Are SSDs reliable enough? The importance of extensive testing under adverse conditions.

Recently, interesting research (see here) from researchers at Ohio State was presented at USENIX. 

To summarize, they tested 15 SSDs, several of them “Enterprise” grade, and subjected them to various power fault conditions. 

Almost all the drives suffered data loss that should not have occurred, and some were so corrupt as to be rendered utterly unusable (could not even be seen on the bus). It’s worth noting that spinning drives used in enterprise arrays would not have suffered the same way.

It’s not just an issue of whether or not the SSD has some supercapacitors in order to de-stage the built-in RAM contents to flash – a certain very prominent SSD vendor was hit with this issue even though the SSDs in question had the supercapacitors, generous overprovisioning and internal RAID. A firmware issue is suspected and this is not fixed yet.

You might ask, why am I mentioning this?

Several storage systems try to lower SSD costs by using cheap SSDs (often consumer models found in laptops, not even eMLC) and then try to get more longevity out said SSDs by using clever write techniques in order to minimize the amount of data written (dedupe, compression) as well as make the most of wear-leveling the flash chips in the box by also writing in flash-friendly ways (more appends, less overwrites, moving data around as needed, and more).

However, all those (perfectly valid) techniques have a razor-sharp focus on the fact that cheaper flash has a very limited number of write/erase cycles, but are utterly unrelated to things like massive corruption stemming from weird power failures or firmware bugs (and, after having lived through multiple UPS and generator failures, I don’t accept those as a complete answer, either).

On the other hand, the Tier 1 storage vendors typically do pretty extensive component testing, including various power failure scenarios, from the normal to the very strange. The system has to withstand those, then come up no matter what. Edge cases are tested as a matter of course – a main reason people buy enterprise storage is how edge cases are handled… 🙂

At NetApp, when we certify SSDs, they go through an extra-rigorous process since we are paranoid and they are still a relatively new technology. We also offer our standard dual-parity RAID, along with multiple ways to safeguard against lost writes, for all media. The last thing one needs is multiple drives failing due to a strange power failure or a firmware bug.

Protection against failures is even more important in storage systems that lack the extra integrity checks NetApp offers. Those non-NetApp systems that use SSDs either as their only storage or as part of a pool can suffer catastrophic failures if the integrity of the SSDs is compromised sufficiently since, by definition, if part of the pool fails, then the whole pool fails, which could mean the entire storage system may have to be restored from backup.

For those systems where cheap SSDs are merely used as an acceleration mechanism, catastrophic performance failures are a very real potential outcome. 1000 VDI users calling the helpdesk is not my idea of fun.

Such component behavior is clearly unacceptable.

Proper testing comes with intelligence, talent, but also experience and extensive battle scarring. Back when NetApp was young, we didn’t know the things we know today, and couldn’t handle some of the fault conditions we can handle today. Test harnesses in most Tier 1 vendors become more comprehensive as new problems are discovered, and sometimes the only way to discover the really weird problems is through sheer numbers (selling many millions of units of a certain component provides some pretty solid statistics regarding its reliability and failure modes).

“With age comes wisdom”.




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