[Qemu-block] 'drive-mirror' vs. 'blockdev-mirror' semantics

[Kashyap Chamarthy]

Hi,

>From the QAPI schema documentation of 'blockdev-mirror':

  # @target: the id or node-name of the block device to mirror to. This
             mustn't be attached to guest.

Why must the target, prepared by 'blockdev-add', be not attached to the
guest?  (Side question: is this via 'device_add' HMP comand, which
handles the guest-visible "frontend" aspect?)  John mentioned on IRC
that this is may be something do with Linux AIO contexts.

>From the DriveMirror structure documentation (and from my own tests of
'drive-mirror' to different targets -- local file, NBD over
UnixSocketAddress, NBD over InetSocketAddress) I don't see such a
limitation.

* * *

I was taking notes on the subtle difference between 'drive-mirror' &
'blockdev-mirror' (I realize this operates with the new 'blockdev-*'
infrastructure, which will be the future, once 'blockdev-add' goes out
of experimental, perhaps).

.---------------------------------------------------------------------.
|     drive-mirror                  |  blockdev-mirror                |
----------------------------------------------------------------------
| 'target' will be created if not   | 'target' needs to be explicitly |
| specified; or an existing target  | created by `blockdev-add`, which|
|  can be used if it exists         | assigns a name to the to-be     |
|                                   | created node                    |
-----------------------------------------------------------------------
| 'target' device can be            | The block-core.json file tells  |
| attached to the guest             | us that  the 'target' "mustn't  |
|                                   | be attached to the guest"       |
-----------------------------------------------------------------------
| Can specify an (optional)         | No 'node-name' parameter,       |
| 'node-name' parameter ("the new   | because `blockdev-add` sets     |
| block driver state node name in   | up the 'node-name'[*]           |
| the graph") for the target        |                                 |
-----------------------------------------------------------------------

[*] On the topic of 'node-name' parameter, I now see that Kevin has
    applied his series "[PATCH v5 00/11] block: Accept node-name in all
    node level QMP commands" will address this, with the below specific
    patch.  [/me noticed that this series is now applied to the
    'block-next' Git branch]:

    PATCH v5 04/11] block: Accept node-name for blockdev-mirror --
    https://lists.nongnu.org/archive/html/qemu-devel/2016-08/msg00540.html


-- 
/kashyap