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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 two SSDs from Mushkin Enhanced behave like any other second-generation SandForce-based product in this test. Garbage collection is not effective at all. They can only restore their performance by means of the TRIM command. However, even TRIM can’t restore their performance to the original level. The steady-state speed of the Mushkin SSDs is going to be somewhat lower than what they can show in their fresh out-of-box state. This only affects the speed of writing, though. The following diagrams show the difference as measured with Crystal Disk Mark 3.0.1. We use random data here.

If you want an SSD which doesn’t slow down over time, you should consider products based on the Marvell 88SS9174 controller. That’s why such SSDs offer a higher write speed than their SandForce-based counterparts when filled with data. The Mushkin SSDs are no different from their SandForce cousins: the Chronos deluxe is comparable to the OCZ Vertex 3 Max IOPS whereas the ordinary Chronos is similar to the Corsair Force 3.

 
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