[$] Asynchronous fsync()

The cost of fsync() is well known to filesystem developers, which is why there are efforts to provide cheaper alternatives. Ric Wheeler wanted to discuss the longstanding idea of adding an asynchronous version of fsync() in a filesystem session at the 2019 Linux Storage, Filesystem, and Memory-Management Summit (LSFMM). It turns out that what he wants may already be available via the new io_uring interface.