HARDWARE

OCZ Vertex 3.20 Solid State Drive Review (Part 3)

8/9/2013 9:22:05 AM

Performance degradation, garbage collection and TRIM

Unfortunately, SSDs do not always work as fast as in their "fresh" state. In many cases their performance drops down after a while and in the real-life use process, we deal with the write speeds that are completely different from what we saw on the diagrams in the previous chapter in our review. The reason for this phenomenon is as follows: since the SSD is out of blank pages in the flash memory, its controller has to erase its memory page blocks before saving the data into them, which causes significant delays. Thus, the modern SSDs usually try to free memory in advance, and not at the time when the write process is carried out. This process usually occurs in the idle mode. At this time the SSD controller can reduce the performance decrease almost entirely by removing unused flash memory ahead of time. The corresponding steps are usually performed in the idle mode, when the controller is likely to fully restore the SSD performance by deleting the flash memory pages that are not used. They utilize two techniques for this process, that is: the garbage in the idle time and TRIM.

The modern SSDs usually try to free memory in advance, and not at the time when the write process is carried out.

The modern SSDs usually try to free memory in advance, and not at the time when the write process is carried out.

An SSD controller does not know which memory pages hold user data, and which are considered free by the operating system. It happens this way since in the file system, the process of removing a file is not related to its actual physical removal. Instead, the corresponding memory marked in the file system is available for writing data. Consequently, without being related to the operating system, a SSD controller is likely to only delete the pages in the reserve pool (if there are any) in advance, which is not accessible by the operating system. For a better solution to this problem, the modern operating systems that feature the TRIM command will improve garbage collection efficiently. TRIM offers the SSD controller with information on data that can potentially be removed without any damage, because it is considered not to be used by the operating system. As a result, the SSD controller is able to increase the pages that are deleted by removing the unnecessary physical data for users not to feel a diminished performance in writing next time.

This is how it will lie in the ideal conditions. However, in reality, the SSDs perform differently in the process of their garbage collection and TRIM implementation. That is why we test the reduced performance of an SSD when moving from the state of just taking it out of the box (flash memory is clean) to the steady state. This testing follows the instructions of SNIA SSSI PTS TWG, which means we measure the write speeds in four cases one by one. First we measure the speed of SSD "fresh". Then we measure the speed after the SSD has been fully filled with data twice. The third test session takes place after a 30 minute rest meanwhile the controller can partially restore operation by running the idle-time garbage collection. And finally, we measure the speed after releasing a TRIM command.

We ran the tests in 1.1.0 RC1 synthetic IOMeter benchmark, where we measured the random write speed when working with 4 KB blocks of data in accordance with flash memory pages at 32 that requires successively. The test data were artificial-random. The chart below shows the history of changes in the relative speed, where 100% mentions the SSD performance in the state of "fresh-out-of-box".

Iometer, Relative Random Write

Iometer, Relative Random Write

The chart shows that the problems of reduced performance only spread at the SandForce-based SSDs meanwhile the other models can recover their speed after the operating system provide them with the TRIM command. However, the OCZ Vertex 3.20 is even worse than the other SSDs with SF-2281 controller: its write speed degrades greatly and garbage collection algorithms are not very effective. As a result, it may be halved the performance during the writing process.

Hopefully, the Vertex 3.20 might improve after the firmware updates. We do not know exactly what reference software OCZ employs in this SSD because OCZ has its own numbering system. One of the latest reference versions had some issues with TRIM, which can display in the Vertex 3.20. If so, they can be removed in the next firmware versions. On the other hand, the SandForce engineers have been unable to perfectly perform their garbage collection algorithms for over 2 years; therefore, their controller is unlikely to avoid some performance decrease anyway, which is its main weak point.

Futuremark PCMark 7

The popular PCMark 7 contains an individual standard disk system. It is not a synthetic test, but is given on the real-life applications. This standard may create the typical disk usage scenarios and measure how fast they are fulfilled for common tasks. Starting with the version 1.4.0, the PCMark 7 disk system test generates raw performance results which do not take into account any delay in the process of queuing commands. Such new results are not compatible with the old ones, but the differences between the performance numbers of the various SSDs are now becoming clearer. That's why we decided to change to the new version for the test sessions from now on.

We ran PCMark 7 on "steady" SSD, which is the state that exists in the real-life computer systems most of the time. Their activities in this case are not only affected by their controller or flash memory speed, but also by the effectiveness of their internal algorithms against the performance decrease.

PCMark 7

PCMark 7

SandForce–based SSDs are always good in this standard since it uses compressed data, but the OCZ Vertex 3.20 is still the worst in some of their products. This might have been anticipated because it was slower than the SandForce-based SSDs with 25nm ONFI or Toggle Mode flash in the synthetic tests.

However, the total score is a more general measure of performance. Now let us examine the individual tests to get a more detailed picture of what our SSDs are capable of working at each operational load level:

The Windows Defender Speed

​​The Windows Defender Speed

The speed of importing images

The speed of ​​importing images

The speed of video editing

The speed ​​of video editing

The speed of Windows Media Center

The speed of Windows Media Center

The speed of adding music

The speed ​​of adding music

The speed of starting applications

The speed of starting applications

The speed of gaming

The speed ​​of gaming

A remarkable unity! The OCZ Vertex 3.20 always stays behind the rest of the SandForce-based SSDs with synchronous MLC NAND flash; consequently, its results are average of the overall ranks. There is no proper script used for this product, and its appeal to the end-users will be likely to depend entirely on its price.

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