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 following diagram shows the history of the relative speed changes, where 100% refers to the SSD performance in “fresh-out-of-box” state.
Plextor SSDs feature True Speed technology which helps ensure high performance even without the TRIM command. The technology refers to an aggressive and intellectual garbage collection technique which is actually employed by many other SSDs with Marvell controllers as well as Corsair's LAMD-based solutions. And it works well. The Plextor SSDs do their background garbage collection perfectly. Their write performance can be restored to 60-80% of the out-of-box speed even when the SSD is filled with data to the brim. That's very good.
It is no wonder then that Plextor’s TRIM-triggered garbage collection works ideally, restoring each drive to its original write performance. It means that Plextor SSDs are going to always deliver consistent and predictable performance in TRIM-supporting environments, e.g. in recent versions of Microsoft OSes.
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.
As the SandForce platform is losing its ground, the diagrams showing steady-state write performance of SSDs become more like the diagrams that show their out-of-box write performance. Performance degradation only plagues the SF-2281 controller while the others handle the TRIM command perfectly and keep their performance high throughout their entire service life.
So, we can’t add anything new about the Marvell-based Plextor SSDs here. Yes, they are not very fast at writing, but typical SSD usage scenarios are all about reading. Moreover, desktop computers do not produce a long disk request queue.