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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.

We praised the Vertex 4 for its good implementation of the TRIM command. The Agility 4 uses the same algorithm and can be restored to its original performance. That's an advantage over SandForce-based products whose steady-state performance is lower.

Background garbage collection works in the Agility 4 disks, too. It prepares flash memory pages for writing when the SSD is idle. However, this technique cannot restore 100% performance. Its result is similar to what the Marvell-based Crucial m4 disk gains from background garbage collection.

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.

I am sure that OCZ’s marketing department could put these diagrams to great use. Thanks to the Everest 2 controller, the Agility 4 series is quite fast at writing and this advantage becomes even larger as the SandForce-based opponents slow down, getting filled with data. As a result, the inexpensive Agility 4 disks with asynchronous memory outperform even those competitors which are equipped with synchronous flash.

The only fact that can spoil this victory is that writes are much less frequent compared to reads in everyday disk usage scenarios. That’s why the advantage of the Everest 2 based SSDs in terms of writing doesn’t mean they are faster overall.

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