RAID selection can be a tough choice, especially when budgets are tight. Learn why RAID 10 might be a better choice than RAID 6 even though it’s more expensive.
Last week, my friend and fellow TechRepublic contributor Rick Vanover provided some compelling reasons why you should opt for RAID 6 instead of RAID 5 for data protection, particularly as individual disk size increases and more and more disks are added to an array. As Rick indicated, RAID 6 provides much greater protection against data loss than RAID 5. In fact, a lot has been written about the growing need to avoid RAID 5 due to its inherent and growing set of limitations.
When it comes to a choice between RAID 5 and RAID 6, I agree with Rick that, from a data protection standpoint, RAID 6 is the better choice. There are, however, some significant tradeoffs, which Rick alluded to in his article. Most importantly, RAID 6 imposes a serious (as in, not insignificant) write performance penalty, even when compared to RAID 5. For every write operation initiated against a RAID 6-based array, six I/O operations are required. For RAID 5, a write operation results in just four I/O operations. In my opinion, these are significant roadblocks even in a relatively equally balanced read/write environment.
Although cost is always an important consideration when buying new storage, growing disk sizes have gone a long way toward making it possible to focus much more on the performance side of the storage equation as opposed to the capacity side. That’s why, in most situations, if given the option, I’d choose RAID 10 (data striping over mirrored data sets) over either RAID 5 or RAID 6. In fact, I recently put my money where my mouth is on the answer to this question when I purchased an expansion disk shelf for our SAN.
Storage capacity didn’t enter into the equation (we have plenty of space); however, we were hitting an IOPS wall. As such, the primary focus was performance – balanced read and write performance. The new disk shelf is configured as a RAID 1+0 array. Whereas RAID 6 imposes that aforementioned 6x write penalty and RAID 5 imposes a 4x penalty, RAID 1+0 imposes just a 2x penalty and has other significant benefits:
Going back to the space cost inherent in making the choice between RAID 6 and RAID 1+0, understand that with RAID 6, you “lose” 2/number-of-disks-in-array worth of capacity to parity. With RAID 1+0, that “lost” space equals a straight 50% of total array capacity, regardless of the number of disks. So, yes, RAID 10 does have a higher space cost, but I believe that the benefits brought to the table (particularly with regard to write performance) are powerful reasons to avoid RAID 6 in favor of RAID 10.
If your storage device is using RAID 6 and you’re not having performance problems — particularly related to writes — there’s no need to blow it away and replace it with a RAID 1+0 configuration. My advice here is intended to be food for thought when it comes to deploying new storage. Don’t simply write off RAID 10-based storage due to its 50% overhead cost; it might be worth the cost trade off, depending on your situation. As always, your selection will be based on your own testing, application needs and risk tolerance.
With close to twenty years of experience in Information Technology, Scott has experience across the board in the industry. Recently, Scott left his position as Vice President and CIO for a small private college to launch a consultancy -- The 1610 Group -- aimed at the SMB space and K-12 and higher education.
