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4 KB Random Performance: Response Time

The OCZ RevoDrive 3 X2 Preview: Second-Gen SandForce Goes PCIe
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It's really wrong to look at data rates (throughput) without taking latency and processing time into account. We're explained this before in tablet reviews with regard to WiFi throughput, but the same concept also applies to storage. Let's go back to the analogy of a phone call, because it easily illustrates why there's more to speed than just throughput.

Throughput is the audio quality. Latency is the amount of time from when you speak into the phone until the person on the other side hears you, and processing time is the delay for the person on the other line to think about what you said before answering back. If we apply this to SSDs, throughput is the amount of data you can send over time, latency is the lag due to data transmission, while processing time is the overhead incurred by the SSD when it receives the data.

Now consider that latency plus processing time equals response time. That's really what we're measuring in Iometer. This is a little confusing because Iometer uses the terms latency and response time interchangeably, but it's really only capable of measuring the latter.

In random reads, the RevoDrive 3 X2 has a response time of .14 ms, which is about 40% slower than the RevoDrive X2. Interestingly, the Vertex 3 is the slowest SSD with a response time of 0.17 ms.

Write response time is another matter. The 240 GB Vertex 3, 256 GB m4, 512 GB m4, and both RevoDrives tie for the top spot with a response time of 0.07 ms.

Keep in mind that response time is a measure of the difference between initiating and completing an operation, while throughput is a measure of the amount of data transferred. These two values affect performance in different ways, but they don't stack up. So it's not like the 64 GB drive "feels" 75% slower than the 128 GB drive (25% slower throughput + 50% slower response time). Throughput and response time are usually correlated, in that you get high throughput with low response time. In the event that they don't, it's more like having a high-quality phone call with a long lag time or vice versa.

The max response time offers a look at the extremes. In random reads, the RevoDrive 3 X2 only beats out the 5400.6 Seagate hard drive. That's about 6x slower than the RevoDrive X2 and Vertex 3.

All of the OCZ SSDs fall behind once we look at the maximum write response time, though you need to keep this in perspective. The improvement over a hard drive is what you'll experience most viscerally, rather than the difference between a Vertex 3 and an m4.

OCZ uses SandForce controllers, which means there's a little garbage collection occurring after every write op. RevoDrive 3 X2 doesn't really seem all that different from what we from its predecessor, which suggests that OCZ didn't change its algorithms to the point of affecting performance in this metric.

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    doive1231 , 29 June 2011 15:37
    Interesting that Crucial's m4 products show up well in benchmarks but are often overlooked in favour of Corsair and OCZ SSDs.
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