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