Is it not the case that io_submit can block in some filesystems to handle filesystem metadata (such as block allocation), even with O_DIRECT, whereas io_uring never blocks the submitting thread?
Yes, in theory. In practice, the way io_submit() is actually used in most systems today would not have that issue, and it is designed that way for other practical reasons. You'd want to use io_uring in a similar way. Even if you ignore the blocking aspect, file system metadata modification at runtime is an edge case factory.
For database-y type software generally, it is increasingly uncommon to even install a file system. You work with the raw block devices directly, virtualized or otherwise.
