Articles: Storage

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Degradation and Consistent Performance

Unfortunately, SSDs are not always as fast as in their “fresh” state as they should be. 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. On the other hand, modern SSD controllers can alleviate the performance hit 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. 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 “used” 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 in four different cases. First we measure the “fresh” SSD speed. Then we measure their 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 4KB data blocks aligned to flash memory pages at 32 requests queue depth. The test data were pseudo-random.

SSDs with SF-2281 controller have a considerable performance hit at writing. Unfortunately, they never return to their fresh-state performance notwithstanding the mentioned techniques against degradation. The Zalman F1 is no exception, behaving exactly like its opponent Corsair Force GT. This performance degradation is a shame especially as the Crucial m4, based on a Marvell controller, shows that an SSD’s performance can indeed be restore by the TRIM command.

It means that the write speeds shown in the diagrams in the previous section of our review only reflect but a fragment of the overall picture. As soon as fresh SSDs turn into used ones, their performance changes completely. Their write speed is different. The next diagrams show this speed as benchmarked by CrystalDiskMark 3.0.1.

The SandForce-based SSDs slow down but the overall picture remains the same. The Zalman F1 doesn’t show any sign of its promised superiority over competitors.

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