The SSD Endurance Experiment: 22TB update


So far, so good
— 10:33 AM on September 6, 2013

Flash memory has limited write endurance. So do the SSDs based on it. How many writes can modern drives take before they expire, and what happens to them as the flash wears out? We're trying to find out by testing a selection of SSDs to failure. You can read all the nitty-gritty details about the experiment in this introductory article. Today, we're checking in on our subjects after 22TB of writes.

22TB might seem like an odd place to pause our testing, but it's a close match for the endurance specification attached to the Intel 335 Series. That drive is rated for 20GB of writes per day for three years. Do the math.

The Intel 335 Series is one of only two drives in our lineup with a published endurance rating. The other, the Kingston HyperX 3K, is supposed to withstand 192TB of writes. Since it'll take a while to push into triple-terabyte digits, we've decided to stop at 22TB before moving on.

Unfortunately—or fortunately, depending on your perspective—we have little to report at this juncture. Anvil's endurance test wrote 22TB to each SSD, and all the drives passed the benchmark's built-in data integrity tests without issues. The SSDs didn't reach the 22TB mark at the same time, though. Getting there required a little less than three days of non-stop testing on the slowest drive. Here are the average write speeds for the first batch of tests.

Now for the requisite sprinkling of salt: to expedite our experiment, we're running the endurance benchmark on all the drives simultaneously. The SSDs are split between two different systems and also between 6Gbps and 3Gbps SATA ports, so the average speeds above aren't entirely comparable. That said, the results provide us with a baseline that may be helpful in assessing the write speeds for subsequent endurance runs. Plus, the numbers give us a sense of how long the next round of testing will take.

(For reference, the Neutron GTX, Intel 335 Series, and the HyperX 3K are all housed in the same system. Another, identical rig contains the Samsung SSDs and a second HyperX 3K that's being tested with compressible data rather than the incompressible payload used for the others. The HyperX 3K SSDs are both connected to 3Gbps SATA ports, while the rest are plugged into 6Gbps ones.)

We can draw more meaningful conclusions from our targeted performance tests. For these benchmarks, the SSDs are tested individually using the same SATA port on the same system. This method should produce data more appropriate for head-to-head comparisons, but we're not particularly concerned with how the drives perform relative to each other in this limited collection of synthetic tests. Instead, we're interested in how each SSD's basic performance characteristics change as the flash wears out.

So far, we haven't observed too much of note. We benched the SSDs before endurance testing began and again after we reached the 22TB mark. The performance differences are summarized below.

The vast majority of our most recent results are within 1-2% of the factory-fresh readings. Given the run-to-run variance associated with these tests, I wouldn't worry about such small differences—not unless they become part of a long-term trend.

Surprisingly, the HyperX SSDs got a lot faster in the random read speed test. You shouldn't need to break in an SSD before it starts delivering peak performance, but the Kingston drives apparently didn't kick into high gear right away. The Intel 335 Series uses the same SandForce controller as the HyperX drives; it also had higher random read performance after 22TB, although only by 8%. Perhaps we're looking at a quirk of the controller or its associated firmware.

We expect flash wear to decrease SSD performance over time, but these drives still have a lot of life left in them. Each SSD has SMART attributes that tally bad blocks, bytes written, and other variables. We're tracking those attributes, and the SSDs are so far free from bad blocks, which means all of their NAND remains intact.

The data we've collected also provide some insight into SandForce's write compression mojo. Again, we're testing two SandForce-based HyperX drives: one with incompressible data like the other SSDs, and another with the endurance benchmark's 46% "applications" compression setting. According to the "lifetime compressed writes" SMART attribute on the HyperX drives, the incompressible data produced 22.8TB of flash writes. The compressible data wrote only 15.5TB to the flash, a savings of 32%.

We can compare those totals to the host write tallies in order to get a sense of write amplification. Both HyperX drives report 21.6TB of host writes, resulting in write amplification factors of 1.05 for the incompressible data and 0.72 for compressible data. The Intel 335 Series registers 22.9TB of total NAND writes on 21.6TB of host writes, closely matching the write amplification of its HyperX counterpart. Unfortunately, the other SSDs don't track NAND writes in addition to host writes, preventing us from calculating their write amplification factors.

Our endurance experiment is still in its infancy, so that's all the analysis we'll indulge for now. Next on the agenda: another 78TB of writes to bring the drives up to 100TB. We'll evaluate the SSDs again at that point, but we may not report back with additional results until something exciting happens. It could take hundreds of terabytes for the first cracks to appear.TR

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Tags: Storage