Re: [PATCH v5 11/12] nbd: Expose actual depth in qemu:allocation-depth

[Eric Blake]

On 10/24/20 4:59 AM, Vladimir Sementsov-Ogievskiy wrote:
> 23.10.2020 21:36, Eric Blake wrote:
>> Preserve the tri-state encoding in the low bits, as that still remains
>> a valuable way to utilize qemu-img map with x-dirty-bitmap for
>> accessing quick information without needing a third-party NBD client.
> 
> Hmm.. that doesn't sound as a good reason for redundant information in
> the protocol. Previously good reason was additional effort needed to
> implement sever part, but you've implemented it. And if we export depth
> anyway, it seems better to hack a bit nbd_client_co_block_status to
> convert extent.flags appropriately if metadata context is
> "qemu:allocation-depth" (to keep x-dirty-bitmap working), than have a
> workaround at the protocol layer.

I'm happy to respin this to expose JUST a depth rather than redundant
information, but time is short if we want it in 5.2 (as soft freeze is
this week).  I'll see what I can get to today; I'll rearrange the series
to put multiple bitmap exports first (as that appears ready), while
saving 'qemu-nbd -A' until we're happy with the qemu:allocation-depth
semantics.  After all, once we release something, we've committed to
that user interface.

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