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It ain't what you do…

Editorial Type: Opinion     Date: 01-2014    Views: 1921   





…It's the way that you do it, especially when it comes to building reliable storage arrays, argues Gavin McLaughlin of X-IO

When looking into storage reliability, it's all too easy to get caught up in the "HDD is unreliable" melodrama being created by many All Flash Array vendors. To argue that one media is more reliable than another is analogous to arguing that cars are more reliable than trucks - they're two different tools for two different jobs.

The good news is that when it comes to hard drives, there's plenty of statistical evidence out there to help organisations architect solutions based upon balancing cost and risk. One such is a recent study by cloud backup provider Backblaze, looking at their 25,000 odd disk drive implementation and plotting out lifecycles. In the past we've seen studies from Google and Carnegie Mellon University; crucially however, Backblaze looked at both consumer and enterprise grade drives.

What they initially saw was of no great surprise to those who work in this space. Their SATA consumer grade drives showed an Annual Failure Rate (AFR) of 5.1% in the first 18 months, and drive failures followed a "bathtub" curve with a dramatic increase in years four and five of the drives' lifecycle. Why do you think the vast majority of storage vendors are happy to give a three year warranty but get a little jumpy when you ask for an inclusive five year one? The simple answer: the drives are much more likely to fail once they get into later life.

You may think: "But these are consumer grade drives - things will be better with enterprise grade…" Well, this is where things get interesting: Backblaze went on to look at their enterprise grade drive implementation and plotted drive years of service against failure rates to show an AFR for enterprise drives of 4.6% against consumer drives one of 4.2%. Now it's not a direct comparison as the enterprise drives are small in number and have only been installed for two years. There's also an interesting point raised by Seagate on their blog that Backblaze created the "perfect storm" with their use case and physical mounting. This proves a point that a select few in the storage industry have been making for a while:"It ain't what you do, it's the way that you do it".

The truth is that hard disk drives are sensitive creatures. Some years ago Sun Microsystems produced a video to show off new software for analysing drive latency, which proved the point that drives are sensitive to vibration - in this case an Australian engineer shouting at them. Vibration and noise aren't the only drive killers - heat and density are a big factor too. Add in the error correcting capabilities of consumer grade drives and you start to see some of the AFRs that Backblaze saw.

So how come some vendors - such as X-IO - have been able to solve this core issue? Well the key here is good old fashioned hardware engineering: acknowledge that drives are sensitive to such elements and deal with them. Stop them vibrating, keep them cool and treat them with a little bit of respect by mounting them evenly and horizontally. X-IO have gone a stage further and use patented software to rebuild the drive in the case of errors, but the fundamental hardware design really does make a difference.

Yes some vendors run predictive failure software and will argue that it's no big deal to send an engineer or just a replacement drive out - but how many people can reel off an anecdote about an engineer swapping the wrong drive, knocking a cable out or hitting the EPO instead of the exit button in your data centre?

The fact is that with the arrival of new approaches such as software defined storage, the temptation to use lower cost components such as cheap commodity desktop drives will grow. When we strip away some of the increasingly unnecessary core controller feature sets of many enterprise storage arrays, we're left with the same OEM disk shelves that everyone has. If they haven't solved the crucial hardware design challenges such as vibration and cooling then you'll have an array that can be unpredictable not only in terms of reliability but also in terms of performance.

Yes you could take the alternative approach of using an all flash array - but do you really need a truck to pick up a loaf of bread from the supermarket?
More info: www.xiostorage.com

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