Re: [Qemu-devel] NBD BLOCK_STATUS

[Eric Blake <eblake@redhat.com>]

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On 11/09/2017 06:42 AM, Vladimir Sementsov-Ogievskiy wrote:
> Hi!
> 
> Interesting fact: list/set_meta_context options are per-export,
> so, in the server we should keep context selection per client per export.
> 
> And it is possible for client to set contexts for one export and than
> proceed
> to transmission phase with another one.

However, we also documented in the spec that

+    A client MUST NOT send `NBD_CMD_BLOCK_STATUS` unless
+    within the negotiation phase it sent `NBD_OPT_SET_META_CONTEXT`
+    at least once, and the final time it was sent, it referred
+    to the export name that was ultimately selected, the server
+    responded without an error, and returned at least one metadata
+    context.

My take on this is if the client calls:

NBD_OPT_SET_META_CONTEXT (export name "foo")
NBD_OPT_GO (export name "bar")

then the server has no contexts to remember, and the client must not
call NBD_CMD_BLOCK_STATUS.  That is, a server is free to track only a
SINGLE list of context ids (namely, the context ids it replied in the
last response to NBD_OPT_SET_META_CONTEXT), and then toss that list on
any further NBD_OPT_SET_META_CONTEXT  or if NBD_OPT_EXPORT_NAME/GO does
not select the same export name, rather than having to track multiple
separate per-export lists while waiting for the client to pick which
export to finally go with.

> 
> This is not really touched in spec, for example
> "Change the set of active metadata contexts. Issuing this command
> replaces all previously-set metadata contexts"
> 
> looks like lacking "for specified export" suffix. May be there are other
> places.

No, it is intentional that it replaces ALL previous contexts (not just
the per-export context), because of the argument that the only context
that matters as a client is the one you use with NBD_OPT_GO, and if you
choose GO with a different export than your last SET_META_CONTEXT, then
you shouldn't be calling NBD_CMD_BLOCK_STATUS.


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


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