SSDs and RAID Levels for Your Hybrid ArrayIf you’re planning to stand up a storage array with both flash storage and spinning disk, you may be wondering about the optimal RAID level to use for your SSD tier.

Often times, when an IT planner architects a new hybrid system, they include too much flash capacity, and it ends up going unused. And even when your SSD tier is full, it may not be fully utilized performance-wise. Excess cache turnover is a bad thing, but having little to no cache turnover can be just as bad.

While there are a few options for RAID level on your SSD tier, each has benefits and drawbacks. Today, we’re going to look at some of the potential RAID levels you can implement, and how they affect SSD performance.

RAID 5 & RAID 6

Going with RAID 5 or RAID 6 on your SSD tier presents a couple of challenges, the first being latency. Due to additional XOR calculations that the storage controller must execute building parity takes extra time.

But how much this extra time impacts performance totally depends on your workload. For many data centers, the overhead may not be a problem since their applications aren’t maxing out their SSDs. SSDs may have IOPS to burn, so the extra parity calculations have no noticeable effect on application response time.

This excess performance capacity is the reason compression and deduplication are usually non-issues with flash arrays. However, if your workload is particularly intense, then latency will be noticeable in a RAID 5 or RAID 6 configuration.

The Write Issue

Another downside of RAID 5 and RAID 6 is extra writes. Once an SSD has reached a steady state, write operations are the slowest part of flash I/O since every write operation requires an erase before the actual data can be recorded.

So with RAID 5 you’re adding one “erase write” for every actual write. And with RAID 6 you’re adding two erase writes for each actual write. The extra operations will affect performance more than any XOR parity-building latency. It can also have an impact on SSD lifespan, since flash cells have an a finite number of write-erase cycles before they fail.

RAID 1 and RAID 10

Generally, either RAID 1 (mirrored) or RAID 10 (striped and mirrored) can be solid configurations for SSDs. While RAID 1 and RAID 10 cut your usable capacity by 50%, if you leverage compression and deduplication, you’ll still be able to recoup the lost capacity.

Final Thoughts

So when you’re architecting your next hybrid array, there are a couple of things you need to take into account. RAID 5 and 6 give you the most fault protection, but, depending on your workload, may degrade the performance of your SSD tier and the longevity of your SSDs. If you’re comfortable going with RAID 1 or RAID 10, both configurations can be successfully leveraged, as long as you’re doing dedupe and compression. Also, be mindful of building in too much SSD capacity that won’t ultimately be used.

If you’re looking for expert advice on how to best leverage flash technology in your hybrid storage array, reach out to one of our storage specialists or give us a call at (404) 551-4534 and we can help walk you through the process.