Comment by benlwalker

On very recent Linux kernels you can open the raw NVMe device and use the NVMe pass thru ioctl to directly send NVMe commands (or you can use SPDK on essentially any Linux kernel) and bypass whatever the fsync implementation is doing. That gives a much more direct test of the hardware (and some vendors have automated tests that do this with SPDK and ip power switches!). There's a bunch of complexity around atomicity of operations during power failure beyond just flush that have to get verified.

But the way you tested is almost certainly valid.