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.
Like every other SSD with a second-generation SandForce controller, the Kingmax SMP35 Client 480GB isn’t good at background garbage collection and can only restore its performance by means of the TRIM command. However, even TRIM can't bring the SSD back to its original state. The steady-state performance of the SMP35 Client 480GB is somewhat lower than its out-of-box speed. This refers to writing only, though. You can see the performance degradation as measured by CrystalDiskMark 3.0.1 in the following diagrams. The SSD is filled with random data here.
The SMP35 Client 480GB faces as many as three unfavorable factors in this test: writing, incompressible data and not being right out of the box. As a result, this SSD falls far behind each of its opponents. The OCZ Vertex 4, based on the Everest 2 controller which doesn’t compress data, is many times as fast as the Kingmax. On the other hand, we should keep it in mind that write operations are less frequent than read ones in real usage scenarios, so the failure of the Kingmax in this test isn’t a catastrophe.