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.
The performance degradation problem is sinking into the past along with SandForce controllers. If the OS supports the TRIM command (which works normally in all modern versions of Windows and Linux), the performance of modern SSDs doesn’t deteriorate below the out-of-box level. The same goes for the OCZ Vector: it supports TRIM without any problems.
If the OS doesn’t have TRIM (e.g. Windows XP and, in some cases, Mac OS X), the writing performance unavoidably worsens, making garbage collection algorithms important. Such algorithms are implemented best in SSDs with Marvell and LAMD controllers whereas the Vector with its new Indilinx Barefoot 3 controller can only get back 40% of its original write speed.
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.
Considering that we carry out our tests in Windows 7, which supports TRIM, we can see substantial changes in the speed of SandForce-based SSDs only. And we can again marvel at the brilliant performance of the new OCZ Vector which takes one of the top places in each of the three diagrams.