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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 Kingston SSDNow V+200 120GB behaves here just like any other SSD with second-generation SandForce controller. They do not do much background garbage collection, so their performance can only be restored by means of the TRIM command. Yet even TRIM can’t bring the SSD back to its original speed: the steady-state performance of the SSDNow V+200 120GB is going to be somewhat lower compared to its out-of-box state. This is limited to writing only, though. You can see the SSD’s write speed degradation in the next diagram, as measured with CrystalDiskMark 3.0.1 (we use random data in this test).

Interestingly, the out-of-box SSDNow V+200 120GB was about as fast as the same-capacity Corsair Force 3 but there is a large gap between them here. We used to consider the Force 3 among the slowest SandForce-based SSDs, but it is now more than 50% faster than the SSDNow V+200 when writing 4KB data blocks. Thus, the SSDNow V+200 is going to be slower than SSDs like ADATA S510, Corsair Force 3, OCZ Agility 3 or Patriot Pyro both at reading and writing.

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