage is block-based, while many SSD type storage is page-based, and less affected by WA.
The Medical device manufacture buyer was frustrated. She was changing cards, and capacities, and suppliers for the 3rd time in 3 years, and her compliance people were frustrated as well. In Medical devices, the approvals and testing is expensive and can take sometimes months. She was demanding at least a 24 month life cycle on this card, rather than the 18 months the Delkin Salesperson was offering her based on our flash roadmap.
The Customer Applications Team explained that most Industrial embedded storage manufactures were buying flash from the same 4 or 5 flash makers, and most were using Toshiba or Micron. Anyone that could promise flash out beyond 18 months was not being honest or was planning on holding flash for her. That was a possibility she was considering, and she was asking how that would work.
Almost as an afterthought, the Delkin consultant asked if she was willing to allow one of our product managers to run a model on her usage, something that would predict flash-endurance, and compare an MLC based SD card with one of our SLC SD cards.
The Delkin models to compare MLC and an SLC SD cards are one of the more sophisticated in the market, and in addition to taking into account the TBW and capacity of the card, also used the usage model and file size and how it was written in our measurement. Along with the target price and the TCO would allow a fair comparison of technologies, which might be the solution the buyer was looking for.
The first step for the Delkin Customer Applications team was a complete understanding of the requirements of the customer. Starting with the usage model of the cards, Delkin was able to determine the P/E cycles, TBW, workload and design of the flash, to come up with an expected life of the flash and flash-endurance in this customer’s application. As in all things, the more accurate the data, the more realistic the time estimate would be.
When the model was sent over to the customer, the results were astounding. If the customer were to switch to a 512MB SLC SD card, from a 8GB MLC card, the P/E cycles would increase by 20 times. This along with the other factors showed that the useful life of this 512MB card was actually 10.2 years, compared to the 8GB MLC cards .6 years useful life. The icing on the cake that convinced this customer that the SLC SD card was the best solution, was the fact that the SLC SD card had a life cycle of more than 4 years, based on the SLC flash roadmap, far exceeding the 8GB card she was considering.
You have to choose carefully when designing in an industrial storage solution for your embedded device. Besides finding one which meets your requirements for physical size, capacity, temperature, cost, functionality, TCO and your usage model, you need to choose a solution that may need some custom requirements of your host application. Industrial flash storage products rely on a strong, modern CPU to bring features your host can use, so consult with Delkin to help you make the right choice on an embedded storage solution that meets your needs.
Lean on Delkin’s Technical Team
Have you had a problem with your host storage? The Delkin Customer Applications team stands at the ready to be your trusted advisor for your host critical storage. We have solved hundreds of complicated host failures and look forward to understanding your usage model for flash-based Rugged Controlled Storage. Contact us today.
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