Solid-state drives have revolutionized the PC storage industry. Their wicked-fast access times deliver a palpable improvement in overall system responsiveness, and prices have fallen enough to make decent-sized drives affordable for all. There's just one catch: due to the nature of flash memory, SSDs have limited endurance.
Flash writes erode the structure of the individual memory cells. Eventually, cells degrade enough that entire blocks of them have to be retired. Those bad blocks are replaced by fresh ones pulled from the SSD's spare area, and business proceeds as usual.
SSDs only have so much spare area at their disposal, though. That area is also used to accelerate performance, so we've been curious about what happens to SSDs as wear accumulates. Do drives burn out or do they fade away—and what happens to performance as write cycling takes a toll on the flash? We're attempting to answer those questions in our SSD Endurance Experiment.
If you're unfamiliar with the experiment, I recommend reading our introductory article on the subject. Here's the short version: we have six SSDs from Corsair, Intel, Kingston, and Samsung, and we're hammering them with writes until they expire. We're also testing performance at regular intervals.
We last checked in on our subjects after 22TB of writes, which works out to 20GB per day for three years. There was no drama to report at the time. However, we've now written 200TB to the drives, and the first cracks are starting to show.
For one of the SSDs, the first signs of weakness appeared when we polled the field after 100TB of writes. The SMART attributes of our Samsung 840 Series 250GB SSD revealed 11 reallocated sectors—bad blocks, in other words. Since the 840 Series' three-bit TLC NAND has lower write endurance than the two-bit MLC flash typically found in consumer-grade SSDs, we weren't surprised that it was the first to exhibit failures. All of our other candidates use MLC flash.
Despite those first bad blocks, the 840 Series' performance and user-accessible capacity remained unchanged. The same was true for the other SSDs, so we set our sights on 200TB.
At the latest milestone, the 840 Series is up to 370 reallocated sectors. It's not the only one with bad blocks, either. One of our Kingston HyperX 3K 240GB drives—the one we're testing with incompressible data like the other SSDs—reports four bad blocks. We also have an identical HyperX drive that's being tested with 46% compressible data, but it remains free of flash failures. That drive has only written 143TB to the flash thanks to its SandForce-powered compression tech, so we're not surprised that it's in better shape than its twin.
The Kingston SSDs have 4MB blocks, so the one with reallocated sectors has lost 16MB of total flash capacity. Samsung has yet to answer our questions about the 840 Series' block size. However, based on information published by AnandTech, that drive appears to have 1.5MB blocks. With 370 of those blocks now retired, the total flash hit works out to 555MB.
Because bad blocks are replaced by flash reserves held in each SSD's spare area, the HyperX and 840 Series drives have the same storage capacities as they did when our testing began. The HyperX offers 224GB in Windows, while the 840 Series serves up 234GB. Both have 256GB of flash onboard, leaving plenty to spare for bad block replacement. We've barely dipped into the HyperX's reserves, and we've only consumed a fraction of the 840 Series' spare area.
According to Samsung's SSD Magician utility, our 840 Series SSD is in "good" health despite the bad block tally. Hard Disk Sentinel, the software we're using to capture SMART data, is less optimistic. Here's how that application rated the health of our contenders at 100 and 200TB:
|Corsair Neutron GTX 240GB||100%||100%|
|Intel 335 Series 240GB||88%||73%|
|Kingston HyperX 3K 240GB||100%||98%|
|Kingston HyperX 3K 240GB (Comp)||100%||100%|
|Samsung 840 Pro 256GB||78%||51%|
|Samsung 840 Series 250GB||66%||19%|
For what it's worth, the Samsung utility says our 840 Pro is also in good health. The same goes for Intel's equivalent app and the 335 Series SSD. Corsair's software doesn't have a general health indicator, and Kingston's utility doesn't work with our test system's storage drivers.
The media wear and SSD life attributes we've been tracking haven't budged since testing began, so it's hard to know which numbers to trust. It's important to keep things in perspective, though. We've written 200TB to the drives—the equivalent of more than 100GB per day for five years—and most of the SSDs are completely intact. Even though a decent-sized portion of the 840 Series' flash has expired, the drive appears to be far from failure.
Now, let's look at the performance picture.