Two PCIe slots looked the same but gave different benchmark results - PCIe-2 vs PCIe-3

I benchmarked a new-old NVMe disk drive I got off eBay and didn't get the performance I expected in a benchmark.  It turns out I used the wrong PCIe slot.  All the connectors looked the same in the dark. The old HP Z820 machine has a bunch of PCIe-3, apparently one PCIe-2, and one classic PCI slot. 

PCIe 3.0 supports a max speed of 8.0 GT/s while PCIe 2.0 supports a max speed of 5.0 GT/s. I have no idea if there was any real-world difference.

PCIe 2.0 vs PCIe 3.0

These are two representative samples with an Intel P3700 in the v2 vs v3 slot.  I ran both of these with the Default setting not the more stressful NVMe CrystalDiskMark configuration. There is a pretty significant boost in sequential performance moving from PCIe-2 to PCIe-3.  The rest of the settings appear to be maxed out because of the test configuration.


Test Configuration Impact - Saturating the Device

CrystalDiskMark 8 has two different test modes, Default and NVMe. The difference is the number of Queues and the number of threads in the middle two tests. It shows that the NVMe drives really need to be saturated to see the differences in some tests.

Random Operation Differences

These results may explain why people don't see huge differences between running O/S operations on an SSD vs PCIe 2.0 vs PCIe 3.0.  Random operation performance is really pretty similar when the device isn't saturated with parallel operations. 

PCIe 3.0 NVMe drives probably make a huge difference in the data center or cloud where higher IOPs or parallel random activity is more common.

SATA 6Gb/s NVMe vs Default tests

Running the same two configurations against a 6Gb/s SATA shows that the single-threaded random test is probably not saturating the drive for read operations. Changing the test configuration to NVMe increases the thread count resulting in about a 2.5x performance boost for that single benchmark.  

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Created 2023 02

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