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Re: [Qemu-block] nbd oldstyle negotiation

From: Eric Blake
Subject: Re: [Qemu-block] nbd oldstyle negotiation
Date: Thu, 16 Aug 2018 14:56:10 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 08/16/2018 02:02 PM, Vladimir Sementsov-Ogievskiy wrote:
Hi Eric!

There is a small problem with our qemu-nbd cmdline interface: people forget to use option -x or don't know about it and face into problems with old protocol version. One of may colleagues after such situation (because of this option, he could not use utility nbd-client, which doesn't support old version) asked me to add comments about version selection by -x option into --help of qemu-nbd. And really, it's not an obvious interface solution...

Now I think, should not we use new version by default? Or, even drop old version support at all? Anybody uses it?

nbdkit 1.3 switched its default to newstyle (Jan 2018):

and you are correct that nbd 3.10 dropped oldstyle long ago (Mar 2015):

oldstyle is severely lacking in features (it can't do TLS or efficient sparse file handling, for example). Do we need a formal qemu deprecation period (where you still get oldstyle if -x was not used, but a nice big warning) before flipping the default in 3.2/3.3 [1], or are we okay flipping it in 3.1?

[1] Okay, I know that our new equally-arbitrary policy on when to bump qemu version number components means that we'll probably see 3.1 in 2018 then 4.0 in 2019, rather than 3.2.

And, if we switch the default now (which I am in favor of), do we want to add a -o switch to still make it possible to select oldstyle for a couple more releases in case someone was relying on it? Then again, if you have to amend your script to use -o to keep things from breaking, why not just amend your setup to use newstyle?

My personal preference: completely drop oldstyle support from qemu, for 3.1, with no deprecation period. Do so by making '-x ""' the default for any qemu-nbd command line that did not otherwise specify an export name. If someone still needs interoperability, they can use nbdkit in the middle, which will still support oldstyle, and provides a plugin for connecting:

newstyle client => nbdkit nbd plugin => oldstyle server
oldstyle client => nbdkit nbd plugin => newstyle server

(well, that plugin could be enhanced to support IP addresses, rather than just Unix sockets, but that's a topic for another mailing list)

But I'm open to counter opinions on whether we need to be more conservative, and will not rip things out without at least getting opinions from others on how aggressive or conservative the switchover should be.

from nbd spec:
 ... the old style negotiation is now no longer developed; starting with version 3.10 of the reference implementation, it is also no longer supported.

it's unsupported, I think it's ok to drop it. I can create patches, if you agree.

If you want to create patches, that's also fine by me.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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