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RAID 0: Great For Benchmarks, Not So Much In The Real World

One SSD Vs. Two In RAID: Which Is Better?
By , Achim Roos

They're a funny thing, SSD benchmarks. You can run synthetics all day long and create these unrealistically demanding workloads that make solid-state storage look one way. Then, you can tinker around with real-world metrics that paint another picture entirely.

For enthusiasts, the truth often lies somewhere in between. A majority of the tasks we perform do tend to involve basic operations like opening Web browsers, editing images, composing email, and watching video. But sometimes we do need big performance from our systems: compiling a big project, moving tens of gigabytes of media files, or capturing uncompressed AVIs for FCAT analysis. In those instances, you want responsiveness on demand.

As we expected, twin SSDs in RAID 0 post phenomenal numbers when we hammer them with sequential reads and writes. Twin 256 GB 840 Pros nearly hit 1 GB/s in both disciplines. Largely as a result of the SATA 6Gb/s interface, single drives max out at a little more than half of those numbers.

The RAID-based configurations undoubtedly scored the first touchdown given exceptional sequential results, but the game didn't end there. Individual SSDs regained ground in the tests that followed, even posting better scores in some of them. Random I/O performance is a good example. Striped drives are certainly better equipped to push more IOPS, but only when you're stacking commands more than four high. Jumping up to a queue depth of 32, 16, or even eight is really uncommon in a desktop or workstation environment. As a result, the performance differences are far less pronounced in the real world.

One SSD on its own scores again in the contrived tests we put together. The performance differences when we boot up and shut down Windows 8, then fire up different applications, are marginal at best and not noticeable in practice. Single drives actually manage to outperform the striped arrays some of the time, even.

If you're planning an upgrade and want to know whether to buy a couple of 128 GB drives and put them in RAID 0 or just grab a single 256 GB SSD, for example, the answer still seems clear enough to us: just grab the large drive and use one. Using Samsung's 840 Pros as an example, a pair of 128 GB drives will run you $300 on Newegg right now. The 256 GB model sells for $240 (maybe that's why it's out of stock currently). There's also the issue of reliability. If one drive in a RAID 0 configuration fails, the entire array is lost. At least for a primary system drive, one SSD on its own is safer.

There are of course exceptions. SATA 6Gb/s currently limits us to 500+ MB/s reads and sub-500 MB/s writes. Sometimes, that's just not enough. Just take those raw AVI captures mentioned earlier as an example. We use four Crucial m4s in RAID 0 to make sure we aren't dropping any frames. In a case like that, RAID 0 is a must-have, and the fact that only captured video resides on the array means that a failure would be a fairly superficial loss (except the cost of the drive). If you have an application like that, well, then you already know what you need, and you know that a large, single drive isn't going to get the job done.

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