docs: incremental backup documentation
Signed-off-by: John Snow <jsnow@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Message-id: 1429314609-29776-2-git-send-email-jsnow@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Kevin Wolf
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<!--
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Copyright 2015 John Snow <jsnow@redhat.com> and Red Hat, Inc.
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All rights reserved.
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This file is licensed via The FreeBSD Documentation License, the full text of
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which is included at the end of this document.
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-->
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# Dirty Bitmaps and Incremental Backup
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* Dirty Bitmaps are objects that track which data needs to be backed up for the
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next incremental backup.
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* Dirty bitmaps can be created at any time and attached to any node
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(not just complete drives.)
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## Dirty Bitmap Names
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* A dirty bitmap's name is unique to the node, but bitmaps attached to different
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nodes can share the same name.
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## Bitmap Modes
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* A Bitmap can be "frozen," which means that it is currently in-use by a backup
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operation and cannot be deleted, renamed, written to, reset,
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etc.
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## Basic QMP Usage
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### Supported Commands ###
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* block-dirty-bitmap-add
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* block-dirty-bitmap-remove
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* block-dirty-bitmap-clear
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### Creation
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* To create a new bitmap, enabled, on the drive with id=drive0:
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```json
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{ "execute": "block-dirty-bitmap-add",
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"arguments": {
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"node": "drive0",
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"name": "bitmap0"
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}
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}
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```
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* This bitmap will have a default granularity that matches the cluster size of
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its associated drive, if available, clamped to between [4KiB, 64KiB].
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The current default for qcow2 is 64KiB.
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* To create a new bitmap that tracks changes in 32KiB segments:
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```json
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{ "execute": "block-dirty-bitmap-add",
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"arguments": {
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"node": "drive0",
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"name": "bitmap0",
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"granularity": 32768
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}
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}
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```
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### Deletion
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* Bitmaps that are frozen cannot be deleted.
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* Deleting the bitmap does not impact any other bitmaps attached to the same
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node, nor does it affect any backups already created from this node.
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* Because bitmaps are only unique to the node to which they are attached,
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you must specify the node/drive name here, too.
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```json
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{ "execute": "block-dirty-bitmap-remove",
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"arguments": {
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"node": "drive0",
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"name": "bitmap0"
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}
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}
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```
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### Resetting
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* Resetting a bitmap will clear all information it holds.
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* An incremental backup created from an empty bitmap will copy no data,
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as if nothing has changed.
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```json
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{ "execute": "block-dirty-bitmap-clear",
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"arguments": {
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"node": "drive0",
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"name": "bitmap0"
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}
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}
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```
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## Transactions (Not yet implemented)
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* Transactional commands are forthcoming in a future version,
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and are not yet available for use. This section serves as
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documentation of intent for their design and usage.
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### Justification
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Bitmaps can be safely modified when the VM is paused or halted by using
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the basic QMP commands. For instance, you might perform the following actions:
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1. Boot the VM in a paused state.
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2. Create a full drive backup of drive0.
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3. Create a new bitmap attached to drive0.
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4. Resume execution of the VM.
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5. Incremental backups are ready to be created.
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At this point, the bitmap and drive backup would be correctly in sync,
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and incremental backups made from this point forward would be correctly aligned
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to the full drive backup.
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This is not particularly useful if we decide we want to start incremental
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backups after the VM has been running for a while, for which we will need to
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perform actions such as the following:
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1. Boot the VM and begin execution.
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2. Using a single transaction, perform the following operations:
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* Create bitmap0.
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* Create a full drive backup of drive0.
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3. Incremental backups are now ready to be created.
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### Supported Bitmap Transactions
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* block-dirty-bitmap-add
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* block-dirty-bitmap-clear
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The usages are identical to their respective QMP commands, but see below
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for examples.
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### Example: New Incremental Backup
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As outlined in the justification, perhaps we want to create a new incremental
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backup chain attached to a drive.
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```json
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{ "execute": "transaction",
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"arguments": {
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"actions": [
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{"type": "block-dirty-bitmap-add",
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"data": {"node": "drive0", "name": "bitmap0"} },
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{"type": "drive-backup",
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"data": {"device": "drive0", "target": "/path/to/full_backup.img",
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"sync": "full", "format": "qcow2"} }
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]
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}
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}
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```
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### Example: New Incremental Backup Anchor Point
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Maybe we just want to create a new full backup with an existing bitmap and
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want to reset the bitmap to track the new chain.
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```json
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{ "execute": "transaction",
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"arguments": {
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"actions": [
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{"type": "block-dirty-bitmap-clear",
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"data": {"node": "drive0", "name": "bitmap0"} },
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{"type": "drive-backup",
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"data": {"device": "drive0", "target": "/path/to/new_full_backup.img",
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"sync": "full", "format": "qcow2"} }
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]
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}
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}
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```
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## Incremental Backups
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The star of the show.
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**Nota Bene!** Only incremental backups of entire drives are supported for now.
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So despite the fact that you can attach a bitmap to any arbitrary node, they are
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only currently useful when attached to the root node. This is because
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drive-backup only supports drives/devices instead of arbitrary nodes.
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### Example: First Incremental Backup
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1. Create a full backup and sync it to the dirty bitmap, as in the transactional
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examples above; or with the VM offline, manually create a full copy and then
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create a new bitmap before the VM begins execution.
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* Let's assume the full backup is named 'full_backup.img'.
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* Let's assume the bitmap you created is 'bitmap0' attached to 'drive0'.
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2. Create a destination image for the incremental backup that utilizes the
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full backup as a backing image.
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* Let's assume it is named 'incremental.0.img'.
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```sh
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# qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2
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```
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3. Issue the incremental backup command:
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```json
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{ "execute": "drive-backup",
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"arguments": {
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"device": "drive0",
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"bitmap": "bitmap0",
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"target": "incremental.0.img",
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"format": "qcow2",
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"sync": "dirty-bitmap",
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"mode": "existing"
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}
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}
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```
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### Example: Second Incremental Backup
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1. Create a new destination image for the incremental backup that points to the
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previous one, e.g.: 'incremental.1.img'
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```sh
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# qemu-img create -f qcow2 incremental.1.img -b incremental.0.img -F qcow2
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```
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2. Issue a new incremental backup command. The only difference here is that we
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have changed the target image below.
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```json
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{ "execute": "drive-backup",
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"arguments": {
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"device": "drive0",
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"bitmap": "bitmap0",
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"target": "incremental.1.img",
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"format": "qcow2",
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"sync": "dirty-bitmap",
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"mode": "existing"
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}
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}
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```
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## Errors
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* In the event of an error that occurs after a backup job is successfully
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launched, either by a direct QMP command or a QMP transaction, the user
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will receive a BLOCK_JOB_COMPLETE event with a failure message, accompanied
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by a BLOCK_JOB_ERROR event.
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* In the case of an event being cancelled, the user will receive a
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BLOCK_JOB_CANCELLED event instead of a pair of COMPLETE and ERROR events.
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* In either case, the incremental backup data contained within the bitmap is
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safely rolled back, and the data within the bitmap is not lost. The image
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file created for the failed attempt can be safely deleted.
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* Once the underlying problem is fixed (e.g. more storage space is freed up),
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you can simply retry the incremental backup command with the same bitmap.
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### Example
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1. Create a target image:
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```sh
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# qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2
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```
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2. Attempt to create an incremental backup via QMP:
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```json
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{ "execute": "drive-backup",
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"arguments": {
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"device": "drive0",
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"bitmap": "bitmap0",
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"target": "incremental.0.img",
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"format": "qcow2",
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"sync": "dirty-bitmap",
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"mode": "existing"
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}
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}
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```
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3. Receive an event notifying us of failure:
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```json
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{ "timestamp": { "seconds": 1424709442, "microseconds": 844524 },
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"data": { "speed": 0, "offset": 0, "len": 67108864,
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"error": "No space left on device",
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"device": "drive1", "type": "backup" },
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"event": "BLOCK_JOB_COMPLETED" }
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```
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4. Delete the failed incremental, and re-create the image.
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```sh
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# rm incremental.0.img
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# qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2
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```
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5. Retry the command after fixing the underlying problem,
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such as freeing up space on the backup volume:
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```json
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{ "execute": "drive-backup",
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"arguments": {
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"device": "drive0",
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"bitmap": "bitmap0",
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"target": "incremental.0.img",
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"format": "qcow2",
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"sync": "dirty-bitmap",
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"mode": "existing"
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}
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}
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```
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6. Receive confirmation that the job completed successfully:
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```json
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{ "timestamp": { "seconds": 1424709668, "microseconds": 526525 },
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"data": { "device": "drive1", "type": "backup",
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"speed": 0, "len": 67108864, "offset": 67108864},
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"event": "BLOCK_JOB_COMPLETED" }
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```
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