Re: [PATCH v4 6/6] io_uring: add support for zone-append

[hch@xxxxxxxxxxxxx]

On Fri, Aug 14, 2020 at 08:27:13AM +0000, Damien Le Moal wrote:
> > 
> > O_APPEND pretty much implies out of order, as there is no way for an
> > application to know which thread wins the race to write the next chunk.
> 
> Yes and no. If the application threads do not synchronize their calls to
> io_submit(), then yes indeed, things can get out of order. But if the
> application threads are synchronized, then the offset set for each append AIO
> will be in sequence of submission, so the user will not see its writes
> completing at different write offsets than this implied offsets.

Nothing gurantees any kind of ordering for two separate io_submit calls.
The kernel may delay one of them for any reason.

Now if you are doing two fully synchronous write calls on an
O_APPEND fd, yes they are serialized.  But using Zone Append won't
change that.