A Question Of Endurance
TH: So the risk is than that I won't be able to write to it. But there’s a linear relationship between endurance and capacity, right? As you double the capacity, you double its expected lifespan?
TC: Yes. Data endurance numbers doubled for the same about data read and written per day.
TH: Given that, how many years are we expecting from the current crop of drives?
LK: Well, that's the hard part. You almost have a sliding scale if we’re talking about client usage models to server usage models. They’re very different. The worst kind of writes that you can apply to an SSD are random. You will wear a drive out quicker that way. If all the writes are sequential, that's the best case scenario for an SSD. A typical client workload is probably a mixture of those—not all random, not all sequential.
TC: For example, with our new V+ SSD, the proper life is like this: usually, we say the effective read/write duty is about 20% of the power-on hours. With this, normal operation is 8,760 hours per year, and this allows you to read and write 20 gigabyte per day in operation. With these numbers, we know that an SSD’s expected product life is actually much better than a traditional hard drive.
TH: Let’s circle back to that difference in the effect on endurance of sequential versus random writes. If the drive controller dictates how every bit gets written to the drive and wears the memory evenly, why there is a difference between sequential and random?
TC: Let's put it this way. Most of the time with your hard drive, you need an operating system. The random reads and sequential reads have a major effect on the system’s behavior. When you boot up your computer, you are doing a sequential read. Same with hibernating and application loads. But a lot of times, you also need to access your information, the user data, and that's a random read because your data is spread out everywhere. Now, with a hard drive, the arm has to move. And with this SSD drive, there are no moving parts, so no chance of mechanical failure. So, compared to the hard drive, SSD would provide better performance.
TH: Does multi-bit MLC enter into this discussion? Is having three—or later, four—bits per cell going to change the endurance dynamic, particularly when weighed against SLC?
TC: Three bits per cell is already in the market, but you also have to look at density—32nm or 25nm. That provides the density for more stack. The use of two or three bits per cell is just a current trend. The NAND semiconductor industry is more interested in how it can provide maximum data per square inch. Typically, we are looking for more cell density than bits on this point.
TH: But will 3-bit have an impact on endurance? With more electrons being pushed through each floating gate, does that erode the oxide layer more quickly?
TC: No, actually, because right now all NAND has ECC correction. You’re talking about losing all data endurance, so you cannot recover it. You’re talking about the voltage converting in the cell.
LT: But Tony, if we were to implement today's 3-level cell NAND on an SSD, would the endurance of that be less compared to a current MLC product?
TC: I would say that would be partially true. With data endurance there are two different issues. One is how long it will last, and one is how to correct errors if they appear.