Re: [PATCH v4 6/9] qemu-img: Add bitmap sub-command

[Eric Blake]

On 5/14/20 1:45 AM, Vladimir Sementsov-Ogievskiy wrote:
> 13.05.2020 04:16, Eric Blake wrote:
> > Include actions for --add, --remove, --clear, --enable, --disable, and
> > --merge (note that --clear is a bit of fluff, because the same can be
> > accomplished by removing a bitmap and then adding a new one in its
> > place, but it matches what QMP commands exist).  Listing is omitted,
> > because it does not require a bitmap name and because it was already
> > possible with 'qemu-img info'.  A single command line can play one or
> > more bitmap commands in sequence on the same bitmap name (although all
> > added bitmaps share the same granularity, and and all merged bitmaps
> > come from the same source file).  Merge defaults to other bitmaps in
> > the primary image, but can also be told to merge bitmaps from a
> > distinct image.

> I'm sorry for asking it only now on v4.. But still. Why do we need it? 

Ease of use.

> We can instead run qemu binary (or even new qemu-storage-daemon) and 
> just use existing qmp commands. Is there a real benefit in developing 
> qemu-img, maintaining two interfaces for the same thing?

If it makes someone's life easier, and is not hard to maintain, then 
yes.  A command line interface that calls into QMP is not hard to 
maintain.  And _I_ certainly found it easier to write iotests with this 
patch in place, so it already has at least one client.

> Of-course, just 
> run qmp commands from terminal is a lot less comfortable than just a 
> qemu img command.. But may be we need some wrapper, which make it simple 
> to run one qmp command on an image?
>
> It's simple to make a python wrapper working like
>
> qemu-qmp block-dirty-bitmap-add '{node: self, name: bitmap0, persistent: 
> true}' /path/to/x.qcow2

This _IS_ such a wrapper.  The whole point of this patch is that it is 
now simpler to run one (or more) QMP command on an offline image from 
the command line.  Just because I wrote it in C instead of python, and 
attached it to an existing tool instead of writing a new tool, doesn't 
change the fact that it is just a wrapper around the existing QMP commands.

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