In the context of RHEL-66064, file-posix gained support for REQ_FUA, which it translates into RWF_DSYNC for aio=native and aio=io_uring. The code path for aio=threads is still unchanged and submits a separate flush request.

The goal here is to improve performance of I/O on guest disks in a writethrough cache mode (such as cache=directsync) or FUA-heavy workloads in the guest by using RWF_DSYNC for thread pool based requests, too (i.e. for all requests with aio=threads, and for misaligned requests in other configurations).