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 type of controller says it all. Every drive based on the SF-2281 chip is far from efficient when it comes to garbage collection and the TRIM command. Once such an SSD is filled with data, its write performance drops and cannot be restored even by the OS’s cleanup. It is only with the Secure Erase procedure, which is rather difficult for an inexperienced user and destructive in itself, that the SSD can be restored to its original speed. The performance hit at writing usually amounts to 10-20% and the Corsair Force GS fits into that range, too.
This is a good result, however. Using the same hardware design, the SanDisk Extreme SSD suffers a heavier performance hit. That’s because the Force GS’s newer firmware is optimized in terms of garbage collection.
Since the characteristics of most SSDs do change once they transition from fresh out-of-the-box state into steady state, we measure their performance once again using CrystalDiskMark 3.0.1 benchmark. The diagrams below show the obtained results. We use random data writing and measure only performance during writes, because read speed remains constant.
The steady-state writing performance of SandForce-based SSDs is lower compared to their competitors. The Force GS cannot do anything about it. It is a typical SandForce-based drive, even though with a rarely used type of flash memory. On the other hand, writing is not as important for real-life application as reading, so the low performance of the Corsair Force GS shown in the three previous diagrams is not a catastrophe.