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 2000 MBps, but these are not backed up by any of our usual synthetic benchmarks. As we shall see further down in this review, the P40 can sustain the advertised numbers only over a very small data size. One can run ATTO with a 256MB file size and CrystalDiskMark with a 1GB span and back up WD's numbers, but those are not the sizes we use for evaluation.
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. For a 32GB file size, optimal write performance is reached for an I/O size of 128KB. Reads are ot affected much, reaching very close to advertised numbers at 512KB and sustaining it beyond that for a queue depth of 4. It is a matter of concern that none of the other 20 Gbps-class PSSDs have a problem with the ATTO file size used in our evaluation.
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CrystalDiskMark 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.
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The P40 can only average around 1250 MBps writes over a 32GB span. Every other PSSD in its class can sustain 1800 MBps+ for similar sequential accesses. The reads and high-queue depth random accesses are a bit of a saving grace for the P40 when compared to other PSSDs. Low queue-depth performance suffers due to the lack of on-board DRAM.
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