20 nm Write Endurance: Probably Not Something To Worry About
Endurance: Kind Of A Big Deal
Write endurance is a term often thrown around in discussions of solid-state storage because we all worry about that point when an SSD is no longer able to reliably hold our data (even though very few of us have actually seen such a thing happen).
If you have an SSD in your notebook or mainstream desktop, write endurance isn't something that should keep you up at night. It's highly improbable that you'll ever write enough data per day to exhaust the rated life space of the NAND flash cells composing your drive. Both Micron and Intel estimate that the average desktop user writes between 7-10 GB worth of information per day. Even if you use an exaggerated number, basic math assures us that you almost can't render your SSD useless within its warranty period. If you were to run into a reliability issue, it'd be far more likely to come from some sort of firmware-oriented bug.
How the rated write endurance of NAND changes at each new manufacturing process is interesting to watch, though, as are the ways vendors address the physical realities of NAND wear-out. Because Intel's SSD 335 is the first drive equipped with IMFT's 20 nm flash, we are of course curious about how much, if any, its endurance level differs from the SSD 330.
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, enterprises place a lot of importance on the MWI because it represents “a safe zone.”
In Theory, Far Fewer Cycles: But Does That Matter?
| Endurance Rating | ||
|---|---|---|
| Sequential Workload, QD=1, 2 MB | Intel SSD 320 | Intel SSD 335 |
| NAND Type | Intel 25 nm MLC | Intel 20 nm MLC |
| RAW NAND Capacity | 320 GB | 256 GB |
| IDEMA Capacity (User Accessible) | 300 GB | 240 GB |
| Overprovisioning | 7% | 7% |
| P/E Cycles Observed (IDEMA) | 5460 | |
| P/E Cycles Observed (Raw) | 5119 | |
| Host Writes per 1% of MWI | 16.38 TB |
Update, November 29, 2012: Intel still doesn’t provide an official endurance rating on its SSD 335, but the company’s recent firmware update (335t) now puts endurance somewhere near 748 TB using incompressible writes. If we work backwards from that result, the math ([Host Writes per 1% of MWI * 100] / Capacity) tells us that IMFT’s 20 nm should be good for somewhere around 3000 P/E cycles. This doubles the result we came away with originally, backing up Intel’s assurance that endurance doesn’t dip as it shifts away from 25 nm lithography.
Now, we’re not huge fans of these updates. We’re reminded that, with a simple firmware, a vendor can change the algorithm used to calculate MWI—an essential SMART field monitored by enterprise customers and power users as a way to quantify the health of their SSDs. Fortunately, if you buy a retail drive, you’re covered for three years no matter what, even if the MWI hits 1. Intel’s OEM SSD 330/335 drives do specify coverage for three years or until the MWI hits 1, though, so our discussion isn’t as purely theoretical as it might sound.
But we weren’t particularly concerned about endurance when we were measuring 1500 P/E-cycle endurance, and we’re still not. Even going by the bugged firmware’s halved rating, assuming 10 GB of writes per day, it’d still take more than 100 years to wear down the drive’s NAND using our workload.
