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

On 15 Sep 2016, at 13:36, Christoph Hellwig [off-list ref] wrote:

On Thu, Sep 15, 2016 at 01:33:20PM +0100, Alex Bligh wrote:

quoted

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.

That's probably right in the case of file-based back ends that
are running on a Linux OS. But gonbdserver for instance supports
(e.g.) Ceph based backends, where each connection might be talking
to a completely separate ceph node, and there may be no cache
consistency between connections.

-- 
Alex Bligh