Endurance: Comparing MLC, eMLC, And SLC
Endurance is a term thrown around a lot in discussions of solid-state storage because we all worry about that point where an SSD is no longer able to reliably store our data. If you have an SSD in your notebook or mainstream desktop, endurance shouldn't be much of a concern. It's unlikely that you'll ever write enough data per day, every day, to exhaust the useable life of the NAND flash cells that make up your drive. Far more likely is a firmware-related issue that results in problematic operation. But even those are fairly rare.
Endurance is a much more important discussion in the enterprise world, though. Demanding workloads force many machines to read or write data continuously, day in and day out. On a conventional hard drive, other issues contribute to eventual failures. But when it comes to SSDs, those business-oriented tasks gradually chip away at the rated number of program/erase cycles that each NAND vendor affixes to its memory products. Because eMLC and SLC flash offer the highest endurance ratings, they're particularly attractive for enterprise-oriented products.
That's not to say multi-level cell NAND is out of place in professional applications. Based on our discussions with data center managers, we know there are plenty of original X25-M and SSD 320s used in mission-critical environments. They are used in such a way that a failure won't result in data loss, though, and they aren't bombarded with writes in the same way one of these Toshiba drives might be.
Evaluating SSD Endurance
Before we take a stab at quantifying the endurance of different flash technologies, we want to discuss our methodology. Our estimates come from monitoring each drive's media wear indicator (referred to as the MWI), which counts down from 100 to 1. Because the number of program-erase cycles a NAND cell can withstand is finite, the MWI is designed to facilitate a rough estimate of endurance.
In theory, once you reach the end of the counter, all of the memory's rated P/E cycles are exhausted. That's not to say something bad happens when you hit the bottom, but nobody wants to entrust irreplaceable data to a drive living on borrowed time, either. Naturally, enterprise customers place a lot of importance into the MWI, then, because it represents “the safe zone.”
|Endurance Rating (Sequential Workload, QD=1, 2 MB)||Intel SSD 320||Intel SSD 710||Toshiba MK4001GRZB|
|NAND Type||Intel 25 nm MLC||Intel 25 nm eMLC (HET)||Toshiba 32 nm SLC|
|RAW NAND Capacity||320 GB||320 GB||512 GB|
|IDEMA Capacity (User Accessible)||300 GB||200 GB||400 GB|
|P/E Cycles Observed (IDEMA)||5460||36 600||225 064|
|P/E Cycles Observed (Raw)||5119||22 875||175 831|
|Host Writes per 1% of MWI||16.38 TB||73.20 TB||900.2 TB|
According to Toshiba's spec sheet, the 100 GB MK100GRZB comes with an endurance rating of 8.2 PB. Each vendor uses its own method of estimating longevity, which is why it’s difficult to compare endurance across different SSD brands and models. Our numbers assume a purely sequential workload, which means we’re ignoring random access. However, this allows us to take a step back and look at SSD and NAND endurance academically.
Look at the numbers. It’s really clear to see why SLC flash remains the crème of the crop. While it continues to fetch a high premium, SLC is also capable of withstanding many more writes than MLC technology. If you remove the effects of overprovisioning, the Toshiba’s SLC NAND has a rating close to 175 000 P/E cycles. That’s 58 times higher than Intel’s 25 nm MLC NAND, which clocks in at ~5000 P/E cycles.
Remember that P/E-cycle ratings apply to each flash cell. But because larger SSDs employ more NAND (and consequently, a lot more flash cells), it takes longer to write across all of them. As a result, larger drives enjoy a higher endurance rating. If we do the math, our 400 GB MK4001GRZB should be capable of writing 88 PB of data sequentially. That’s insanely high. And perhaps it explains why Toshiba doesn’t provide endurance ratings on its higher-capacity SSDs. Instead, the 200 GB and 400 GB models come with a guarantee that you won’t have to worry about endurance during the company's five-year warranty period (a telling promise, indeed).