docs/interop/live-block-operations.rst

..
    Copyright (C) 2017 Red Hat Inc.

    This work is licensed under the terms of the GNU GPL, version 2 or
    later.  See the COPYING file in the top-level directory.

============================
Live Block Device Operations
============================

QEMU Block Layer currently (as of QEMU 2.9) supports four major kinds of
live block device jobs -- stream, commit, mirror, and backup.  These can
be used to manipulate disk image chains to accomplish certain tasks,
namely: live copy data from backing files into overlays; shorten long
disk image chains by merging data from overlays into backing files; live
synchronize data from a disk image chain (including current active disk)
to another target image; and point-in-time (and incremental) backups of
a block device.  Below is a description of the said block (QMP)
primitives, and some (non-exhaustive list of) examples to illustrate
their use.

.. note::
    The file ``qapi/block-core.json`` in the QEMU source tree has the
    canonical QEMU API (QAPI) schema documentation for the QMP
    primitives discussed here.

.. todo (kashyapc):: Remove the ".. contents::" directive when Sphinx is
                     integrated.

.. contents::

Disk image backing chain notation
---------------------------------

A simple disk image chain.  (This can be created live using QMP
``blockdev-snapshot-sync``, or offline via ``qemu-img``)::

                   (Live QEMU)
                        |
                        .
                        V

            [A] <----- [B]

    (backing file)    (overlay)

The arrow can be read as: Image [A] is the backing file of disk image
[B].  And live QEMU is currently writing to image [B], consequently, it
is also referred to as the "active layer".

There are two kinds of terminology that are common when referring to
files in a disk image backing chain:

(1) Directional: 'base' and 'top'.  Given the simple disk image chain
    above, image [A] can be referred to as 'base', and image [B] as
    'top'.  (This terminology can be seen in in QAPI schema file,
    block-core.json.)

(2) Relational: 'backing file' and 'overlay'.  Again, taking the same
    simple disk image chain from the above, disk image [A] is referred
    to as the backing file, and image [B] as overlay.

   Throughout this document, we will use the relational terminology.

.. important::
    The overlay files can generally be any format that supports a
    backing file, although QCOW2 is the preferred format and the one
    used in this document.


Brief overview of live block QMP primitives
-------------------------------------------

The following are the four different kinds of live block operations that
QEMU block layer supports.

(1) ``block-stream``: Live copy of data from backing files into overlay
    files.

    .. note:: Once the 'stream' operation has finished, three things to
              note:

                (a) QEMU rewrites the backing chain to remove
                    reference to the now-streamed and redundant backing
                    file;

                (b) the streamed file *itself* won't be removed by QEMU,
                    and must be explicitly discarded by the user;

                (c) the streamed file remains valid -- i.e. further
                    overlays can be created based on it.  Refer the
                    ``block-stream`` section further below for more
                    details.

(2) ``block-commit``: Live merge of data from overlay files into backing
    files (with the optional goal of removing the overlay file from the
    chain).  Since QEMU 2.0, this includes "active ``block-commit``"
    (i.e. merge the current active layer into the base image).

    .. note:: Once the 'commit' operation has finished, there are three
              things to note here as well:

                (a) QEMU rewrites the backing chain to remove reference
                    to now-redundant overlay images that have been
                    committed into a backing file;

                (b) the committed file *itself* won't be removed by QEMU
                    -- it ought to be manually removed;

                (c) however, unlike in the case of ``block-stream``, the
                    intermediate images will be rendered invalid -- i.e.
                    no more further overlays can be created based on
                    them.  Refer the ``block-commit`` section further
                    below for more details.

(3) ``drive-mirror`` (and ``blockdev-mirror``): Synchronize a running
    disk to another image.

(4) ``drive-backup`` (and ``blockdev-backup``): Point-in-time (live) copy
    of a block device to a destination.


.. _`Interacting with a QEMU instance`:

Interacting with a QEMU instance
--------------------------------

To show some example invocations of command-line, we will use the
following invocation of QEMU, with a QMP server running over UNIX
socket:

.. parsed-literal::

  $ |qemu_system| -display none -no-user-config -nodefaults \\
    -m 512 -blockdev \\
    node-name=node-A,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./a.qcow2 \\
    -device virtio-blk,drive=node-A,id=virtio0 \\
    -monitor stdio -qmp unix:/tmp/qmp-sock,server=on,wait=off

The ``-blockdev`` command-line option, used above, is available from
QEMU 2.9 onwards.  In the above invocation, notice the ``node-name``
parameter that is used to refer to the disk image a.qcow2 ('node-A') --
this is a cleaner way to refer to a disk image (as opposed to referring
to it by spelling out file paths).  So, we will continue to designate a
``node-name`` to each further disk image created (either via
``blockdev-snapshot-sync``, or ``blockdev-add``) as part of the disk
image chain, and continue to refer to the disks using their
``node-name`` (where possible, because ``block-commit`` does not yet, as
of QEMU 2.9, accept ``node-name`` parameter) when performing various
block operations.

To interact with the QEMU instance launched above, we will use the
``qmp-shell`` utility (located at: ``qemu/scripts/qmp``, as part of the
QEMU source directory), which takes key-value pairs for QMP commands.
Invoke it as below (which will also print out the complete raw JSON
syntax for reference -- examples in the following sections)::

    $ ./qmp-shell -v -p /tmp/qmp-sock
    (QEMU)

.. note::
    In the event we have to repeat a certain QMP command, we will: for
    the first occurrence of it, show the ``qmp-shell`` invocation, *and*
    the corresponding raw JSON QMP syntax; but for subsequent
    invocations, present just the ``qmp-shell`` syntax, and omit the
    equivalent JSON output.


Example disk image chain
------------------------

We will use the below disk image chain (and occasionally spelling it
out where appropriate) when discussing various primitives::

    [A] <-- [B] <-- [C] <-- [D]

Where [A] is the original base image; [B] and [C] are intermediate
overlay images; image [D] is the active layer -- i.e. live QEMU is
writing to it.  (The rule of thumb is: live QEMU will always be pointing
to the rightmost image in a disk image chain.)

The above image chain can be created by invoking
``blockdev-snapshot-sync`` commands as following (which shows the
creation of overlay image [B]) using the ``qmp-shell`` (our invocation
also prints the raw JSON invocation of it)::

    (QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2
    {
        "execute": "blockdev-snapshot-sync",
        "arguments": {
            "node-name": "node-A",
            "snapshot-file": "b.qcow2",
            "format": "qcow2",
            "snapshot-node-name": "node-B"
        }
    }

Here, "node-A" is the name QEMU internally uses to refer to the base
image [A] -- it is the backing file, based on which the overlay image,
[B], is created.

To create the rest of the overlay images, [C], and [D] (omitting the raw
JSON output for brevity)::

    (QEMU) blockdev-snapshot-sync node-name=node-B snapshot-file=c.qcow2 snapshot-node-name=node-C format=qcow2
    (QEMU) blockdev-snapshot-sync node-name=node-C snapshot-file=d.qcow2 snapshot-node-name=node-D format=qcow2


A note on points-in-time vs file names
--------------------------------------

In our disk image chain::

    [A] <-- [B] <-- [C] <-- [D]

We have *three* points in time and an active layer:

- Point 1: Guest state when [B] was created is contained in file [A]
- Point 2: Guest state when [C] was created is contained in [A] + [B]
- Point 3: Guest state when [D] was created is contained in
  [A] + [B] + [C]
- Active layer: Current guest state is contained in [A] + [B] + [C] +
  [D]

Therefore, be aware with naming choices:

- Naming a file after the time it is created is misleading -- the
  guest data for that point in time is *not* contained in that file
  (as explained earlier)
- Rather, think of files as a *delta* from the backing file


Live block streaming --- ``block-stream``
-----------------------------------------

The ``block-stream`` command allows you to do live copy data from backing
files into overlay images.

Given our original example disk image chain from earlier::

    [A] <-- [B] <-- [C] <-- [D]

The disk image chain can be shortened in one of the following different
ways (not an exhaustive list).

.. _`Case-1`:

(1) Merge everything into the active layer: I.e. copy all contents from
    the base image, [A], and overlay images, [B] and [C], into [D],
    *while* the guest is running.  The resulting chain will be a
    standalone image, [D] -- with contents from [A], [B] and [C] merged
    into it (where live QEMU writes go to)::

        [D]

.. _`Case-2`:

(2) Taking the same example disk image chain mentioned earlier, merge
    only images [B] and [C] into [D], the active layer.  The result will
    be contents of images [B] and [C] will be copied into [D], and the
    backing file pointer of image [D] will be adjusted to point to image
    [A].  The resulting chain will be::

        [A] <-- [D]

.. _`Case-3`:

(3) Intermediate streaming (available since QEMU 2.8): Starting afresh
    with the original example disk image chain, with a total of four
    images, it is possible to copy contents from image [B] into image
    [C].  Once the copy is finished, image [B] can now be (optionally)
    discarded; and the backing file pointer of image [C] will be
    adjusted to point to [A].  I.e. after performing "intermediate
    streaming" of [B] into [C], the resulting image chain will be (where
    live QEMU is writing to [D])::

        [A] <-- [C] <-- [D]


QMP invocation for ``block-stream``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For `Case-1`_, to merge contents of all the backing files into the
active layer, where 'node-D' is the current active image (by default
``block-stream`` will flatten the entire chain); ``qmp-shell`` (and its
corresponding JSON output)::

    (QEMU) block-stream device=node-D job-id=job0
    {
        "execute": "block-stream",
        "arguments": {
            "device": "node-D",
            "job-id": "job0"
        }
    }

For `Case-2`_, merge contents of the images [B] and [C] into [D], where
image [D] ends up referring to image [A] as its backing file::

    (QEMU) block-stream device=node-D base-node=node-A job-id=job0

And for `Case-3`_, of "intermediate" streaming", merge contents of
images [B] into [C], where [C] ends up referring to [A] as its backing
image::

    (QEMU) block-stream device=node-C base-node=node-A job-id=job0

Progress of a ``block-stream`` operation can be monitored via the QMP
command::

    (QEMU) query-block-jobs
    {
        "execute": "query-block-jobs",
        "arguments": {}
    }


Once the ``block-stream`` operation has completed, QEMU will emit an
event, ``BLOCK_JOB_COMPLETED``.  The intermediate overlays remain valid,
and can now be (optionally) discarded, or retained to create further
overlays based on them.  Finally, the ``block-stream`` jobs can be
restarted at anytime.


Live block commit --- ``block-commit``
--------------------------------------

The ``block-commit`` command lets you merge live data from overlay
images into backing file(s).  Since QEMU 2.0, this includes "live active
commit" (i.e. it is possible to merge the "active layer", the right-most
image in a disk image chain where live QEMU will be writing to, into the
base image).  This is analogous to ``block-stream``, but in the opposite
direction.

Again, starting afresh with our example disk image chain, where live
QEMU is writing to the right-most image in the chain, [D]::

    [A] <-- [B] <-- [C] <-- [D]

The disk image chain can be shortened in one of the following ways:

.. _`block-commit_Case-1`:

(1) Commit content from only image [B] into image [A].  The resulting
    chain is the following, where image [C] is adjusted to point at [A]
    as its new backing file::

        [A] <-- [C] <-- [D]

(2) Commit content from images [B] and [C] into image [A].  The
    resulting chain, where image [D] is adjusted to point to image [A]
    as its new backing file::

        [A] <-- [D]

.. _`block-commit_Case-3`:

(3) Commit content from images [B], [C], and the active layer [D] into
    image [A].  The resulting chain (in this case, a consolidated single
    image)::

        [A]

(4) Commit content from image only image [C] into image [B].  The
    resulting chain::

	[A] <-- [B] <-- [D]

(5) Commit content from image [C] and the active layer [D] into image
    [B].  The resulting chain::

	[A] <-- [B]


QMP invocation for ``block-commit``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For :ref:`Case-1 <block-commit_Case-1>`, to merge contents only from
image [B] into image [A], the invocation is as follows::

    (QEMU) block-commit device=node-D base=a.qcow2 top=b.qcow2 job-id=job0
    {
        "execute": "block-commit",
        "arguments": {
            "device": "node-D",
            "job-id": "job0",
            "top": "b.qcow2",
            "base": "a.qcow2"
        }
    }

Once the above ``block-commit`` operation has completed, a
``BLOCK_JOB_COMPLETED`` event will be issued, and no further action is
required.  As the end result, the backing file of image [C] is adjusted
to point to image [A], and the original 4-image chain will end up being
transformed to::

    [A] <-- [C] <-- [D]

.. note::
    The intermediate image [B] is invalid (as in: no more further
    overlays based on it can be created).

    Reasoning: An intermediate image after a 'stream' operation still
    represents that old point-in-time, and may be valid in that context.
    However, an intermediate image after a 'commit' operation no longer
    represents any point-in-time, and is invalid in any context.


However, :ref:`Case-3 <block-commit_Case-3>` (also called: "active
``block-commit``") is a *two-phase* operation: In the first phase, the
content from the active overlay, along with the intermediate overlays,
is copied into the backing file (also called the base image).  In the
second phase, adjust the said backing file as the current active image
-- possible via issuing the command ``block-job-complete``.  Optionally,
the ``block-commit`` operation can be cancelled by issuing the command
``block-job-cancel``, but be careful when doing this.

Once the ``block-commit`` operation has completed, the event
``BLOCK_JOB_READY`` will be emitted, signalling that the synchronization
has finished.  Now the job can be gracefully completed by issuing the
command ``block-job-complete`` -- until such a command is issued, the
'commit' operation remains active.

The following is the flow for :ref:`Case-3 <block-commit_Case-3>` to
convert a disk image chain such as this::

    [A] <-- [B] <-- [C] <-- [D]

Into::

    [A]

Where content from all the subsequent overlays, [B], and [C], including
the active layer, [D], is committed back to [A] -- which is where live
QEMU is performing all its current writes).

Start the "active ``block-commit``" operation::

    (QEMU) block-commit device=node-D base=a.qcow2 top=d.qcow2 job-id=job0
    {
        "execute": "block-commit",
        "arguments": {
            "device": "node-D",
            "job-id": "job0",
            "top": "d.qcow2",
            "base": "a.qcow2"
        }
    }


Once the synchronization has completed, the event ``BLOCK_JOB_READY`` will
be emitted.

Then, optionally query for the status of the active block operations.
We can see the 'commit' job is now ready to be completed, as indicated
by the line *"ready": true*::

    (QEMU) query-block-jobs
    {
        "execute": "query-block-jobs",
        "arguments": {}
    }
    {
        "return": [
            {
                "busy": false,
                "type": "commit",
                "len": 1376256,
                "paused": false,
                "ready": true,
                "io-status": "ok",
                "offset": 1376256,
                "device": "job0",
                "speed": 0
            }
        ]
    }

Gracefully complete the 'commit' block device job::

    (QEMU) block-job-complete device=job0
    {
        "execute": "block-job-complete",
        "arguments": {
            "device": "job0"
        }
    }
    {
        "return": {}
    }

Finally, once the above job is completed, an event
``BLOCK_JOB_COMPLETED`` will be emitted.

.. note::
    The invocation for rest of the cases (2, 4, and 5), discussed in the
    previous section, is omitted for brevity.


Live disk synchronization --- ``drive-mirror`` and ``blockdev-mirror``
----------------------------------------------------------------------

Synchronize a running disk image chain (all or part of it) to a target
image.

Again, given our familiar disk image chain::

    [A] <-- [B] <-- [C] <-- [D]

The ``drive-mirror`` (and its newer equivalent ``blockdev-mirror``)
allows you to copy data from the entire chain into a single target image
(which can be located on a different host), [E].

.. note::

    When you cancel an in-progress 'mirror' job *before* the source and
    target are synchronized, ``block-job-cancel`` will emit the event
    ``BLOCK_JOB_CANCELLED``.  However, note that if you cancel a
    'mirror' job *after* it has indicated (via the event
    ``BLOCK_JOB_READY``) that the source and target have reached
    synchronization, then the event emitted by ``block-job-cancel``
    changes to ``BLOCK_JOB_COMPLETED``.

    Besides the 'mirror' job, the "active ``block-commit``" is the only
    other block device job that emits the event ``BLOCK_JOB_READY``.
    The rest of the block device jobs ('stream', "non-active
    ``block-commit``", and 'backup') end automatically.

So there are two possible actions to take, after a 'mirror' job has
emitted the event ``BLOCK_JOB_READY``, indicating that the source and
target have reached synchronization:

(1) Issuing the command ``block-job-cancel`` (after it emits the event
    ``BLOCK_JOB_COMPLETED``) will create a point-in-time (which is at
    the time of *triggering* the cancel command) copy of the entire disk
    image chain (or only the top-most image, depending on the ``sync``
    mode), contained in the target image [E]. One use case for this is
    live VM migration with non-shared storage.

(2) Issuing the command ``block-job-complete`` (after it emits the event
    ``BLOCK_JOB_COMPLETED``) will adjust the guest device (i.e. live
    QEMU) to point to the target image, [E], causing all the new writes
    from this point on to happen there.

About synchronization modes: The synchronization mode determines
*which* part of the disk image chain will be copied to the target.
Currently, there are four different kinds:

(1) ``full`` -- Synchronize the content of entire disk image chain to
    the target

(2) ``top`` -- Synchronize only the contents of the top-most disk image
    in the chain to the target

(3) ``none`` -- Synchronize only the new writes from this point on.

    .. note:: In the case of ``drive-backup`` (or ``blockdev-backup``),
              the behavior of ``none`` synchronization mode is different.
              Normally, a ``backup`` job consists of two parts: Anything
              that is overwritten by the guest is first copied out to
              the backup, and in the background the whole image is
              copied from start to end. With ``sync=none``, it's only
              the first part.

(4) ``incremental`` -- Synchronize content that is described by the
    dirty bitmap

.. note::
    Refer to the :doc:`bitmaps` document in the QEMU source
    tree to learn about the detailed workings of the ``incremental``
    synchronization mode.


QMP invocation for ``drive-mirror``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To copy the contents of the entire disk image chain, from [A] all the
way to [D], to a new target (``drive-mirror`` will create the destination
file, if it doesn't already exist), call it [E]::

    (QEMU) drive-mirror device=node-D target=e.qcow2 sync=full job-id=job0
    {
        "execute": "drive-mirror",
        "arguments": {
            "device": "node-D",
            "job-id": "job0",
            "target": "e.qcow2",
            "sync": "full"
        }
    }

The ``"sync": "full"``, from the above, means: copy the *entire* chain
to the destination.

Following the above, querying for active block jobs will show that a
'mirror' job is "ready" to be completed (and QEMU will also emit an
event, ``BLOCK_JOB_READY``)::

    (QEMU) query-block-jobs
    {
        "execute": "query-block-jobs",
        "arguments": {}
    }
    {
        "return": [
            {
                "busy": false,
                "type": "mirror",
                "len": 21757952,
                "paused": false,
                "ready": true,
                "io-status": "ok",
                "offset": 21757952,
                "device": "job0",
                "speed": 0
            }
        ]
    }

And, as noted in the previous section, there are two possible actions
at this point:

(a) Create a point-in-time snapshot by ending the synchronization.  The
    point-in-time is at the time of *ending* the sync.  (The result of
    the following being: the target image, [E], will be populated with
    content from the entire chain, [A] to [D])::

        (QEMU) block-job-cancel device=job0
        {
            "execute": "block-job-cancel",
            "arguments": {
                "device": "job0"
            }
        }

(b) Or, complete the operation and pivot the live QEMU to the target
    copy::

        (QEMU) block-job-complete device=job0

In either of the above cases, if you once again run the
`query-block-jobs` command, there should not be any active block
operation.

Comparing 'commit' and 'mirror': In both then cases, the overlay images
can be discarded.  However, with 'commit', the *existing* base image
will be modified (by updating it with contents from overlays); while in
the case of 'mirror', a *new* target image is populated with the data
from the disk image chain.


QMP invocation for live storage migration with ``drive-mirror`` + NBD
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Live storage migration (without shared storage setup) is one of the most
common use-cases that takes advantage of the ``drive-mirror`` primitive
and QEMU's built-in Network Block Device (NBD) server.  Here's a quick
walk-through of this setup.

Given the disk image chain::

    [A] <-- [B] <-- [C] <-- [D]

Instead of copying content from the entire chain, synchronize *only* the
contents of the *top*-most disk image (i.e. the active layer), [D], to a
target, say, [TargetDisk].

.. important::
    The destination host must already have the contents of the backing
    chain, involving images [A], [B], and [C], visible via other means
    -- whether by ``cp``, ``rsync``, or by some storage array-specific
    command.)

Sometimes, this is also referred to as "shallow copy" -- because only
the "active layer", and not the rest of the image chain, is copied to
the destination.

.. note::
    In this example, for the sake of simplicity, we'll be using the same
    ``localhost`` as both source and destination.

As noted earlier, on the destination host the contents of the backing
chain -- from images [A] to [C] -- are already expected to exist in some
form (e.g. in a file called, ``Contents-of-A-B-C.qcow2``).  Now, on the
destination host, let's create a target overlay image (with the image
``Contents-of-A-B-C.qcow2`` as its backing file), to which the contents
of image [D] (from the source QEMU) will be mirrored to::

    $ qemu-img create -f qcow2 -b ./Contents-of-A-B-C.qcow2 \
        -F qcow2 ./target-disk.qcow2

And start the destination QEMU (we already have the source QEMU running
-- discussed in the section: `Interacting with a QEMU instance`_)
instance, with the following invocation.  (As noted earlier, for
simplicity's sake, the destination QEMU is started on the same host, but
it could be located elsewhere):

.. parsed-literal::

  $ |qemu_system| -display none -no-user-config -nodefaults \\
    -m 512 -blockdev \\
    node-name=node-TargetDisk,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./target-disk.qcow2 \\
    -device virtio-blk,drive=node-TargetDisk,id=virtio0 \\
    -S -monitor stdio -qmp unix:./qmp-sock2,server=on,wait=off \\
    -incoming tcp:localhost:6666

Given the disk image chain on source QEMU::

    [A] <-- [B] <-- [C] <-- [D]

On the destination host, it is expected that the contents of the chain
``[A] <-- [B] <-- [C]`` are *already* present, and therefore copy *only*
the content of image [D].

(1) [On *destination* QEMU] As part of the first step, start the
    built-in NBD server on a given host (local host, represented by
    ``::``)and port::

        (QEMU) nbd-server-start addr={"type":"inet","data":{"host":"::","port":"49153"}}
        {
            "execute": "nbd-server-start",
            "arguments": {
                "addr": {
                    "data": {
                        "host": "::",
                        "port": "49153"
                    },
                    "type": "inet"
                }
            }
        }

(2) [On *destination* QEMU] And export the destination disk image using
    QEMU's built-in NBD server::

        (QEMU) nbd-server-add device=node-TargetDisk writable=true
        {
            "execute": "nbd-server-add",
            "arguments": {
                "device": "node-TargetDisk"
            }
        }

(3) [On *source* QEMU] Then, invoke ``drive-mirror`` (NB: since we're
    running ``drive-mirror`` with ``mode=existing`` (meaning:
    synchronize to a pre-created file, therefore 'existing', file on the
    target host), with the synchronization mode as 'top' (``"sync:
    "top"``)::

        (QEMU) drive-mirror device=node-D target=nbd:localhost:49153:exportname=node-TargetDisk sync=top mode=existing job-id=job0
        {
            "execute": "drive-mirror",
            "arguments": {
                "device": "node-D",
                "mode": "existing",
                "job-id": "job0",
                "target": "nbd:localhost:49153:exportname=node-TargetDisk",
                "sync": "top"
            }
        }

(4) [On *source* QEMU] Once ``drive-mirror`` copies the entire data, and the
    event ``BLOCK_JOB_READY`` is emitted, issue ``block-job-cancel`` to
    gracefully end the synchronization, from source QEMU::

        (QEMU) block-job-cancel device=job0
        {
            "execute": "block-job-cancel",
            "arguments": {
                "device": "job0"
            }
        }

(5) [On *destination* QEMU] Then, stop the NBD server::

        (QEMU) nbd-server-stop
        {
            "execute": "nbd-server-stop",
            "arguments": {}
        }

(6) [On *destination* QEMU] Finally, resume the guest vCPUs by issuing the
    QMP command `cont`::

        (QEMU) cont
        {
            "execute": "cont",
            "arguments": {}
        }

.. note::
    Higher-level libraries (e.g. libvirt) automate the entire above
    process (although note that libvirt does not allow same-host
    migrations to localhost for other reasons).


Notes on ``blockdev-mirror``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``blockdev-mirror`` command is equivalent in core functionality to
``drive-mirror``, except that it operates at node-level in a BDS graph.

Also: for ``blockdev-mirror``, the 'target' image needs to be explicitly
created (using ``qemu-img``) and attach it to live QEMU via
``blockdev-add``, which assigns a name to the to-be created target node.

E.g. the sequence of actions to create a point-in-time backup of an
entire disk image chain, to a target, using ``blockdev-mirror`` would be:

(0) Create the QCOW2 overlays, to arrive at a backing chain of desired
    depth

(1) Create the target image (using ``qemu-img``), say, ``e.qcow2``

(2) Attach the above created file (``e.qcow2``), run-time, using
    ``blockdev-add`` to QEMU

(3) Perform ``blockdev-mirror`` (use ``"sync": "full"`` to copy the
    entire chain to the target).  And notice the event
    ``BLOCK_JOB_READY``

(4) Optionally, query for active block jobs, there should be a 'mirror'
    job ready to be completed

(5) Gracefully complete the 'mirror' block device job, and notice the
    the event ``BLOCK_JOB_COMPLETED``

(6) Shutdown the guest by issuing the QMP ``quit`` command so that
    caches are flushed

(7) Then, finally, compare the contents of the disk image chain, and
    the target copy with ``qemu-img compare``.  You should notice:
    "Images are identical"


QMP invocation for ``blockdev-mirror``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Given the disk image chain::

    [A] <-- [B] <-- [C] <-- [D]

To copy the contents of the entire disk image chain, from [A] all the
way to [D], to a new target, call it [E].  The following is the flow.

Create the overlay images, [B], [C], and [D]::

    (QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2
    (QEMU) blockdev-snapshot-sync node-name=node-B snapshot-file=c.qcow2 snapshot-node-name=node-C format=qcow2
    (QEMU) blockdev-snapshot-sync node-name=node-C snapshot-file=d.qcow2 snapshot-node-name=node-D format=qcow2

Create the target image, [E]::

    $ qemu-img create -f qcow2 e.qcow2 39M

Add the above created target image to QEMU, via ``blockdev-add``::

    (QEMU) blockdev-add driver=qcow2 node-name=node-E file={"driver":"file","filename":"e.qcow2"}
    {
        "execute": "blockdev-add",
        "arguments": {
            "node-name": "node-E",
            "driver": "qcow2",
            "file": {
                "driver": "file",
                "filename": "e.qcow2"
            }
        }
    }

Perform ``blockdev-mirror``, and notice the event ``BLOCK_JOB_READY``::

    (QEMU) blockdev-mirror device=node-B target=node-E sync=full job-id=job0
    {
        "execute": "blockdev-mirror",
        "arguments": {
            "device": "node-D",
            "job-id": "job0",
            "target": "node-E",
            "sync": "full"
        }
    }

Query for active block jobs, there should be a 'mirror' job ready::

    (QEMU) query-block-jobs
    {
        "execute": "query-block-jobs",
        "arguments": {}
    }
    {
        "return": [
            {
                "busy": false,
                "type": "mirror",
                "len": 21561344,
                "paused": false,
                "ready": true,
                "io-status": "ok",
                "offset": 21561344,
                "device": "job0",
                "speed": 0
            }
        ]
    }

Gracefully complete the block device job operation, and notice the
event ``BLOCK_JOB_COMPLETED``::

    (QEMU) block-job-complete device=job0
    {
        "execute": "block-job-complete",
        "arguments": {
            "device": "job0"
        }
    }
    {
        "return": {}
    }

Shutdown the guest, by issuing the ``quit`` QMP command::

    (QEMU) quit
    {
        "execute": "quit",
        "arguments": {}
    }


Live disk backup --- ``drive-backup`` and ``blockdev-backup``
-------------------------------------------------------------

The ``drive-backup`` (and its newer equivalent ``blockdev-backup``) allows
you to create a point-in-time snapshot.

In this case, the point-in-time is when you *start* the ``drive-backup``
(or its newer equivalent ``blockdev-backup``) command.


QMP invocation for ``drive-backup``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Yet again, starting afresh with our example disk image chain::

    [A] <-- [B] <-- [C] <-- [D]

To create a target image [E], with content populated from image [A] to
[D], from the above chain, the following is the syntax.  (If the target
image does not exist, ``drive-backup`` will create it)::

    (QEMU) drive-backup device=node-D sync=full target=e.qcow2 job-id=job0
    {
        "execute": "drive-backup",
        "arguments": {
            "device": "node-D",
            "job-id": "job0",
            "sync": "full",
            "target": "e.qcow2"
        }
    }

Once the above ``drive-backup`` has completed, a ``BLOCK_JOB_COMPLETED`` event
will be issued, indicating the live block device job operation has
completed, and no further action is required.


Notes on ``blockdev-backup``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``blockdev-backup`` command is equivalent in functionality to
``drive-backup``, except that it operates at node-level in a Block Driver
State (BDS) graph.

E.g. the sequence of actions to create a point-in-time backup
of an entire disk image chain, to a target, using ``blockdev-backup``
would be:

(0) Create the QCOW2 overlays, to arrive at a backing chain of desired
    depth

(1) Create the target image (using ``qemu-img``), say, ``e.qcow2``

(2) Attach the above created file (``e.qcow2``), run-time, using
    ``blockdev-add`` to QEMU

(3) Perform ``blockdev-backup`` (use ``"sync": "full"`` to copy the
    entire chain to the target).  And notice the event
    ``BLOCK_JOB_COMPLETED``

(4) Shutdown the guest, by issuing the QMP ``quit`` command, so that
    caches are flushed

(5) Then, finally, compare the contents of the disk image chain, and
    the target copy with ``qemu-img compare``.  You should notice:
    "Images are identical"

The following section shows an example QMP invocation for
``blockdev-backup``.

QMP invocation for ``blockdev-backup``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Given a disk image chain of depth 1 where image [B] is the active
overlay (live QEMU is writing to it)::

    [A] <-- [B]

The following is the procedure to copy the content from the entire chain
to a target image (say, [E]), which has the full content from [A] and
[B].

Create the overlay [B]::

    (QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2
    {
        "execute": "blockdev-snapshot-sync",
        "arguments": {
            "node-name": "node-A",
            "snapshot-file": "b.qcow2",
            "format": "qcow2",
            "snapshot-node-name": "node-B"
        }
    }


Create a target image that will contain the copy::

    $ qemu-img create -f qcow2 e.qcow2 39M

Then add it to QEMU via ``blockdev-add``::

    (QEMU) blockdev-add driver=qcow2 node-name=node-E file={"driver":"file","filename":"e.qcow2"}
    {
        "execute": "blockdev-add",
        "arguments": {
            "node-name": "node-E",
            "driver": "qcow2",
            "file": {
                "driver": "file",
                "filename": "e.qcow2"
            }
        }
    }

Then invoke ``blockdev-backup`` to copy the contents from the entire
image chain, consisting of images [A] and [B] to the target image
'e.qcow2'::

    (QEMU) blockdev-backup device=node-B target=node-E sync=full job-id=job0
    {
        "execute": "blockdev-backup",
        "arguments": {
            "device": "node-B",
            "job-id": "job0",
            "target": "node-E",
            "sync": "full"
        }
    }

Once the above 'backup' operation has completed, the event,
``BLOCK_JOB_COMPLETED`` will be emitted, signalling successful
completion.

Next, query for any active block device jobs (there should be none)::

    (QEMU) query-block-jobs
    {
        "execute": "query-block-jobs",
        "arguments": {}
    }

Shutdown the guest::

    (QEMU) quit
    {
            "execute": "quit",
                "arguments": {}
    }
            "return": {}
    }

.. note::
    The above step is really important; if forgotten, an error, "Failed
    to get shared "write" lock on e.qcow2", will be thrown when you do
    ``qemu-img compare`` to verify the integrity of the disk image
    with the backup content.


The end result will be the image 'e.qcow2' containing a
point-in-time backup of the disk image chain -- i.e. contents from
images [A] and [B] at the time the ``blockdev-backup`` command was
initiated.

One way to confirm the backup disk image contains the identical content
with the disk image chain is to compare the backup and the contents of
the chain, you should see "Images are identical".  (NB: this is assuming
QEMU was launched with ``-S`` option, which will not start the CPUs at
guest boot up)::

    $ qemu-img compare b.qcow2 e.qcow2
    Warning: Image size mismatch!
    Images are identical.

NOTE: The "Warning: Image size mismatch!" is expected, as we created the
target image (e.qcow2) with 39M size.