Re: [PATCH] io_uring: add support for barrier fsync

On Tue, Apr 09, 2019 at 10:27:43AM -0600, Jens Axboe wrote:

It's a quite common use case to issue a bunch of writes, then an fsync
or fdatasync when they complete. Since io_uring doesn't guarantee any
type of ordering, the application must track issued writes and wait
with the fsync issue until they have completed.

Add an IORING_FSYNC_BARRIER flag that helps with this so the application
doesn't have to do this manually. If this flag is set for the fsync
request, we won't issue it until pending IO has already completed.

I think we need a much more detailed explanation of the semantics,
preferably in man page format.

Barrier at least in Linux traditionally means all previously submitted
requests have finished and no new ones are started until the
barrier request finishes, which is very heavy handed.  Is that what
this is supposed to do?  If not what are the exact guarantees vs
ordering and or barrier semantics?