Live Block Device Operations — QEMU 5.1.0 (v5.1.0-11824-g8699890d91-dirty) documentation

# 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.

Contents

- Live Block Device Operations

  - Disk image backing chain notation

  - Brief overview of live block QMP primitives

  - Interacting with a QEMU instance

  - Example disk image chain

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

  - Live block streaming — `block-stream`

    - QMP invocation for `block-stream`

  - Live block commit — `block-commit`

    - QMP invocation for `block-commit`

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

    - QMP invocation for `drive-mirror`

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

    - Notes on `blockdev-mirror`

    - QMP invocation for `blockdev-mirror`

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

    - QMP invocation for `drive-backup`

    - Notes on `blockdev-backup`

    - QMP invocation for `blockdev-backup`

## 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:

   > 1. QEMU rewrites the backing chain to remove reference to the now-streamed and redundant backing file;

   > 2. the streamed file itself won’t be removed by QEMU, and must be explicitly discarded by the user;

   > 3. 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:

   > 1. QEMU rewrites the backing chain to remove reference to now-redundant overlay images that have been committed into a backing file;

   > 2. the committed file itself won’t be removed by QEMU – it ought to be manually removed;

   > 3. 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

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

    $ ./x86_64-softmmu/qemu-system-x86_64 -display none -no-user-config \
        -M q35 -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,nowait

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).

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]

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]

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:

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]

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 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, 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 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 Dirty Bitmaps and Incremental Backup 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:

1. 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"
           }
       }

2. 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):

    $ ./x86_64-softmmu/qemu-system-x86_64 -display none -no-user-config \
        -M q35 -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,nowait \
        -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.