Re: [PATCH] qapi: deprecate drive-backup

[Kashyap Chamarthy <kchamart@redhat.com>]

On Mon, Apr 26, 2021 at 01:34:17PM -0400, John Snow wrote:
> On 4/23/21 8:59 AM, Vladimir Sementsov-Ogievskiy wrote:
> > Modern way is using blockdev-add + blockdev-backup, which provides a
> > lot more control on how target is opened.

[...]

> 1) Let's add a sphinx reference to
> https://qemu-project.gitlab.io/qemu/interop/live-block-operations.html#live-disk-backup-drive-backup-and-blockdev-backup
> 
> 
> 2) Just a thought, not a request: We also may wish to update
> https://qemu-project.gitlab.io/qemu/interop/bitmaps.html to use the
> new, preferred method. However, this doc is a bit old and is in need
> of an overhaul anyway (Especially to add the NBD pull workflow.) Since
> the doc is in need of an overhaul anyway, can we ask Kashyap to help
> us here, if he has time?

Yes, I should be able to make time and help here; been putting it off on
the back burner.  Thanks for the reminder.  :) I'd like to update both
these:

    https://qemu-project.gitlab.io/qemu/interop/bitmaps.html
    https://qemu-project.gitlab.io/qemu/interop/live-block-operations.html

Both of them, as you know, refer to 'drive-backup'.  They also need
other adjustments / additions.  Also perhaps break the larger doc into a
couple of smaller ones.

I'll start working on it the end of this week.  First I need to tinker
with some of the recent improvements to refresh my memory and get a
sense of the modifications involved.  So please bear with me.

> 3) Let's add a small explanation here that outlines the differences in
> using these two commands. Here's a suggestion:
> 
> This change primarily separates the creation/opening process of the
> backup target with explicit, separate steps. BlockdevBackup uses
> mostly the same arguments as DriveBackup, except the "format" and
> "mode" options are removed in favor of using explicit
> "blockdev-create" and "blockdev-add" calls.
> 
> The "target" argument changes semantics. It no longer accepts
> filenames, and will now additionally accept arbitrary node names in
> addition to device names.

Yeah; this is something I had figure out by some trial-and-error when I
was playing with it in the past.

> 4) Also not a request: If we want to go above and beyond, it might be nice
> to spell out the exact steps required to transition from the old interface
> to the new one. Here's a (hasty) suggestion for how that might look:
> 
> - The MODE argument is deprecated.
>   - "existing" is replaced by using "blockdev-add" commands.
>   - "absolute-paths" is replaced by using "blockdev-add" and
>     "blockdev-create" commands.
> 
> - The FORMAT argument is deprecated.
>   - Format information is given to "blockdev-add"/"blockdev-create".
> 
> - The TARGET argument has new semantics:
>   - Filenames are no longer supported, use blockdev-add/blockdev-create
>     as necessary instead.
>   - Device targets remain supported.

Agreed; docs like these can be useful for management tools that rely on
the old behaviour and are not as up-to-date as libvirt in keeping track
of QEMU developments.

[...]

-- 
/kashyap