Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Christoph Hellwig]

On Thu, Sep 15, 2016 at 01:33:20PM +0100, Alex Bligh wrote:
> At an implementation level that is going to be a little difficult
> for some NBD servers, e.g. ones that fork() a different process per
> connection. There is in general no IPC to speak of between server
> instances. Such servers would thus be unsafe with more than one
> connection if FLUSH is in use.
> 
> I believe such servers include the reference server where there is
> process per connection (albeit possibly with several threads).
> 
> Even single process servers (including mine - gonbdserver) would
> require logic to pair up multiple connections to the same
> device.

Why?  If you only send the completion after your I/O syscall returned
your are fine if fsync comes from a difference process, no matter
if you're using direct or buffered I/O underneath.