Out-of-the-Box Performance Evaluation

Silicon Motion claims read speeds of up to 895 MBps and write speeds in excess of 700 MBps in their marketing material for the SM2708. However, these speeds are heavily dependent on the NAND flash in the card. For the sampled reference design, Silicon Motion mentioned that they were seeing 895 MBps reads and around 420 MBps writes in terms of peak performance. Real-world speeds are bound to be much lower, depending on the particulars of the access trace.

CrystalDiskMark [Fresh]

CrystalDiskMark serves as a quick check to ensure that the card can meet the performance claims of the manufacturer. The workloads were processed for the SM2708 card in both SD Express and UHS-I modes.

CrystalDiskMark [Fresh] Benchmarks

In the out-of-the-box case, the read and write speeds in SD Express mode match Silicon Motion's numbers - coming in at 890 MBps and 418 MBps respectively. The bump in the random access IOPS in multi-threaded scenarios indicates that the SM2708 controller is pretty much a NVMe SSD controller with the legacy UHS-I controller functions addedin another form. The UHS-I numbers are around the 100 MBps mark, as expected. In any case, UHS-I operation is for backwards compatibility, and performance is not much of a concern in that mode.

Sequential Access - fio Workload

One key aspect to note with SD Express moving forward is that it is going to be pretty much impossible for the cards to maintain their peak speeds beyond the initial SLC cache region. In effect, the claimed speeds are going to be only for burst scenarios. For most applications, that really doesn't matter as long as the card is capable of sustaining the maximum possible rate at which the camera it is used in dumps data. We use fio workloads to emulate typical camera recording conditions. We first run the workload on a fresh card, and also after simulating extended usage (covered in a later section). Instantaneous bandwidth numbers are graphed.

fio Sequential Workload [Fresh]

The reference design has a SLC cache of around 5.5 GB up to which write speeds of up to 375 MBps can be sustained. Beyond that, we have a 75 MBps region, and further on, 30 MBps. Once the controller has been subject to this traffic, the reads start off around 200 MBps before moving higher and higher and ending up at around 700 MBps. On the other hand, the SLC caching effect is a bit more difficult to track in the UHS-I mode. Write speeds vary from as low as 10 MBps to peaks around 62 MBps. Reads are consistent at 72 MBps.

Next Gen SD Card Review: SM2708 Simulating Extended Usage
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