Endurance Testing: Write Amplification And Estimated Lifespan

SandForce's Technology: Very Low Write Amplification

According to SandForce, SSD manufacturers can tweak firmware in a number of different ways. Naturally, then, we were curious to see whether Intel altered the way SandForce's compression technology worked.

Gauging this requires us to calculate write amplification. Usually, we'd need to endure days of testing in order to generate the numbers used for this calculation. Fortunately, all SandForce-based SSDs come with SMART counters for host writes (E9) and NAND writes (F1). Intel's SSD 520 features the same counters, so it's really only a matter of setting up Iometer to write a compressible sequential workload. Once you look them up, it's pretty easy to calculate write amplification: just divide host writes by NAND writes.

Intel doesn't appear to be changing the behavior of DuraWrite, which is perfectly fine. Though the company says the SSD 520's firmware is completely its own, this particular aspect of the controller is supplied by SandForce in perfect working order, necessitating nothing in the way of tuning.

Now, at a queue depth of one, an SSD with a non-SandForce controller in it always incurs write amplification greater than or equal to one, meaning flash cells wear faster than on a SandForce-based drive. By compressing data, the SSD 520 and its contemporaries are able to write less data and extend overall endurance.

Endurance: Even Better With SandForce's Compression Technology

By minimizing write amplification, endurance is positively impacted. We can't really understate this effect. Don't believe us? Fortunately, you don't have to take our word alone. All of Intel's latest SSDs come with workload counts that allow you to estimate the life-span of your SSD.

Think of Intel's workload counters similar to a car's trip counter. Instead of distance, they measure endurance over time. We apply a three-hour workload to the drive in order to generate enough data to be meaningful.

Before we dive in, we want to clarify a few things so that you don't misinterpret what we're saying here. 

First, the media wear indicator on an SSD counts down from 100 to 1. Because the number of program-erase cycles a NAND cell can withstand is limited, the MWI is designed to facilitate a rough estimate of endurance. In theory, once you reach the end of the counter, the memory's rated P/E cycles have been exhausted, though that's not to say anything bad will happen immediately after.

Second, using workload counters to estimate endurance is still a tenuous measurement (and without running any of our drives down, we're presenting this information academically, rather than practically). Iometer runs so fast and writes so much that we're essentially condensing months worth of activity into hours. Both Micron and Intel estimate that the average desktop user writes between 7-10 GB worth of information per day. So, we're basing our real-world estimates on at least 7 GB of writes by the host.

Finally, P/E-cycle ratings apply to each flash cell. But because larger SSDs employ more NAND, it takes longer to write across all cells. So, they consequently enjoy a higher endurance rating. The numbers below apply to Intel's 60 GB SSD 520, specifically.

Now, we're able to look at the following data without freaking out about SSD longevity. This is really about SandForce's technology and its effect on write amplification, and, in turn, endurance.

Presented with completely compressible data, Intel's 60 GB SSD 520 is told to write 583 GB of data, and actually writes 100 GB to flash. This translates into a write amplification of 0.17x. That's downright incredible considering non-SandForce will generally end up with write amplification that looks more like our incompressible workload, where 211 GB of data is written as 616 GB of to the NAND (yielding amplification of 2.9x).

In reality, you'll probably never see either of the extremes presented here. We're taxing the heck out of these SSDs, allowing no idle time for background garbage collection to affect the drive. That's an important process, responsible for further minimizing write amplification. As a result, in normal use, endurance really isn't your indicator of reliability. Again, we're really just trying to illustrate how SandForce's compression technology, which is sometimes maligned for its variable impact on performance, might also help extend endurance for SSD vendors who use lower-binned NAND.