Re: [Qemu-devel] [PATCH 1/3] block: Expose protocol-specific data to 'qemu-img info'

[Eric Blake]

On 1/17/19 10:39 AM, Kevin Wolf wrote:
> Am 17.01.2019 um 16:33 hat Eric Blake geschrieben:
>> 'qemu-img info' is useful for showing additional information
>> about an image - but sometimes the interesting information is
>> specific to the protocol rather than to the format layer.  Set
>> the groundwork for showing this information; further patches
>> will then enable specific pieces of information.
>>
>> Signed-off-by: Eric Blake <address@hidden>
> 
> Isn't this making invalid assumptions about the structure of the block
> graph? Who says that I don't have a quorum node below the format layer
> with five different protocol layer children? Some of which may have
> filter nodes on top?

Does quorum have bs->file?  If not, then this field is not populated for
quorum, so it doesn't make invalid assumptions.

You do have a point about bs->file pointing to filters being an
interesting case, though.  Max had a series proposal back in August that
tries to do smarter role-based descent through the graph rather than
just hard-coding assumptions based on bs->file:

https://lists.gnu.org/archive/html/qemu-devel/2018-08/msg01644.html

Maybe we need to get that in place first, at which point it then becomes
easier to reason about what related BDS information to visit in addition
to the format layer.

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

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