Cumulative followup to various insightful replies.

I wrote:

>>>> No, it's not really possible currently and we have no plans to add
>>> such support since it would not be of any long-term value.
>> The long-term value would be the ability to migrate volumes from, say, a 
>> replicated pool to an an EC pool without extended downtime.

Among the replies:

> That's why the Mimic release should offer a specific set of "rbd
> migration XYX" actions to perform minimal downtime migrations.

That'd be awesome, if it can properly handle client-side attachments.

> I was saying that hacking around rbd-mirror to add such a feature has
> limited long-term value given the release plan. Plus, in the immediate
> term you can use a kernel md RAID device or QEMU+librbd to perform the
> migration for you w/ minimal downtime (albeit with potentially more
> hands-on setup involved).

Great ideas if one controls both the back end and the guest OS.  In our case we 
can't go mucking around inside the guests.

> if you use qemu, it's also possible to use drive-mirror feature from qemu.
> (can mirror and migrate from 1 storage to another storage without downtime).

Interesting idea, and a qemu function I was not previously aware of.  Has some 
potential wrinkles though:

o Considerably more network traffic and I/O load to/from the hypervisor (or 
whatever you call the hosts where your guest VMs run)
o Scaling to thousands of volumes each potentially TBs in size
o Handling unattached volumes
o Co-ordination with / prevention of user attach/detach operations during the 

-- aad

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