Re: [RFC PATCH 07/22] block/export: Remove magic from block-export-add

[Eric Blake]

On 8/20/20 6:05 AM, Kevin Wolf wrote:

> As long as we can keep the compatibility code local to qmp_nbd_*(), I
> don't think it's too bad. In particular because it's already written.
>
> Instead of adjusting libvirt to changes in the nbd-* commands, I'd
> rather have it change over to block-export-*. I would like to see the
> nbd-server-add/remove commands deprecated soon after we have the
> replacements.

Makes sense to me. So deprecate nbd-server-add in 5.2, and require 
block-export in 6.1.


> > > +    /*
> > > +     * nbd-server-add doesn't complain when a read-only device should be
> > > +     * exported as writable, but simply downgrades it. This is an error with
> > > +     * block-export-add.
> >
> > I'd be happy with either marking this deprecated now (and fixing it in two
> > releases), or declaring it a bug in nbd-server-add now (and fixing it
> > outright).
>
> How about deprecating nbd-server-add completely?

Works for me. Keeping the warts backwards-compatible in nbd-server-add 
is more palatable if we know we are going to drop it wholesale down the 
road.

> > > +    /*
> > > +     * nbd-server-add removes the export when the named BlockBackend used for
> > > +     * @device goes away.
> > > +     */
> > > +    on_eject_blk = blk_by_name(arg->device);
> > > +    if (on_eject_blk) {
> > > +        nbd_export_set_on_eject_blk(export, on_eject_blk);
> > > +    }
> >
> > Wait - is the magic export removal tied only to exporting a drive name, and
> > not a node name?  So as long as libvirt is using only node names whwen
> > adding exports, a drive being unplugged won't interfere?
>
> Yes, seems so. It's the existing behaviour, I'm only moving the code
> around.
>
> > Overall, the change makes sense to me, although I'd love to see if we could
> > go further on the writable vs. read-only issue.
>
> If nbd-server-add will be going away relatively soon, it's probably not
> worth the trouble. But if you have reasons to keep it, maybe we should
> consider it.

No, I'm fine with the idea of getting rid of nbd-server-add, at which 
point changing it before removal is pointless.


--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org