[luchs]

The optimal block size is probably the the amount of data which can be transferred with one DMA operation.

For NVMe disks on Linux, you can find out this size with the nvme-cli [0] tool. Use "nvme id-ctrl" to find the Maximum Data Transfer Size (MDTS) in disk (LBA) blocks and "nvme id-ns" to find the LBA Data Size (LBADS). The value is then 2^MDTS * 2^LBADS byte.

For example, the Intel SSD 450 can transfer 32 blocks of 4096 byte per NVMe command, so you'd want a block size of 128 kiB.

[0] https://github.com/linux-nvme/nvme-cli