Data Centers

Understand when RAID 60 is overkill

When it comes to RAID 60, Scott Lowe advises IT pros not to have a one-size-fits-all mentality. He explains the potential benefits and downsides of using RAID 60.

In a recent TechRepublic column, I extolled the merits of RAID 50 and discussed how I think that, by borrowing certain aspects of both RAID 5 and RAID 10, RAID 50 hits a sweet spot between RAID 5 and RAID 10 by providing a good balance between capacity, performance, and reliability. My RAID 50 column generated a lot of comments, including a couple about RAID 60; specifically, it was mentioned that disks often fail in groups, so from a data protection standpoint, why not go with RAID 60 to further protect your data? I'll provide a brief overview of RAID 60 and then examine both sides of the RAID 60 equation.

How RAID 60 works

The diagram in Figure A gives you an idea of how a RAID 60 array is constructed. Like RAID 50, RAID 60 is a multi-level disk set; you start with a bunch of RAID 6 sets, and then these sets are aggregated at a higher level into a RAID 0 array that has no redundancy on its own. However, each RAID 60 set does have redundancy and can withstand the loss of up to two disks in each parity set. In theory, in the diagram below, you could lose six of the 12 total disks (two in each set) and still have an operational array. As soon as you lose more than two disks in a single parity set, though, your world would come crashing down, as the RAID 0 set breaks, and you're back to data recovery mode.

Figure A

RAID 60 diagram

An additional note: RAID 60 requires a minimum of eight disks in order to operate — you need at least two four disk RAID 6 sets to comprise a single RAID 60 set.

RAID 60 benefits and drawbacks

I like RAID 50 a lot, so you might think that I'm a big believer in taking that to the next level and that I would like RAID 60 even more; to be honest, I'm not sure. I see RAID 50 as a great balance between capacity, performance, and reliability, and I see RAID 60 as potentially imbalanced on the capacity side (to the negative) in order to support the increased reliability inherent in RAID 60.

I don't think you should simply avoid RAID 60 at all costs; instead, make the decision on a case by case basis with an understanding of the tradeoffs that you'll face. In fact, you might find that RAID 60 is a great fit when you need higher usable capacity and better reliability and can trade a little in write performance for it.

With RAID 60, you're going to lose anywhere from around 12% to 50% of your usable space to parity information. This is not a bad thing, and the whole design of RAID 6 is built around the idea that using more space (two disk's worth to be exact) to enhance reliability is a good thing. If you're ultra-concerned about reliability, are you more likely to use fewer disks per individual RAID 6 set? If so, this would decrease the overall usable capacity of the solution. In fact, in the diagram above, you'd lose 50% of your disk space to parity, so why not just go with RAID 10 in that scenario?

With RAID 6, you will take a performance hit (more so than with RAID 50) when it comes to writes, but reads will be boosted, as is the case with RAID 10 and RAID 50. The exact performance hit you take with writes under RAID 60 is largely dependent on the quality of your RAID controller and on what you're doing. If you're considering implementing a RAID 60 that eats 50% of your space in overhead, it's time to consider just using RAID 10, which will provide similar read performance and better overall write performance and provide similar levels of redundancy.

From a pure reliability perspective, a RAID 60 array is orders of magnitude more reliable than even RAID 50 arrays due largely to the extra parity disk employed in RAID 60.

The more disks you add to each individual RAID 6 set in a RAID 60 array, the higher percentage of usable space you get from the overall RAID 60 array. Perhaps the biggest tradeoff in RAID 60 is that you can build larger individual RAID 60 sets in a safer manner than is possible under RAID 50, so from that perspective, perhaps you can get more safely usable space from a RAID 60 array.

Conclusion

When it comes to RAID 60, I don't think IT pros should have a one-size-fits-all mentality. And before you jump on the RAID 60 train, be aware that there are potential downsides for usable space and performance that need to be considered, so choose wisely. For more information, I recommend checking out IBM's article and chart about selecting a RAID level.

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About

Since 1994, Scott Lowe has been providing technology solutions to a variety of organizations. After spending 10 years in multiple CIO roles, Scott is now an independent consultant, blogger, author, owner of The 1610 Group, and a Senior IT Executive w...

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