Re: [pve-devel] [RFC qemu-server] migration: nbd export: switch away from deprecated QMP command

[Thomas Lamprecht <t.lamprecht@proxmox.com>]

On 02/12/2022 13:54, Fiona Ebner wrote:
> The 'nbd-server-add' QMP command has been deprecated since QEMU 5.2 in
> favor of a more general 'block-export-add'.
> 
> When using 'nbd-server-add', QEMU internally converts the parameters
> and calls blk_exp_add() which is also used by 'block-export-add'. It
> does one more thing, namely calling nbd_export_set_on_eject_blk() to
> auto-remove the export from the server when the backing drive goes
> away. But that behavior is not needed in our case, stopping the NBD
> server removes the exports anyways.
> 
> It was checked with a debugger that the parameters to blk_exp_add()
> are still the same after this change. Well, the block node names are
> autogenerated and not consistent across invocations.
> 
> The alternative to using 'query-block' would be specifying a
> predictable 'node-name' for our '-drive' commandline. It's not that
> difficult for this use case, but in general one needs to be careful
> (e.g. it can't be specified for an empty CD drive, but would need to
> be set when inserting a CD later). Querying the actual 'node-name'
> seemed a bit more future-proof.

are there other examples than the CD-Drive, as that one wouldn't be
really relevant for us here or?

IMO they key disadvantage of switching to computed node-names is the
upgrade path, as I'd expect that for running VMs we cannot change them
to computed ones and thus we would need to still query (or keep the
legacy command) for those...