HN Reader

7 GB/s at 512 KB block size is only ~14,000 IO/s which is a whopping ~70 us/IO. That is a trivial rate for even synchronous IO. You should only need one inflight operation (prefetch 1) to overlap your memory copy (to avoid serializing the IO with the memory copy) to get the full IO bandwidth.

Their referenced previous post [1] demonstrates ~240,000 IO/s when using basic settings. Even that seems pretty low, but is still more than enough to completely trivialize this benchmark and saturate the hardware IO with zero tuning.

[1] https://steelcake.com/blog/comparing-io-uring/

why not use page allocator to get aligned memory instead of overallocating?

I'm not very familiar with zig and was kind a struggling to follow the code and maybe that's why I couldn't find where the setting was being set up, but in case it's not, be sure to also use registered file descriptors with io_uring as they make a fairly big difference.

Interesting how an implementation using FreeBSDs AIO would compare.

I see you use a hard-coded constant ALIGN = 512. Many NVMe drives actually allow you to raise the logical block size to 4096 by re-formatting (nvme-format(1)) the drive.

Zig is currently undergoing lots of breaking changes in the IO API and implementation. Any post about IO in zig should also mention the zig version used.

I see it’s 0.15.1 in the zon file, but that should also be part of the post somewhere.

> The result I get from fio is 4.083 GB/s write, … The result for the diorw example in zig is 3.802 GB/s write….

4.083 / 3.802 = 1.0739

2^30 / 10^9 = 1.0737

I think the same rate was likely achieved but there is confusion between GiB and GB.