SanDisk Professional PRO-G40 Thunderbolt / USB Dual-Mode Portable SSD Reviewby Ganesh T S on October 6, 2022 8:00 AM EST
Synthetic Benchmarks - ATTO and CrystalDiskMark
Benchmarks such as ATTO and CrystalDiskMark help provide a quick look at the performance of the direct-attached storage device. The results translate to the instantaneous performance numbers that consumers can expect for specific workloads, but do not account for changes in behavior when the unit is subject to long-term conditioning and/or thermal throttling. Yet another use of these synthetic benchmarks is the ability to gather information regarding support for specific storage device features that affect performance.
Western Digital claims read and write speeds of 2700 MBps and 1900 MBps respectively, and these are backed up by the ATTO benchmarks provided below. In fact, WD has been quite conservative in their claims, and has actually put in numbers that are more representative of sustained usage for the PRO-G40's use-case. ATTO benchmarking is restricted to a single configuration in terms of queue depth, and is only representative of a small sub-set of real-world workloads. It does allow the visualization of change in transfer rates as the I/O size changes, with optimal performance being reached around 1 MB for a queue depth of 4. The effect of enabling write caching are also evident at higher I/O sizes, with the feature enabling better consistency.
CrystalDiskMark. for example, uses four different access traces for reads and writes over a configurable region size. Two of the traces are sequential accesses, while two are 4K random accesses. Internally, CrystalDiskMark uses the Microsoft DiskSpd storage testing tool. The 'Seq128K Q32T1' sequential traces use 128K block size with a queue depth of 32 from a single thread, while the '4K Q32T16' one does random 4K accesses with the same queue configuration, but from multiple threads. The 'Seq1M' traces use a 1MiB block size. The plain 'Rnd4K' one uses only a single queue and single thread . Comparing the '4K Q32T16' and '4K Q1T1' numbers can quickly tell us whether the storage device supports NCQ (native command queuing) / UASP (USB-attached SCSI protocol). If the numbers for the two access traces are in the same ballpark, NCQ / UASP is not supported. This assumes that the host port / drivers on the PC support UASP.
The 32GB data span used in our portable SSD reviews fits in nicely with the amount of SLC cache present in the PRO-G40 (as we shall see further down in this review). Read and write numbers are well beyond the advertised values, and the performance scales with the host interface - acting as a 1GBps-class drive with non-Thunderbolt ports, and as a 2.5GBps+ drive when connected to a Thunderbolt 3 host.