Doubts Over SSD Reliability

Arthur Cole

The reliability of SSDs has been an article of faith since the technology made the jump from laptops and mobile devices to enterprise settings. But that view is coming under increasing scrutiny now that real-world deployments are becoming more common.

At best, the issue has proven to be a lot more complex than initially thought, with a wide variety of factors and operational considerations entering into reliability calculations.

Some critical hard data came out earlier this month from tech analysts at French website Based on sales and return data from an unnamed storage retailer, the group showed that between October 2009 and April 2010, SSDs saw a return rate of 2.05 percent compared to 1.94 percent for HDDs. That's close enough to be a statistical dead heat, although the HDD figure represents a jump from the previous six-month survey when the return rate was only 1.63 percent.

The complexity comes when you parse the numbers. As a rule, failure rates were higher in large capacity models. In hard drive technology, this is to be expected as more capacity usually comes at the expense of pushing mechanical systems to their theoretical limits. SSDs, though, have no moving parts, so capacity should be limited only by the number of Flash cells that can be crammed into a desired form factor.

But as storage analyst Robin Harris points out, there are other factors at play within the Flash chip. A key consideration is power. When you start piling up multiple chips, each with their own dedicated capacitors and oscillators, you can quickly breach the insulating layers between chips, producing the inevitable failure.

Another problem in assessing reliability is the lack of standards for key components that makes it difficult to verify vendors' claims or gauge one device's performance against another. The fact is, differences in memory structure, controller design, surrounding electronics, even the ASIC itself, can cause an SSD to excel or fail under different workloads. The JEDEC Solid State Association recently published standards for SSD endurance and reliability, but it is too early to tell whether they will receive broad industry adoption.

In the meantime, expect vendors to continue pitching improved reliability in SSD designs through a range of data integrity and error correction technologies, particularly in MLC designs that have long been tagged as the less rugged version of more traditional SLC technology. At the same time, we're seeing devices intended for more extreme industrial and embedded applications tricked out with more enterprise-friendly components, like Super Talent's recent addition of a SATA interface to the DureDrive AT2 device.

At the end of the day, however, the equation for deploying SSDs in the enterprise has not changed. They still provide faster performance at a lower operating cost than HDDs, making them suitable primarily for high-volume, time-sensitive applications but less appropriate for long-term bulk storage.

It seems clear, though, that we shouldn't expect them to last any longer than magnetic drives.

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