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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. The following diagram shows the history of the relative speed changes, where 100% refers to the SSD performance in “fresh-out-of-box” state.

Corsair’s Neutron drives are both very good in terms of garbage collection and TRIM. Being experienced in developing controllers for enterprise-level SSDs, LAMD has implemented optimized algorithms in its LM87800 controller. Besides a perfect implementation of the TRIM command, which brings the SSD’s performance back to its original level, the new Corsair SSDs can boast efficient background garbage collection, so the write performance of the Neutron GTX and Neutron can be restored well enough even without TRIM. Not to the out-of-box state, of course, but well enough. We’ve only seen this with Plextor SSDs that have Marvell controllers and True Speed technology. But unlike Marvell-based SSDs, the Neutron GTX and Neutron have a large reserve pool, up to 13% of the total capacity, which helps ensure such a high level of write performance regeneration.

All of this means that Corsair SSDs can be recommended for computing environments that do not support TRIM, which is good news for Windows XP users.

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 a matter of fact, performance degradation only plagues SandForce-based SSDs. Other platforms can eliminate this problem by means of the TRIM command. So, if data on your SSD is supposed to be frequently updated, you may want to avoid SandForce-based solutions.

 
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