Re: [Nbd] [Qemu-devel] [PATCH v3] doc: Add NBD_CMD_BLOCK_STATUS extension

[Alex Bligh <alex@...872...>]

> On 2 Dec 2016, at 20:39, John Snow <jsnow@...696...> wrote:
> 
> OK. We do certainly support multiple bitmaps being active at a time in
> QEMU, but I had personally always envisioned that you'd associate them
> one-at-a-time when starting the NBD export of a particular device.
> 
> I don't have a use case in my head where two distinct bitmaps being
> exposed simultaneously offer any particular benefit, but maybe there is
> something. I'm sure there is.

The obvious case is an allocation bitmap, and a dirty bitmap.

It's possible one might want more than one dirty bitmap at once.
Perhaps two sorts of backup, or perhaps live migrate of storage
backed by NBD, or perhaps inspecting the *state* of live migration
via NBD and a bitmap, or perhaps determining what extents in
a QCOW image are in that image itself (as opposed to the image
on which it is based).

I tried to pick some QEMU-like ones, but I am sure there are
examples that would work outside of QEMU.

-- 
Alex Bligh