Re: [Qemu-block] [PATCH v2 3/6] nbd: Allow bitmap export during QMP nbd-server-add

[Eric Blake]

On 1/10/19 9:11 AM, Nikolay Shirokovskiy wrote:

>>> The experimental command included an optional 'bitmap-export-name'
>>> field for remapping the name exposed over NBD to be different from
>>> the bitmap name stored on disk.
>>
>>
>> Nikolay, do you have comments on this?
>>
>>
>>    However, my libvirt demo code
>>> for implementing differential backups on top of persistent bitmaps
>>> did not need to take advantage of that feature (it is instead
>>> possible to create a new temporary bitmap with the desired name,
>>> use block-dirty-bitmap-merge to merge one or more persistent
>>> bitmaps into the temporary, then associate the temporary with the
>>> NBD export, if control is needed over the exported bitmap name).
> 
> With bitmap-export-name we can use shorter bitmap names in NBD export.
> For example it can be just UUID of corresponding snapshot. With merge
> strategy this should be at least export-$UUID because $UUID is already
> used for bitmaps created during snapshots.
> 
> Nikolay

If I need to add in an x-bitmap-export-name, as a separate patch, we can
still do that; but I still think libvirt found it fairly easy to just
always create a non-persistent bitmap with the desired name, merge in
the bitmap(s) that belong to the incremental or differential snapshots,
and then expose that temporary bitmap (at which point, you are no longer
tied to the name of the persistent bitmaps, and thus don't need
bitmap-export-name to provide a shorter export name of an actual
persistent bitmap).  Be aware that the rest of this series drops
x-nbd-server-add-bitmap entirely; as well as John's pending changes that
drop the x- prefix from dirty-bitmap-enable/merge/disable, and changed
the merge semantics to take an array; so even if you've had demo code
working against the experimental names, you do have to plan to make some
tweaks. Speak up now [well, before 4.0 is final] if we are about to lock
something into stable that you can't manage to work with (as I have been
doing the same testing with my libvirt code, updating it to use the
stable names and retesting).

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

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