# Dirty Bitmaps and Incremental Backup

Dirty Bitmaps are in-memory objects that track writes to block devices. They can be used in conjunction with various block job operations to perform incremental or differential backup regimens.

This document explains the conceptual mechanisms, as well as up-to-date, complete and comprehensive documentation on the API to manipulate them. (Hopefully, the “why”, “what”, and “how”.)

The intended audience for this document is developers who are adding QEMU backup features to management applications, or power users who run and administer QEMU directly via QMP.

Contents

- Dirty Bitmaps and Incremental Backup

  - Overview

  - Supported Image Formats

  - Dirty Bitmap Names

  - Bitmap Status

  - Basic QMP Usage

    - Supported Commands

    - Creation: block-dirty-bitmap-add

    - Deletion: block-dirty-bitmap-remove

    - Resetting: block-dirty-bitmap-clear

    - Enabling: block-dirty-bitmap-enable

    - Enabling: block-dirty-bitmap-disable

    - Merging, Copying: block-dirty-bitmap-merge

    - Querying: query-block

  - Bitmap Persistence

  - Transactions

    - Justification

    - Supported Bitmap Transactions

  - Incremental Backups - Push Model

    - Example: New Incremental Backup Anchor Point

    - Example: Resetting an Incremental Backup Anchor Point

    - Example: First Incremental Backup

    - Example: Second Incremental Backup

    - Example: Incremental Push Backups without Backing Files

    - Example: Multi-drive Incremental Backup

  - Push Backup Errors & Recovery

    - Example: Individual Failures

    - Example: Partial Transactional Failures

    - Example: Grouped Completion Mode

## Overview

Bitmaps are bit vectors where each ‘1’ bit in the vector indicates a modified (“dirty”) segment of the corresponding block device. The size of the segment that is tracked is the granularity of the bitmap. If the granularity of a bitmap is 64K, each ‘1’ bit means that a 64K region as a whole may have changed in some way, possibly by as little as one byte.

Smaller granularities mean more accurate tracking of modified disk data, but requires more computational overhead and larger bitmap sizes. Larger granularities mean smaller bitmap sizes, but less targeted backups.

The size of a bitmap (in bytes) can be computed as such:

  `size` = ceil(ceil(`image_size` / `granularity`) / 8)

e.g. the size of a 64KiB granularity bitmap on a 2TiB image is:
  `size` = ((2147483648K / 64K) / 8)

    = 4194304B = 4MiB.

QEMU uses these bitmaps when making incremental backups to know which sections of the file to copy out. They are not enabled by default and must be explicitly added in order to begin tracking writes.

Bitmaps can be created at any time and can be attached to any arbitrary block node in the storage graph, but are most useful conceptually when attached to the root node attached to the guest’s storage device model.

That is to say: It’s likely most useful to track the guest’s writes to disk, but you could theoretically track things like qcow2 metadata changes by attaching the bitmap elsewhere in the storage graph. This is beyond the scope of this document.

QEMU supports persisting these bitmaps to disk via the qcow2 image format. Bitmaps which are stored or loaded in this way are called “persistent”, whereas bitmaps that are not are called “transient”.

QEMU also supports the migration of both transient bitmaps (tracking any arbitrary image format) or persistent bitmaps (qcow2) via live migration.

## Supported Image Formats

QEMU supports all documented features below on the qcow2 image format.

However, qcow2 is only strictly necessary for the persistence feature, which writes bitmap data to disk upon close. If persistence is not required for a specific use case, all bitmap features excepting persistence are available for any arbitrary image format.

For example, Dirty Bitmaps can be combined with the ‘raw’ image format, but any changes to the bitmap will be discarded upon exit.

Warning

Transient bitmaps will not be saved on QEMU exit! Persistent bitmaps are available only on qcow2 images.

## Dirty Bitmap Names

Bitmap objects need a method to reference them in the API. All API-created and managed bitmaps have a human-readable name chosen by the user at creation time.

- A bitmap’s name is unique to the node, but bitmaps attached to different nodes can share the same name. Therefore, all bitmaps are addressed via their (node, name) pair.

- The name of a user-created bitmap cannot be empty (“”).

- Transient bitmaps can have JSON unicode names that are effectively not length limited. (QMP protocol may restrict messages to less than 64MiB.)

- Persistent storage formats may impose their own requirements on bitmap names and namespaces. Presently, only qcow2 supports persistent bitmaps. See docs/interop/qcow2.txt for more details on restrictions. Notably:

  > - qcow2 bitmap names are limited to between 1 and 1023 bytes long.

  > - No two bitmaps saved to the same qcow2 file may share the same name.

- QEMU occasionally uses bitmaps for internal use which have no name. They are hidden from API query calls, cannot be manipulated by the external API, are never persistent, nor ever migrated.

## Bitmap Status

Dirty Bitmap objects can be queried with the QMP command query-block, and are visible via the BlockDirtyInfo QAPI structure.

This struct shows the name, granularity, and dirty byte count for each bitmap. Additionally, it shows several boolean status indicators:

- `recording`: This bitmap is recording writes.

- `busy`: This bitmap is in-use by an operation.

- `persistent`: This bitmap is a persistent type.

- `inconsistent`: This bitmap is corrupted and cannot be used.

The `+busy` status prohibits you from deleting, clearing, or otherwise modifying a bitmap, and happens when the bitmap is being used for a backup operation or is in the process of being loaded from a migration. Many of the commands documented below will refuse to work on such bitmaps.

The `+inconsistent` status similarly prohibits almost all operations, notably allowing only the `block-dirty-bitmap-remove` operation.

There is also a deprecated `status` field of type DirtyBitmapStatus. A bitmap historically had five visible states:

> 1. `Frozen`: This bitmap is currently in-use by an operation and is immutable. It can’t be deleted, renamed, reset, etc.

>    (This is now `+busy`.)

> 2. `Disabled`: This bitmap is not recording new writes.

>    (This is now `-recording -busy`.)

> 3. `Active`: This bitmap is recording new writes.

>    (This is now `+recording -busy`.)

> 4. `Locked`: This bitmap is in-use by an operation, and is immutable. The difference from “Frozen” was primarily implementation details.

>    (This is now `+busy`.)

> 5. `Inconsistent`: This persistent bitmap was not saved to disk correctly, and can no longer be used. It remains in memory to serve as an indicator of failure.

>    (This is now `+inconsistent`.)

These states are directly replaced by the status indicators and should not be used. The difference between `Frozen` and `Locked` is an implementation detail and should not be relevant to external users.

## Basic QMP Usage

The primary interface to manipulating bitmap objects is via the QMP interface. If you are not familiar, see docs/interop/qmp-intro.txt for a broad overview, and qemu-qmp-ref for a full reference of all QMP commands.

### Supported Commands

There are six primary bitmap-management API commands:

- `block-dirty-bitmap-add`

- `block-dirty-bitmap-remove`

- `block-dirty-bitmap-clear`

- `block-dirty-bitmap-disable`

- `block-dirty-bitmap-enable`

- `block-dirty-bitmap-merge`

And one related query command:

- `query-block`

### Creation: block-dirty-bitmap-add

block-dirty-bitmap-add:

Creates a new bitmap that tracks writes to the specified node. granularity, persistence, and recording state can be adjusted at creation time.

Example

to create a new, actively recording persistent bitmap:

    -> { "execute": "block-dirty-bitmap-add",
         "arguments": {
           "node": "drive0",
           "name": "bitmap0",
           "persistent": true,
         }
       }

    <- { "return": {} }

- This bitmap will have a default granularity that matches the cluster size of its associated drive, if available, clamped to between [4KiB, 64KiB]. The current default for qcow2 is 64KiB.

Example

To create a new, disabled (`-recording`), transient bitmap that tracks changes in 32KiB segments:

    -> { "execute": "block-dirty-bitmap-add",
         "arguments": {
           "node": "drive0",
           "name": "bitmap1",
           "granularity": 32768,
           "disabled": true
         }
       }

    <- { "return": {} }

### Deletion: block-dirty-bitmap-remove

block-dirty-bitmap-remove:

Deletes a bitmap. Bitmaps that are `+busy` cannot be removed.

- Deleting a bitmap does not impact any other bitmaps attached to the same node, nor does it affect any backups already created from this bitmap or node.

- Because bitmaps are only unique to the node to which they are attached, you must specify the node/drive name here, too.

- Deleting a persistent bitmap will remove it from the qcow2 file.

Example

Remove a bitmap named `bitmap0` from node `drive0`:

    -> { "execute": "block-dirty-bitmap-remove",
         "arguments": {
           "node": "drive0",
           "name": "bitmap0"
         }
       }

    <- { "return": {} }

### Resetting: block-dirty-bitmap-clear

block-dirty-bitmap-clear:

Clears all dirty bits from a bitmap. `+busy` bitmaps cannot be cleared.

- An incremental backup created from an empty bitmap will copy no data, as if nothing has changed.

Example

Clear all dirty bits from bitmap `bitmap0` on node `drive0`:

    -> { "execute": "block-dirty-bitmap-clear",
         "arguments": {
           "node": "drive0",
           "name": "bitmap0"
         }
       }

    <- { "return": {} }

### Enabling: block-dirty-bitmap-enable

block-dirty-bitmap-enable:

“Enables” a bitmap, setting the `recording` bit to true, causing writes to begin being recorded. `+busy` bitmaps cannot be enabled.

- Bitmaps default to being enabled when created, unless configured otherwise.

- Persistent enabled bitmaps will remember their `+recording` status on load.

Example

To set `+recording` on bitmap `bitmap0` on node `drive0`:

    -> { "execute": "block-dirty-bitmap-enable",
         "arguments": {
           "node": "drive0",
           "name": "bitmap0"
         }
       }

    <- { "return": {} }

### Enabling: block-dirty-bitmap-disable

block-dirty-bitmap-disable:

“Disables” a bitmap, setting the `recording` bit to false, causing further writes to begin being ignored. `+busy` bitmaps cannot be disabled.

Warning

This is potentially dangerous: QEMU makes no effort to stop any writes if there are disabled bitmaps on a node, and will not mark any disabled bitmaps as `+inconsistent` if any such writes do happen. Backups made from such bitmaps will not be able to be used to reconstruct a coherent image.

- Disabling a bitmap may be useful for examining which sectors of a disk changed during a specific time period, or for explicit management of differential backup windows.

- Persistent disabled bitmaps will remember their `-recording` status on load.

Example

To set `-recording` on bitmap `bitmap0` on node `drive0`:

    -> { "execute": "block-dirty-bitmap-disable",
         "arguments": {
           "node": "drive0",
           "name": "bitmap0"
         }
       }

    <- { "return": {} }

### Merging, Copying: block-dirty-bitmap-merge

block-dirty-bitmap-merge:

Merges one or more bitmaps into a target bitmap. For any segment that is dirty in any one source bitmap, the target bitmap will mark that segment dirty.

- Merge takes one or more bitmaps as a source and merges them together into a single destination, such that any segment marked as dirty in any source bitmap(s) will be marked dirty in the destination bitmap.

- Merge does not create the destination bitmap if it does not exist. A blank bitmap can be created beforehand to achieve the same effect.

- The destination is not cleared prior to merge, so subsequent merge operations will continue to cumulatively mark more segments as dirty.

- If the merge operation should fail, the destination bitmap is guaranteed to be unmodified. The operation may fail if the source or destination bitmaps are busy, or have different granularities.

- Bitmaps can only be merged on the same node. There is only one “node” argument, so all bitmaps must be attached to that same node.

- Copy can be achieved by merging from a single source to an empty destination.

Example

Merge the data from `bitmap0` into the bitmap `new_bitmap` on node `drive0`. If `new_bitmap` was empty prior to this command, this achieves a copy.

    -> { "execute": "block-dirty-bitmap-merge",
         "arguments": {
           "node": "drive0",
           "target": "new_bitmap",
           "bitmaps": [ "bitmap0" ]
         }
       }

    <- { "return": {} }

### Querying: query-block

query-block:

Not strictly a bitmaps command, but will return information about any bitmaps attached to nodes serving as the root for guest devices.

- The “inconsistent” bit will not appear when it is false, appearing only when the value is true to indicate there is a problem.

Example

Query the block sub-system of QEMU. The following json has trimmed irrelevant keys from the response to highlight only the bitmap-relevant portions of the API. This result highlights a bitmap `bitmap0` attached to the root node of device `drive0`.

    -> {
         "execute": "query-block",
         "arguments": {}
       }

    <- {
         "return": [ {
           "dirty-bitmaps": [ {
             "status": "active",
             "count": 0,
             "busy": false,
             "name": "bitmap0",
             "persistent": false,
             "recording": true,
             "granularity": 65536
           } ],
           "device": "drive0",
         } ]
       }

## Bitmap Persistence

As outlined in Supported Image Formats, QEMU can persist bitmaps to qcow2 files. Demonstrated in Creation: block-dirty-bitmap-add, passing `persistent: true` to `block-dirty-bitmap-add` will persist that bitmap to disk.

Persistent bitmaps will be automatically loaded into memory upon load, and will be written back to disk upon close. Their usage should be mostly transparent.

However, if QEMU does not get a chance to close the file cleanly, the bitmap will be marked as `+inconsistent` at next load and considered unsafe to use for any operation. At this point, the only valid operation on such bitmaps is `block-dirty-bitmap-remove`.

Losing a bitmap in this way does not invalidate any existing backups that have been made from this bitmap, but no further backups will be able to be issued for this chain.

## Transactions

Transactions are a QMP feature that allows you to submit multiple QMP commands at once, being guaranteed that they will all succeed or fail atomically, together. The interaction of bitmaps and transactions are demonstrated below.

See transaction in the QMP reference for more details.

### Justification

Bitmaps can generally be modified at any time, but certain operations often only make sense when paired directly with other commands. When a VM is paused, it’s easy to ensure that no guest writes occur between individual QMP commands. When a VM is running, this is difficult to accomplish with individual QMP commands that may allow guest writes to occur between each command.

For example, using only individual QMP commands, we could:

1. Boot the VM in a paused state.

2. Create a full drive backup of drive0.

3. Create a new bitmap attached to drive0, confident that nothing has been written to drive0 in the meantime.

4. Resume execution of the VM.

5. At a later point, issue incremental backups from `bitmap0`.

At this point, the bitmap and drive backup would be correctly in sync, and incremental backups made from this point forward would be correctly aligned to the full drive backup.

This is not particularly useful if we decide we want to start incremental backups after the VM has been running for a while, for which we would want to perform actions such as the following:

1. Boot the VM and begin execution.

2. Using a single transaction, perform the following operations:

   - Create `bitmap0`.

   - Create a full drive backup of `drive0`.

3. At a later point, issue incremental backups from `bitmap0`.

Note

As a consideration, if `bitmap0` is created prior to the full drive backup, incremental backups can still be authored from this bitmap, but they will copy extra segments reflecting writes that occurred prior to the backup operation. Transactions allow us to narrow critical points in time to reduce waste, or, in the other direction, to ensure that no segments are omitted.

### Supported Bitmap Transactions

- `block-dirty-bitmap-add`

- `block-dirty-bitmap-clear`

- `block-dirty-bitmap-enable`

- `block-dirty-bitmap-disable`

- `block-dirty-bitmap-merge`

The usages for these commands are identical to their respective QMP commands, but see the sections below for concrete examples.

## Incremental Backups - Push Model

Incremental backups are simply partial disk images that can be combined with other partial disk images on top of a base image to reconstruct a full backup from the point in time at which the incremental backup was issued.

The “Push Model” here references the fact that QEMU is “pushing” the modified blocks out to a destination. We will be using the drive-backup and blockdev-backup QMP commands to create both full and incremental backups.

Both of these commands are jobs, which have their own QMP API for querying and management documented in Background jobs.

### Example: New Incremental Backup Anchor Point

As outlined in the Transactions - Justification section, perhaps we want to create a new incremental backup chain attached to a drive.

This example creates a new, full backup of “drive0” and accompanies it with a new, empty bitmap that records writes from this point in time forward.

Note

Any new writes that happen after this command is issued, even while the backup job runs, will be written locally and not to the backup destination. These writes will be recorded in the bitmap accordingly.

    -> {
         "execute": "transaction",
         "arguments": {
           "actions": [
             {
               "type": "block-dirty-bitmap-add",
               "data": {
                 "node": "drive0",
                 "name": "bitmap0"
               }
             },
             {
               "type": "drive-backup",
               "data": {
                 "device": "drive0",
                 "target": "/path/to/drive0.full.qcow2",
                 "sync": "full",
                 "format": "qcow2"
               }
             }
           ]
         }
       }

    <- { "return": {} }

    <- {
         "timestamp": {
           "seconds": 1555436945,
           "microseconds": 179620
         },
         "data": {
           "status": "created",
           "id": "drive0"
         },
         "event": "JOB_STATUS_CHANGE"
       }

    ...

    <- {
         "timestamp": {...},
         "data": {
           "device": "drive0",
           "type": "backup",
           "speed": 0,
           "len": 68719476736,
           "offset": 68719476736
         },
         "event": "BLOCK_JOB_COMPLETED"
       }

    <- {
         "timestamp": {...},
         "data": {
           "status": "concluded",
           "id": "drive0"
         },
         "event": "JOB_STATUS_CHANGE"
       }

    <- {
         "timestamp": {...},
         "data": {
           "status": "null",
           "id": "drive0"
         },
         "event": "JOB_STATUS_CHANGE"
       }

A full explanation of the job transition semantics and the JOB_STATUS_CHANGE event are beyond the scope of this document and will be omitted in all subsequent examples; above, several more events have been omitted for brevity.

Note

Subsequent examples will omit all events except BLOCK_JOB_COMPLETED except where necessary to illustrate workflow differences.

Omitted events and json objects will be represented by ellipses: `...`

### Example: Resetting an Incremental Backup Anchor Point

If we want to start a new backup chain with an existing bitmap, we can also use a transaction to reset the bitmap while making a new full backup:

    -> {
         "execute": "transaction",
         "arguments": {
           "actions": [
           {
             "type": "block-dirty-bitmap-clear",
             "data": {
               "node": "drive0",
               "name": "bitmap0"
             }
           },
           {
             "type": "drive-backup",
             "data": {
               "device": "drive0",
               "target": "/path/to/drive0.new_full.qcow2",
               "sync": "full",
               "format": "qcow2"
             }
           }
         ]
       }
     }

    <- { "return": {} }

    ...

    <- {
         "timestamp": {...},
         "data": {
           "device": "drive0",
           "type": "backup",
           "speed": 0,
           "len": 68719476736,
           "offset": 68719476736
         },
         "event": "BLOCK_JOB_COMPLETED"
       }

    ...

The result of this example is identical to the first, but we clear an existing bitmap instead of adding a new one.

Tip

In both of these examples, “bitmap0” is tied conceptually to the creation of new, full backups. This relationship is not saved or remembered by QEMU; it is up to the operator or management layer to remember which bitmaps are associated with which backups.

### Example: First Incremental Backup

1. Create a full backup and sync it to a dirty bitmap using any method:

   - Either of the two live backup method demonstrated above,

   - Using QMP commands with the VM paused as in the Justification section, or

   - With the VM offline, manually copy the image and start the VM in a paused state, careful to add a new bitmap before the VM begins execution.

   Whichever method is chosen, let’s assume that at the end of this step:

   - The full backup is named `drive0.full.qcow2`.

   - The bitmap we created is named `bitmap0`, attached to `drive0`.

2. Create a destination image for the incremental backup that utilizes the full backup as a backing image.

   - Let’s assume the new incremental image is named `drive0.inc0.qcow2`:

       $ qemu-img create -f qcow2 drive0.inc0.qcow2 \
         -b drive0.full.qcow2 -F qcow2

3. Issue an incremental backup command:

       -> {
            "execute": "drive-backup",
            "arguments": {
              "device": "drive0",
              "bitmap": "bitmap0",
              "target": "drive0.inc0.qcow2",
              "format": "qcow2",
              "sync": "incremental",
              "mode": "existing"
            }
          }

       <- { "return": {} }

       ...

       <- {
            "timestamp": {...},
            "data": {
              "device": "drive0",
              "type": "backup",
              "speed": 0,
              "len": 68719476736,
              "offset": 68719476736
            },
            "event": "BLOCK_JOB_COMPLETED"
          }

       ...

This copies any blocks modified since the full backup was created into the `drive0.inc0.qcow2` file. During the operation, `bitmap0` is marked `+busy`. If the operation is successful, `bitmap0` will be cleared to reflect the “incremental” backup regimen, which only copies out new changes from each incremental backup.

Note

Any new writes that occur after the backup operation starts do not get copied to the destination. The backup’s “point in time” is when the backup starts, not when it ends. These writes are recorded in a special bitmap that gets re-added to bitmap0 when the backup ends so that the next incremental backup can copy them out.

### Example: Second Incremental Backup

1. Create a new destination image for the incremental backup that points to the previous one, e.g.: `drive0.inc1.qcow2`

       $ qemu-img create -f qcow2 drive0.inc1.qcow2 \
         -b drive0.inc0.qcow2 -F qcow2

2. Issue a new incremental backup command. The only difference here is that we have changed the target image below.

       -> {
            "execute": "drive-backup",
            "arguments": {
              "device": "drive0",
              "bitmap": "bitmap0",
              "target": "drive0.inc1.qcow2",
              "format": "qcow2",
              "sync": "incremental",
              "mode": "existing"
            }
          }

       <- { "return": {} }

       ...

       <- {
            "timestamp": {...},
            "data": {
              "device": "drive0",
              "type": "backup",
              "speed": 0,
              "len": 68719476736,
              "offset": 68719476736
            },
            "event": "BLOCK_JOB_COMPLETED"
          }

       ...

Because the first incremental backup from the previous example completed successfully, `bitmap0` was synchronized with `drive0.inc0.qcow2`. Here, we use `bitmap0` again to create a new incremental backup that targets the previous one, creating a chain of three images:

Diagram

    +-------------------+   +-------------------+   +-------------------+
    | drive0.full.qcow2 |<--| drive0.inc0.qcow2 |<--| drive0.inc1.qcow2 |
    +-------------------+   +-------------------+   +-------------------+

Each new incremental backup re-synchronizes the bitmap to the latest backup authored, allowing a user to continue to “consume” it to create new backups on top of an existing chain.

In the above diagram, neither drive0.inc1.qcow2 nor drive0.inc0.qcow2 are complete images by themselves, but rely on their backing chain to reconstruct a full image. The dependency terminates with each full backup.

Each backup in this chain remains independent, and is unchanged by new entries made later in the chain. For instance, drive0.inc0.qcow2 remains a perfectly valid backup of the disk as it was when that backup was issued.

### Example: Incremental Push Backups without Backing Files

Backup images are best kept off-site, so we often will not have the preceding backups in a chain available to link against. This is not a problem at backup time; we simply do not set the backing image when creating the destination image:

1. Create a new destination image with no backing file set. We will need to specify the size of the base image, because the backing file isn’t available for QEMU to use to determine it.

       $ qemu-img create -f qcow2 drive0.inc2.qcow2 64G

   Note

   Alternatively, you can omit `mode: "existing"` from the push backup commands to have QEMU create an image without a backing file for you, but you lose control over format options like compatibility and preallocation presets.

2. Issue a new incremental backup command. Apart from the new destination image, there is no difference from the last two examples.

       -> {
            "execute": "drive-backup",
            "arguments": {
              "device": "drive0",
              "bitmap": "bitmap0",
              "target": "drive0.inc2.qcow2",
              "format": "qcow2",
              "sync": "incremental",
              "mode": "existing"
            }
          }

       <- { "return": {} }

       ...

       <- {
            "timestamp": {...},
            "data": {
              "device": "drive0",
              "type": "backup",
              "speed": 0,
              "len": 68719476736,
              "offset": 68719476736
            },
            "event": "BLOCK_JOB_COMPLETED"
          }

       ...

The only difference from the perspective of the user is that you will need to set the backing image when attempting to restore the backup:

    $ qemu-img rebase drive0.inc2.qcow2 \
      -u -b drive0.inc1.qcow2

This uses the “unsafe” rebase mode to simply set the backing file to a file that isn’t present.

It is also possible to use `--image-opts` to specify the entire backing chain by hand as an ephemeral property at runtime, but that is beyond the scope of this document.

### Example: Multi-drive Incremental Backup

Assume we have a VM with two drives, “drive0” and “drive1” and we wish to back both of them up such that the two backups represent the same crash-consistent point in time.

1. For each drive, create an empty image:

       $ qemu-img create -f qcow2 drive0.full.qcow2 64G
       $ qemu-img create -f qcow2 drive1.full.qcow2 64G

2. Create a full (anchor) backup for each drive, with accompanying bitmaps:

       -> {
            "execute": "transaction",
            "arguments": {
              "actions": [
                {
                  "type": "block-dirty-bitmap-add",
                  "data": {
                    "node": "drive0",
                    "name": "bitmap0"
                  }
                },
                {
                  "type": "block-dirty-bitmap-add",
                  "data": {
                    "node": "drive1",
                    "name": "bitmap0"
                  }
                },
                {
                  "type": "drive-backup",
                  "data": {
                    "device": "drive0",
                    "target": "/path/to/drive0.full.qcow2",
                    "sync": "full",
                    "format": "qcow2"
                  }
                },
                {
                  "type": "drive-backup",
                  "data": {
                    "device": "drive1",
                    "target": "/path/to/drive1.full.qcow2",
                    "sync": "full",
                    "format": "qcow2"
                  }
                }
              ]
            }
          }

       <- { "return": {} }

       ...

       <- {
            "timestamp": {...},
            "data": {
              "device": "drive0",
              "type": "backup",
              "speed": 0,
              "len": 68719476736,
              "offset": 68719476736
            },
            "event": "BLOCK_JOB_COMPLETED"
          }

       ...

       <- {
            "timestamp": {...},
            "data": {
              "device": "drive1",
              "type": "backup",
              "speed": 0,
              "len": 68719476736,
              "offset": 68719476736
            },
            "event": "BLOCK_JOB_COMPLETED"
          }

       ...

3. Later, create new destination images for each of the incremental backups that point to their respective full backups:

       $ qemu-img create -f qcow2 drive0.inc0.qcow2 \
         -b drive0.full.qcow2 -F qcow2
       $ qemu-img create -f qcow2 drive1.inc0.qcow2 \
         -b drive1.full.qcow2 -F qcow2

4. Issue a multi-drive incremental push backup transaction:

       -> {
            "execute": "transaction",
            "arguments": {
              "actions": [
                {
                  "type": "drive-backup",
                  "data": {
                    "device": "drive0",
                    "bitmap": "bitmap0",
                    "format": "qcow2",
                    "mode": "existing",
                    "sync": "incremental",
                    "target": "drive0.inc0.qcow2"
                  }
                },
                {
                  "type": "drive-backup",
                  "data": {
                    "device": "drive1",
                    "bitmap": "bitmap0",
                    "format": "qcow2",
                    "mode": "existing",
                    "sync": "incremental",
                    "target": "drive1.inc0.qcow2"
                  }
                },
              ]
            }
          }

       <- { "return": {} }

       ...

       <- {
            "timestamp": {...},
            "data": {
              "device": "drive0",
              "type": "backup",
              "speed": 0,
              "len": 68719476736,
              "offset": 68719476736
            },
            "event": "BLOCK_JOB_COMPLETED"
          }

       ...

       <- {
            "timestamp": {...},
            "data": {
              "device": "drive1",
              "type": "backup",
              "speed": 0,
              "len": 68719476736,
              "offset": 68719476736
            },
            "event": "BLOCK_JOB_COMPLETED"
          }

       ...

## Push Backup Errors & Recovery

In the event of an error that occurs after a push backup job is successfully launched, either by an individual QMP command or a QMP transaction, the user will receive a `BLOCK_JOB_COMPLETE` event with a failure message, accompanied by a `BLOCK_JOB_ERROR` event.

In the case of a job being cancelled, the user will receive a `BLOCK_JOB_CANCELLED` event instead of a pair of COMPLETE and ERROR events.

In either failure case, the bitmap used for the failed operation is not cleared. It will contain all of the dirty bits it did at the start of the operation, plus any new bits that got marked during the operation.

Effectively, the “point in time” that a bitmap is recording differences against is kept at the issuance of the last successful incremental backup, instead of being moved forward to the start of this now-failed backup.

Once the underlying problem is addressed (e.g. more storage space is allocated on the destination), the incremental backup command can be retried with the same bitmap.

### Example: Individual Failures

Incremental Push Backup jobs that fail individually behave simply as described above. This example demonstrates the single-job failure case:

1. Create a target image:

       $ qemu-img create -f qcow2 drive0.inc0.qcow2 \
         -b drive0.full.qcow2 -F qcow2

2. Attempt to create an incremental backup via QMP:

       -> {
            "execute": "drive-backup",
            "arguments": {
              "device": "drive0",
              "bitmap": "bitmap0",
              "target": "drive0.inc0.qcow2",
              "format": "qcow2",
              "sync": "incremental",
              "mode": "existing"
            }
          }

       <- { "return": {} }

3. Receive a pair of events indicating failure:

       <- {
            "timestamp": {...},
            "data": {
              "device": "drive0",
              "action": "report",
              "operation": "write"
            },
            "event": "BLOCK_JOB_ERROR"
          }

       <- {
            "timestamp": {...},
            "data": {
              "speed": 0,
              "offset": 0,
              "len": 67108864,
              "error": "No space left on device",
              "device": "drive0",
              "type": "backup"
            },
            "event": "BLOCK_JOB_COMPLETED"
          }

4. Delete the failed image, and re-create it.

       $ rm drive0.inc0.qcow2
       $ qemu-img create -f qcow2 drive0.inc0.qcow2 \
         -b drive0.full.qcow2 -F qcow2

5. Retry the command after fixing the underlying problem, such as freeing up space on the backup volume:

       -> {
            "execute": "drive-backup",
            "arguments": {
              "device": "drive0",
              "bitmap": "bitmap0",
              "target": "drive0.inc0.qcow2",
              "format": "qcow2",
              "sync": "incremental",
              "mode": "existing"
            }
          }

       <- { "return": {} }

6. Receive confirmation that the job completed successfully:

       <- {
            "timestamp": {...},
            "data": {
              "device": "drive0",
              "type": "backup",
              "speed": 0,
              "len": 67108864,
              "offset": 67108864
            },
            "event": "BLOCK_JOB_COMPLETED"
          }

### Example: Partial Transactional Failures

QMP commands like drive-backup conceptually only start a job, and so transactions containing these commands may succeed even if the job it created later fails. This might have surprising interactions with notions of how a “transaction” ought to behave.

This distinction means that on occasion, a transaction containing such job launching commands may appear to succeed and return success, but later individual jobs associated with the transaction may fail. It is possible that a management application may have to deal with a partial backup failure after a “successful” transaction.

If multiple backup jobs are specified in a single transaction, if one of those jobs fails, it will not interact with the other backup jobs in any way by default. The job(s) that succeeded will clear the dirty bitmap associated with the operation, but the job(s) that failed will not. It is therefore not safe to delete any incremental backups that were created successfully in this scenario, even though others failed.

This example illustrates a transaction with two backup jobs, where one fails and one succeeds:

1. Issue the transaction to start a backup of both drives.

       -> {
            "execute": "transaction",
            "arguments": {
              "actions": [
              {
                "type": "drive-backup",
                "data": {
                  "device": "drive0",
                  "bitmap": "bitmap0",
                  "format": "qcow2",
                  "mode": "existing",
                  "sync": "incremental",
                  "target": "drive0.inc0.qcow2"
                }
              },
              {
                "type": "drive-backup",
                "data": {
                  "device": "drive1",
                  "bitmap": "bitmap0",
                  "format": "qcow2",
                  "mode": "existing",
                  "sync": "incremental",
                  "target": "drive1.inc0.qcow2"
                }
              }]
            }
          }

2. Receive notice that the Transaction was accepted, and jobs were launched:

       <- { "return": {} }

3. Receive notice that the first job has completed:

       <- {
            "timestamp": {...},
            "data": {
              "device": "drive0",
              "type": "backup",
              "speed": 0,
              "len": 67108864,
              "offset": 67108864
            },
            "event": "BLOCK_JOB_COMPLETED"
          }

4. Receive notice that the second job has failed:

       <- {
            "timestamp": {...},
            "data": {
              "device": "drive1",
              "action": "report",
              "operation": "read"
            },
            "event": "BLOCK_JOB_ERROR"
          }

       ...

       <- {
            "timestamp": {...},
            "data": {
              "speed": 0,
              "offset": 0,
              "len": 67108864,
              "error": "Input/output error",
              "device": "drive1",
              "type": "backup"
            },
            "event": "BLOCK_JOB_COMPLETED"
          }

At the conclusion of the above example, `drive0.inc0.qcow2` is valid and must be kept, but `drive1.inc0.qcow2` is incomplete and should be deleted. If a VM-wide incremental backup of all drives at a point-in-time is to be made, new backups for both drives will need to be made, taking into account that a new incremental backup for drive0 needs to be based on top of `drive0.inc0.qcow2`.

For this example, an incremental backup for `drive0` was created, but not for `drive1`. The last VM-wide crash-consistent backup that is available in this case is the full backup:

    [drive0.full.qcow2] <-- [drive0.inc0.qcow2]
    [drive1.full.qcow2]

To repair this, issue a new incremental backup across both drives. The result will be backup chains that resemble the following:

    [drive0.full.qcow2] <-- [drive0.inc0.qcow2] <-- [drive0.inc1.qcow2]
    [drive1.full.qcow2] <-------------------------- [drive1.inc1.qcow2]

### Example: Grouped Completion Mode

While jobs launched by transactions normally complete or fail individually, it’s possible to instruct them to complete or fail together as a group. QMP transactions take an optional properties structure that can affect the behavior of the transaction.

The `completion-mode` transaction property can be either `individual` which is the default legacy behavior described above, or `grouped`, detailed below.

In `grouped` completion mode, no jobs will report success until all jobs are ready to report success. If any job fails, all other jobs will be cancelled.

Regardless of if a participating incremental backup job failed or was cancelled, their associated bitmaps will all be held at their existing points-in-time, as in individual failure cases.

Here’s the same multi-drive backup scenario from Example: Partial Transactional Failures, but with the `grouped` completion-mode property applied:

1. Issue the multi-drive incremental backup transaction:

       -> {
            "execute": "transaction",
            "arguments": {
              "properties": {
                "completion-mode": "grouped"
              },
              "actions": [
              {
                "type": "drive-backup",
                "data": {
                  "device": "drive0",
                  "bitmap": "bitmap0",
                  "format": "qcow2",
                  "mode": "existing",
                  "sync": "incremental",
                  "target": "drive0.inc0.qcow2"
                }
              },
              {
                "type": "drive-backup",
                "data": {
                  "device": "drive1",
                  "bitmap": "bitmap0",
                  "format": "qcow2",
                  "mode": "existing",
                  "sync": "incremental",
                  "target": "drive1.inc0.qcow2"
                }
              }]
            }
          }

2. Receive notice that the Transaction was accepted, and jobs were launched:

       <- { "return": {} }

3. Receive notification that the backup job for `drive1` has failed:

       <- {
            "timestamp": {...},
            "data": {
              "device": "drive1",
              "action": "report",
              "operation": "read"
            },
            "event": "BLOCK_JOB_ERROR"
          }

       <- {
            "timestamp": {...},
            "data": {
              "speed": 0,
              "offset": 0,
              "len": 67108864,
              "error": "Input/output error",
              "device": "drive1",
              "type": "backup"
            },
            "event": "BLOCK_JOB_COMPLETED"
          }

4. Receive notification that the job for `drive0` has been cancelled:

       <- {
            "timestamp": {...},
            "data": {
              "device": "drive0",
              "type": "backup",
              "speed": 0,
              "len": 67108864,
              "offset": 16777216
            },
            "event": "BLOCK_JOB_CANCELLED"
          }

At the conclusion of this example, both jobs have been aborted due to a failure. Both destination images should be deleted and are no longer of use.

The transaction as a whole can simply be re-issued at a later time.