Plextor M5 Pro Xtreme 256GB / M5S 256GB SSD review: one is cheap, the other faster?

One is cheaper, but is the other one faster?

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Continuous Activity test

We recently began testing SSDs to find out how well they perform when they are in continuous use for 15 minutes. This is the test where the Intel DC S3700 should really shine according to Intel's specifications. Intel claims that IOPS scores should vary no more than 10 percent during prolonged, constant activity. That would be an achievement, because our test has proven that virtually all SSDs exhibit a performance drop after a certain amount of time, often after about six to seven minutes without pause. Continuous activity is common for servers, but not as much for consumer applications.

SSDs write data on a so-called page, in 4 kB portions. For an SSD to be able to write, the data cells first have to be emptied. This can only be one per block, and a block consists of 128 pages and is therefore 512 kB. 

SSDs therefore have to employ some smart tricks. When a number of pages of data have to be removed, the rest of the data first has to be copied to another block, after which the entire block can be emptied. In practice that means that SSD controllers save up writes as much as possible. When the SSD is idle, the controller activates garbage collection, which removes the data from the chips and combines the left-over data as much as possible in complete blocks. This ensures that as many blocks as possible are made available for write actions.

However, if the SSD is continuously in use, the garbage collector doesn't get the chance to do this. Eventually this will cause all blocks to be occupied, which will force the SSD to employ garbage collection between commands. This has a negative effect on performance.

To illustrate this impact on performance, we performed a 15-minute test on the Intel DC S3700 with the IOMeter benchmark. The settings were as follows: 32 simultaneous instructions (queue-depth 32), with 50% reading and 50% writing. The instructions consists of 25% 4 kB, 25% 64 kB, 25% 128 kB and 25% is 1 MB. We configured IOmeter to occupy 60 percent of the SSD, which means 160 GB for 256 GB SSDs and 320 GB for 512 GB SSDs.

The M5 Pro is a little faster than the M3, but both drop at around 8 minutes to 175 MB/s, which is pretty low.


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