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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.

Unfortunately, the SanDisk Extreme brings no improvements in terms of the TRIM command. Every time we test a new SandForce-based product we have to complain that the controller developers can’t make their garbage collection algorithms as efficient as in other controllers. The steady-state performance of every SandForce-based SSD is lower than its out-of-box performance, which is sad.

And the saddest thing about the SanDisk Extreme is that its TRIM and garbage collection are even worse than in other SSDs with the same controller, so its performance hit is heavier. Well, this downside is rooted in firmware, so we can expect the SanDisk Extreme to improve with firmware updates. It currently uses reference firmware 5.0.1 whereas most other SandForce-based SSDs have already transitioned to firmware 5.0.2 in which this problem has been somewhat alleviated.

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 diagrams confirm our point. The steady-state performance of the SanDisk Extreme is much lower than what it might deliver with the newer firmware. The current version is dated the end of April and SanDisk doesn’t seem to be too eager to upgrade it.

 
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