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Degradation and Steady-State Performance

Unfortunately, SSDs are not always as fast as in their “fresh” state. In most cases their performance goes down after some time and in real life we deal with completely different write speeds than what we see on the diagrams in the previous chapter of our review. The reason for this phenomenon is the following: as the SSD runs out of free pages in the flash memory, its controller has to clear memory page blocks before saving data into them, which causes substantial delays. Although, modern SSD controllers can alleviate the performance drop by erasing unused flash memory pages ahead of time, when idle. They use two techniques for that: idle-time garbage collection and TRIM.

Of course, users are more interested in the consistent performance of their SSDs over a long period of time rather than the peak speed they are going to see only during the initial short-term usage period, while the drive is still “fresh”. The SSD makers, however, declare the speed characteristics of “fresh” SSDs for marketing reasons. That’s why we decided to test the performance hit that occurs when a “fresh” SSD becomes a “steady” one.

To get a complete picture of SSD performance degradation we ran special tests based on the SNIA SSSI TWG PTS (Solid State Storage Performance Test Specification) methodology. The main idea of this approach is to measure write speed consecutively in four different cases. First we measure the “fresh” SSD speed. Then we measure the speed after the SSD has been fully filled with data twice. The third test occurs after a 30-minute break during which the controller can partially restore performance by running the idle-time garbage collection. And finally, we measure the speed after issuing a TRIM command.

We ran the tests in synthetic IOMeter 1.1.0 RC1 benchmark, where we measured random write speed when working with 4 KB data blocks aligned to flash memory pages at 32 requests queue depth. The test data were pseudo-random.

The results suggest that the OCZ Vertex 4 do not do background garbage collection but their TRIM implementation is excellent. After TRIM, their performance returns to the original level, as opposed to the numerous SandForce-based products. Thus, you are unlikely to see any performance degradation with a Vertex 4 SSD running under a TRIM-supporting OS, e.g. in an ordinary Windows 7 computer. Of course, there should be some free space on the SSD for TRIM to be effective.

Since the majority of SSDs do change their characteristics when they transition from the FOB state into steady state, we retest their write speed using Crystal Disk Mark 3.0.1. The diagrams below show the obtained results. In this case we use random data and only pay attention two performance during writes, because the read speed remains unaffected.

Anyway, the OCZ Vertex 4 series is much better than the rest of the SSDs, many of which have slowed down, in sequential and random writing and at any request queue depth. Take note that the Everest 2 based SSDs are absolutely different from the first-generation Everest. It’s hard to believe that these solutions have something in common.

 
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