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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.

Here Patriot Pyro SE behaves just like all other SandForce based SSDs. Background garbage collection barely works in them and the only way to restore performance is by launching the TRIM. However, even TRIM can’t bring the original performance back: Patriot Pyro SE in steady state will still be slower than the new SSD. However, it only shows in writes. The results of the second round of tests in CrystalDiskMark 3.0.1 are given on the diagram below:

If you prefer solid state drives that won’t lose any of their speed over time, then you should go with the products using Marvell 88SS9174 controller. In fact, this peculiarity makes them demonstrate higher write speeds than the SandForce based drives after they have been completely filled with data. As for the Patriot Pyro SE, this SSD is not any different than the other drives like this. Its results are identical to those of Corsair Force GT in fresh as well as steady state, i.e. it will be identical to all other SandForce based products using synchronous flash memory from Micron or Intel.

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