On Wed, Sep 15, 2021 at 10:15:20AM +0300, Vladimir Sementsov-Ogievskiy wrote: > 14.09.2021 19:32, Richard W.M. Jones wrote: > >On Tue, Sep 14, 2021 at 06:21:58PM +0300, Vladimir Sementsov-Ogievskiy wrote: > >>14.09.2021 17:52, Richard W.M. Jones wrote: > >>>On the > >>>server side when the server receives NBD_CMD_DISC it must complete any > >>>in-flight requests, but there's no requirement for the server to > >>>commit anything to disk. IOW you can still lose data even though you > >>>took the time to disconnect. > >>> > >>>So I don't think there's any reason for libnbd to always gracefully > >> > >>Hmm. Me go to NBD spec :) > >> > >>I think, there is a reason: > >> > >>"The client and the server MUST NOT initiate any form of disconnect other > >>than in one of the above circumstances." > >> > >>And the only possibility for client to initiate a hard disconnect listed > >>above is "if it detects a violation by the other party of a mandatory > >>condition within this document". > >> > >>So at least, nbdsh violates NBD protocol. May be spec should be updated to > >>satisfy your needs. > > > >I would say the spec is at best contradictory, but if you read other > >parts of the spec, then it's pretty clear that we're allowed to drop > >the connection whenever we like. This section says as much: > > > >https://github.com/NetworkBlockDevice/nbd/blob/5805b25ad3da96e7c0b3160cda51ea19eb518d5b/doc/proto.md#terminating-the-transmission-phase > > Hmm, that was exactly the section I read and quoted :) > > > > > There are two methods of terminating the transmission phase: > > ... > > "The client or the server drops the TCP session (in which case it > > SHOULD shut down the TLS session first). This is referred to as > > 'initiating a hard disconnect'." > > Right. Here the method is defined, no word that client can do it at any time.
I don't read this as a definition. But we should probably clarify the spec to note that dropping the connection is fine, because it is - no one has made any argument so far that there's an actual reason to waste time on NBD_CMD_DISC. Rich. > Next, in same section: > > Either side MAY initiate a hard disconnect if it detects a violation by > the other party of a mandatory condition within this document. > > Next > The client MAY issue a soft disconnect at any time > > And finally: > > The client and the server MUST NOT initiate any form of disconnect other > than in one of the above circumstances. > > > Or do you mean some other spec section I missed? > > > > >Anyway we're dropping the TCP connection because sometimes we are just > >interrogating an NBD server eg to find out what it supports, and doing > >a graceful shutdown is a waste of time and internet. > > > >>>shut down (especially in this case where there are no in-flight > >>>requests), and anyway it would break ABI to make that change and slow > >>>down the client in cases when there's nothing to clean up. > >> > >>Which ABI will it break? > > > >Our contract with callers using nbd_close(3), if nbd_shutdown(3) is > >not called beforehand. > >https://libguestfs.org/nbd_shutdown.3.html > >https://libguestfs.org/nbd_create.3.html (really nbd_close ...) > > > >Rich. > > > > > -- > Best regards, > Vladimir -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://people.redhat.com/~rjones/virt-df/
