Performance Comparisons – Mixed Burst

These are the Mixed Burst results introduced in the Samsung 850 EVO 4TB Review. Some tweaks have been made, namely, QD reduced to a more realistic value of 2. Read bursts have been increased to 400MB each. 'Download' speed remains unchanged.

In an attempt to better represent the true performance of hybrid (SLC+TLC) SSDs and to include some general trace-style testing, I’m trying out a new test methodology. First, all tested SSDs are sequentially filled to near maximum capacity. Then the first 8GB span is preconditioned with 4KB random workload, resulting in the condition called out for in many of Intel’s client SSD testing guides. The idea is that most of the data on an SSD is sequential in nature (installed applications, MP3, video, etc), while some portions of the SSD have been written to in a random fashion (MFT, directory structure, log file updates, other randomly written files, etc). The 8GB figure is reasonably practical since 4KB random writes across the whole drive is not a workload that client SSDs are optimized for (it is reserved for enterprise). We may try larger spans in the future, but for now, we’re sticking with the 8GB random write area.

Using that condition as a base for our workload, we now needed a workload! I wanted to start with some background activity, so I captured a BitTorrent download:

burst-1.png

This download was over a saturated 300 Mbit link. While the average download speed was reported as 30 MB/s, the application’s own internal caching meant the writes to disk were more ‘bursty’ in nature. We’re trying to adapt this workload to one that will allow SLC+TLC (caching) SSDs some time to unload their cache between write bursts, so I came to a simple pattern of 40 MB written every 2 seconds. These accesses are more random than sequential, so we will apply it to the designated 8GB span of our pre-conditioned SSD.

Now for the more important part. Since the above ‘download workload’ is a background task that would likely go unnoticed by the user, we also need is a workload that the user *would* be sensitive to. The times where someone really notices their SSD speed is when they are waiting for it to complete a task, and the most common tasks are application and game/level loads. I observed a round of different tasks and came to a 200MB figure for the typical amount of data requested when launching a modern application. Larger games can pull in as much as 2GB (or more), varying with game and level, so we will repeat the 200MB request 10 times during the recorded portion of the run. We will assume 64KB sequential access for this portion of the workload.

Assuming a max Queue Depth of 4 (reasonable for typical desktop apps), we end up with something that looks like this when applied to a couple of SSDs:

burst-2.png

The OCZ Trion 150 (left) is able to keep up with the writes (dashed line) throughout the 60 seconds pictured, but note that the read requests occasionally catch it off guard. Apparently, if some SSDs are busy with a relatively small stream of incoming writes, read performance can suffer, which is exactly the sort of thing we are looking for here.

When we applied the same workload to the 4TB 850 EVO (right), we see an extremely consistent and speedy response to all IOs, regardless of if they are writes or reads. The 200MB read bursts are so fast that they all occur within the same second, and none of them spill over due to other delays caused by the simultaneous writes taking place.

Now for the results:

mixed-1.png

From our Latency Percentile data, we are able to derive the total service time for both reads and writes, and independently show the throughputs seen for both. Remember that these workloads are being applied simultaneously, as to simulate launching apps or games during a 20 MB/s download. The above figures are not simple averages – they represent only the speed *during* each burst. Idle time is not counted.

660p does ok here, falling short of the higher performing parts but holding pace with the WD Black and the Toshiba XG5.

mixed-2.png

Now we are going to focus only on reads, and present some different data. I’ve added up the total service time seen during the 10x 400MB reads that take place during the recorded portion of the test. These figures represent how long you would be sitting there waiting for 4GB of data to be read, but remember this is happening while a download (or another similar background task) is simultaneously writing to the SSD. This metric should closely equate to the 'feel' of using each SSD in a moderate to heavy load. Total read service times should hopefully help you grasp the actual time spent waiting for such a task to complete in the face of background writes taking place.

Since the 660p has to work harder than TLC or MLC SSDs, that ends up being reflected in the read service times. Performance is better with a lighter workload (no background downloads), but this test was meant to be a bit more on the demanding side, so it's no surprise to see the 660p fall where it did in these results.

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