Random IO

Random read and write performance matters if you copy a lot of small files or use the drive as storage for games or applications that are directly started off the drive.





While overall random IO performance is very low, it's surprising to see that random writes run much faster than random reads. Given the encryption support I would have expected the opposite.

Sequential IO

Sequential performance is relevant when you copy large files—videos, for example.





Here we see the Kingston drive reaching the promised 250 MB/s, even at a queue depth of 1, which is important. When compared against other portable SSDs (without encryption), these speeds are quite low though.

Sustained Write Speeds

Write speeds start out at the rated 250 MB/s, but after 250 GB written, they drop considerably, to less than 50 MB/s—this is VERY slow! The underlying reason seems to be that the QLC SSD runs out of SLC cache at this point, and is just suuuuuper slow to write from here on.


To further research this, I took the A400 QLC SSD out of the case and hooked it up right to my host PC, so no encryption or anything. The SSD worked perfectly, but here, too, it showed the huge drop in performance once it's 25% full. This makes sense, because QLC writes four bits per cell, which means in SLC mode, the capacity gets used up four times as fast. The A400 SSD simply fills all its capacity in SLC mode and then has to juggle data out of SLC into QLC and at the same time handle incoming writes.


I found out that you can replace the SSD with another model from Kingston, not any other 2.5" SATA SSD. The controller firmware will look for a "Kingston" drive on boot-up and reject all others. So I installed an old Kingston SH100 240 GB MLC drive, and as you can see, the speeds are much better. Had I installed a modern Kingston SATA SSD with TLC, I'm sure the speeds would be even better. No way to exceed 250 MB/s though—that's a limitation of the encryption controller.