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提交 f9dada2b 编写于 作者: P Peter Maydell
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Merge remote-tracking branch 'remotes/cody/tags/block-pull-request' into staging 

# gpg: Signature made Tue 18 Jul 2017 05:15:03 BST
# gpg:                using RSA key 0xBDBE7B27C0DE3057
# gpg: Good signature from "Jeffrey Cody <jcody@redhat.com>"
# gpg:                 aka "Jeffrey Cody <jeff@codyprime.org>"
# gpg:                 aka "Jeffrey Cody <codyprime@gmail.com>"
# Primary key fingerprint: 9957 4B4D 3474 90E7 9D98  D624 BDBE 7B27 C0DE 3057

* remotes/cody/tags/block-pull-request:
  live-block-ops.txt: Rename, rewrite, and improve it
  bitmaps.md: Convert to rST; move it into 'interop' dir
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
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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.
* Dirty bitmaps created for internal use by QEMU may be anonymous and have no
name, but any user-created bitmaps may not be. There can be any number of
anonymous bitmaps per node.
* The name of a user-created bitmap must not be empty ("").
## 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.
* The normal operating mode for a bitmap is "active."
## 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
### 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": "incremental",
"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": "incremental",
"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": "incremental",
"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": "incremental",
"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" }
```
### Partial Transactional Failures
* Sometimes, a transaction will succeed in launching and return success,
but then later the backup jobs themselves 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, when one of
them fails, it will not interact with the other backup jobs in any way.
* The job(s) that succeeded will clear the dirty bitmap associated with the
operation, but the job(s) that failed will not. It is not "safe" to delete
any incremental backups that were created successfully in this scenario,
even though others failed.
#### Example
* QMP example highlighting two backup jobs:
```json
{ "execute": "transaction",
"arguments": {
"actions": [
{ "type": "drive-backup",
"data": { "device": "drive0", "bitmap": "bitmap0",
"format": "qcow2", "mode": "existing",
"sync": "incremental", "target": "d0-incr-1.qcow2" } },
{ "type": "drive-backup",
"data": { "device": "drive1", "bitmap": "bitmap1",
"format": "qcow2", "mode": "existing",
"sync": "incremental", "target": "d1-incr-1.qcow2" } },
]
}
}
```
* QMP example response, highlighting one success and one failure:
* Acknowledgement that the Transaction was accepted and jobs were launched:
```json
{ "return": {} }
```
* Later, QEMU sends notice that the first job was completed:
```json
{ "timestamp": { "seconds": 1447192343, "microseconds": 615698 },
"data": { "device": "drive0", "type": "backup",
"speed": 0, "len": 67108864, "offset": 67108864 },
"event": "BLOCK_JOB_COMPLETED"
}
```
* Later yet, QEMU sends notice that the second job has failed:
```json
{ "timestamp": { "seconds": 1447192399, "microseconds": 683015 },
"data": { "device": "drive1", "action": "report",
"operation": "read" },
"event": "BLOCK_JOB_ERROR" }
```
```json
{ "timestamp": { "seconds": 1447192399, "microseconds": 685853 },
"data": { "speed": 0, "offset": 0, "len": 67108864,
"error": "Input/output error",
"device": "drive1", "type": "backup" },
"event": "BLOCK_JOB_COMPLETED" }
* In the above example, "d0-incr-1.qcow2" is valid and must be kept,
but "d1-incr-1.qcow2" is invalid 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
"d0-incr-1.qcow2."
### Grouped Completion Mode
* While jobs launched by transactions normally complete or fail on their own,
it is possible to instruct them to complete or fail together as a group.
* QMP transactions take an optional properties structure that can affect
the semantics of the transaction.
* The "completion-mode" transaction property can be either "individual"
which is the default, legacy behavior described above, or "grouped,"
a new behavior detailed below.
* Delayed Completion: In grouped completion mode, no jobs will report
success until all jobs are ready to report success.
* Grouped failure: If any job fails in grouped completion mode, all remaining
jobs will be cancelled. Any incremental backups will restore their dirty
bitmap objects as if no backup command was ever issued.
* Regardless of if QEMU reports a particular incremental backup job as
CANCELLED or as an ERROR, the in-memory bitmap will be restored.
#### Example
* Here's the same example scenario from above with the new property:
```json
{ "execute": "transaction",
"arguments": {
"actions": [
{ "type": "drive-backup",
"data": { "device": "drive0", "bitmap": "bitmap0",
"format": "qcow2", "mode": "existing",
"sync": "incremental", "target": "d0-incr-1.qcow2" } },
{ "type": "drive-backup",
"data": { "device": "drive1", "bitmap": "bitmap1",
"format": "qcow2", "mode": "existing",
"sync": "incremental", "target": "d1-incr-1.qcow2" } },
],
"properties": {
"completion-mode": "grouped"
}
}
}
```
* QMP example response, highlighting a failure for drive2:
* Acknowledgement that the Transaction was accepted and jobs were launched:
```json
{ "return": {} }
```
* Later, QEMU sends notice that the second job has errored out,
but that the first job was also cancelled:
```json
{ "timestamp": { "seconds": 1447193702, "microseconds": 632377 },
"data": { "device": "drive1", "action": "report",
"operation": "read" },
"event": "BLOCK_JOB_ERROR" }
```
```json
{ "timestamp": { "seconds": 1447193702, "microseconds": 640074 },
"data": { "speed": 0, "offset": 0, "len": 67108864,
"error": "Input/output error",
"device": "drive1", "type": "backup" },
"event": "BLOCK_JOB_COMPLETED" }
```
```json
{ "timestamp": { "seconds": 1447193702, "microseconds": 640163 },
"data": { "device": "drive0", "type": "backup", "speed": 0,
"len": 67108864, "offset": 16777216 },
"event": "BLOCK_JOB_CANCELLED" }
```
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docs/interop/bitmaps.rst 0 → 100644
浏览文件 @ f9dada2b
..
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).
.. contents::
Dirty Bitmap Names
------------------
- A dirty bitmap's name is unique to the node, but bitmaps attached to
different nodes can share the same name.
- Dirty bitmaps created for internal use by QEMU may be anonymous and
have no name, but any user-created bitmaps must have a name. There
can be any number of anonymous bitmaps per node.
- The name of a user-created bitmap must not be empty ("").
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.
- The normal operating mode for a bitmap is "active."
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:
.. code:: 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:
.. code:: 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.
.. code:: 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.
.. code:: json
{ "execute": "block-dirty-bitmap-clear",
"arguments": {
"node": "drive0",
"name": "bitmap0"
}
}
Transactions
------------
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.
.. code:: 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.
.. code:: 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 the new incremental image is named
``incremental.0.img``.
.. code:: bash
$ qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2
3. Issue the incremental backup command:
.. code:: json
{ "execute": "drive-backup",
"arguments": {
"device": "drive0",
"bitmap": "bitmap0",
"target": "incremental.0.img",
"format": "qcow2",
"sync": "incremental",
"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``
.. code:: bash
$ 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.
.. code:: json
{ "execute": "drive-backup",
"arguments": {
"device": "drive0",
"bitmap": "bitmap0",
"target": "incremental.1.img",
"format": "qcow2",
"sync": "incremental",
"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:
.. code:: bash
$ qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2
2. Attempt to create an incremental backup via QMP:
.. code:: json
{ "execute": "drive-backup",
"arguments": {
"device": "drive0",
"bitmap": "bitmap0",
"target": "incremental.0.img",
"format": "qcow2",
"sync": "incremental",
"mode": "existing"
}
}
3. Receive an event notifying us of failure:
.. code:: 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.
.. code:: bash
$ 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:
.. code:: json
{ "execute": "drive-backup",
"arguments": {
"device": "drive0",
"bitmap": "bitmap0",
"target": "incremental.0.img",
"format": "qcow2",
"sync": "incremental",
"mode": "existing"
}
}
6. Receive confirmation that the job completed successfully:
.. code:: json
{ "timestamp": { "seconds": 1424709668, "microseconds": 526525 },
"data": { "device": "drive1", "type": "backup",
"speed": 0, "len": 67108864, "offset": 67108864},
"event": "BLOCK_JOB_COMPLETED" }
Partial Transactional Failures
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- Sometimes, a transaction will succeed in launching and return
success, but then later the backup jobs themselves 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, when
one of them fails, it will not interact with the other backup jobs in
any way.
- The job(s) that succeeded will clear the dirty bitmap associated with
the operation, but the job(s) that failed will not. It is not "safe"
to delete any incremental backups that were created successfully in
this scenario, even though others failed.
Example
^^^^^^^
- QMP example highlighting two backup jobs:
.. code:: json
{ "execute": "transaction",
"arguments": {
"actions": [
{ "type": "drive-backup",
"data": { "device": "drive0", "bitmap": "bitmap0",
"format": "qcow2", "mode": "existing",
"sync": "incremental", "target": "d0-incr-1.qcow2" } },
{ "type": "drive-backup",
"data": { "device": "drive1", "bitmap": "bitmap1",
"format": "qcow2", "mode": "existing",
"sync": "incremental", "target": "d1-incr-1.qcow2" } },
]
}
}
- QMP example response, highlighting one success and one failure:
- Acknowledgement that the Transaction was accepted and jobs were
launched:
.. code:: json
{ "return": {} }
- Later, QEMU sends notice that the first job was completed:
.. code:: json
{ "timestamp": { "seconds": 1447192343, "microseconds": 615698 },
"data": { "device": "drive0", "type": "backup",
"speed": 0, "len": 67108864, "offset": 67108864 },
"event": "BLOCK_JOB_COMPLETED"
}
- Later yet, QEMU sends notice that the second job has failed:
.. code:: json
{ "timestamp": { "seconds": 1447192399, "microseconds": 683015 },
"data": { "device": "drive1", "action": "report",
"operation": "read" },
"event": "BLOCK_JOB_ERROR" }
.. code:: json
{ "timestamp": { "seconds": 1447192399, "microseconds":
685853 }, "data": { "speed": 0, "offset": 0, "len": 67108864,
"error": "Input/output error", "device": "drive1", "type":
"backup" }, "event": "BLOCK_JOB_COMPLETED" }
- In the above example, ``d0-incr-1.qcow2`` is valid and must be kept,
but ``d1-incr-1.qcow2`` is invalid 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
``d0-incr-1.qcow2``.
Grouped Completion Mode
~~~~~~~~~~~~~~~~~~~~~~~
- While jobs launched by transactions normally complete or fail on
their own, it is possible to instruct them to complete or fail
together as a group.
- QMP transactions take an optional properties structure that can
affect the semantics of the transaction.
- The "completion-mode" transaction property can be either "individual"
which is the default, legacy behavior described above, or "grouped,"
a new behavior detailed below.
- Delayed Completion: In grouped completion mode, no jobs will report
success until all jobs are ready to report success.
- Grouped failure: If any job fails in grouped completion mode, all
remaining jobs will be cancelled. Any incremental backups will
restore their dirty bitmap objects as if no backup command was ever
issued.
- Regardless of if QEMU reports a particular incremental backup job
as CANCELLED or as an ERROR, the in-memory bitmap will be
restored.
Example
^^^^^^^
- Here's the same example scenario from above with the new property:
.. code:: json
{ "execute": "transaction",
"arguments": {
"actions": [
{ "type": "drive-backup",
"data": { "device": "drive0", "bitmap": "bitmap0",
"format": "qcow2", "mode": "existing",
"sync": "incremental", "target": "d0-incr-1.qcow2" } },
{ "type": "drive-backup",
"data": { "device": "drive1", "bitmap": "bitmap1",
"format": "qcow2", "mode": "existing",
"sync": "incremental", "target": "d1-incr-1.qcow2" } },
],
"properties": {
"completion-mode": "grouped"
}
}
}
- QMP example response, highlighting a failure for ``drive2``:
- Acknowledgement that the Transaction was accepted and jobs were
launched:
.. code:: json
{ "return": {} }
- Later, QEMU sends notice that the second job has errored out, but
that the first job was also cancelled:
.. code:: json
{ "timestamp": { "seconds": 1447193702, "microseconds": 632377 },
"data": { "device": "drive1", "action": "report",
"operation": "read" },
"event": "BLOCK_JOB_ERROR" }
.. code:: json
{ "timestamp": { "seconds": 1447193702, "microseconds": 640074 },
"data": { "speed": 0, "offset": 0, "len": 67108864,
"error": "Input/output error",
"device": "drive1", "type": "backup" },
"event": "BLOCK_JOB_COMPLETED" }
.. code:: json
{ "timestamp": { "seconds": 1447193702, "microseconds": 640163 },
"data": { "device": "drive0", "type": "backup", "speed": 0,
"len": 67108864, "offset": 16777216 },
"event": "BLOCK_JOB_CANCELLED" }
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docs/interop/live-block-operations.rst 0 → 100644
浏览文件 @ f9dada2b
..
Copyright (C) 2017 Red Hat Inc.
This work is licensed under the terms of the GNU GPL, version 2 or
later. See the COPYING file in the top-level directory.
============================
Live Block Device Operations
============================
QEMU Block Layer currently (as of QEMU 2.9) supports four major kinds of
live block device jobs -- stream, commit, mirror, and backup. These can
be used to manipulate disk image chains to accomplish certain tasks,
namely: live copy data from backing files into overlays; shorten long
disk image chains by merging data from overlays into backing files; live
synchronize data from a disk image chain (including current active disk)
to another target image; and point-in-time (and incremental) backups of
a block device. Below is a description of the said block (QMP)
primitives, and some (non-exhaustive list of) examples to illustrate
their use.
.. note::
The file ``qapi/block-core.json`` in the QEMU source tree has the
canonical QEMU API (QAPI) schema documentation for the QMP
primitives discussed here.
.. todo (kashyapc):: Remove the ".. contents::" directive when Sphinx is
integrated.
.. contents::
Disk image backing chain notation
---------------------------------
A simple disk image chain. (This can be created live using QMP
``blockdev-snapshot-sync``, or offline via ``qemu-img``)::
(Live QEMU)
|
.
V
[A] <----- [B]
(backing file) (overlay)
The arrow can be read as: Image [A] is the backing file of disk image
[B]. And live QEMU is currently writing to image [B], consequently, it
is also referred to as the "active layer".
There are two kinds of terminology that are common when referring to
files in a disk image backing chain:
(1) Directional: 'base' and 'top'. Given the simple disk image chain
above, image [A] can be referred to as 'base', and image [B] as
'top'. (This terminology can be seen in in QAPI schema file,
block-core.json.)
(2) Relational: 'backing file' and 'overlay'. Again, taking the same
simple disk image chain from the above, disk image [A] is referred
to as the backing file, and image [B] as overlay.
Throughout this document, we will use the relational terminology.
.. important::
The overlay files can generally be any format that supports a
backing file, although QCOW2 is the preferred format and the one
used in this document.
Brief overview of live block QMP primitives
-------------------------------------------
The following are the four different kinds of live block operations that
QEMU block layer supports.
(1) ``block-stream``: Live copy of data from backing files into overlay
files.
.. note:: Once the 'stream' operation has finished, three things to
note:
(a) QEMU rewrites the backing chain to remove
reference to the now-streamed and redundant backing
file;
(b) the streamed file *itself* won't be removed by QEMU,
and must be explicitly discarded by the user;
(c) the streamed file remains valid -- i.e. further
overlays can be created based on it. Refer the
``block-stream`` section further below for more
details.
(2) ``block-commit``: Live merge of data from overlay files into backing
files (with the optional goal of removing the overlay file from the
chain). Since QEMU 2.0, this includes "active ``block-commit``"
(i.e. merge the current active layer into the base image).
.. note:: Once the 'commit' operation has finished, there are three
things to note here as well:
(a) QEMU rewrites the backing chain to remove reference
to now-redundant overlay images that have been
committed into a backing file;
(b) the committed file *itself* won't be removed by QEMU
-- it ought to be manually removed;
(c) however, unlike in the case of ``block-stream``, the
intermediate images will be rendered invalid -- i.e.
no more further overlays can be created based on
them. Refer the ``block-commit`` section further
below for more details.
(3) ``drive-mirror`` (and ``blockdev-mirror``): Synchronize a running
disk to another image.
(4) ``drive-backup`` (and ``blockdev-backup``): Point-in-time (live) copy
of a block device to a destination.
.. _`Interacting with a QEMU instance`:
Interacting with a QEMU instance
--------------------------------
To show some example invocations of command-line, we will use the
following invocation of QEMU, with a QMP server running over UNIX
socket::
$ ./x86_64-softmmu/qemu-system-x86_64 -display none -nodefconfig \
-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).
.. _`Case-1`:
(1) Merge everything into the active layer: I.e. copy all contents from
the base image, [A], and overlay images, [B] and [C], into [D],
*while* the guest is running. The resulting chain will be a
standalone image, [D] -- with contents from [A], [B] and [C] merged
into it (where live QEMU writes go to)::
[D]
.. _`Case-2`:
(2) Taking the same example disk image chain mentioned earlier, merge
only images [B] and [C] into [D], the active layer. The result will
be contents of images [B] and [C] will be copied into [D], and the
backing file pointer of image [D] will be adjusted to point to image
[A]. The resulting chain will be::
[A] <-- [D]
.. _`Case-3`:
(3) Intermediate streaming (available since QEMU 2.8): Starting afresh
with the original example disk image chain, with a total of four
images, it is possible to copy contents from image [B] into image
[C]. Once the copy is finished, image [B] can now be (optionally)
discarded; and the backing file pointer of image [C] will be
adjusted to point to [A]. I.e. after performing "intermediate
streaming" of [B] into [C], the resulting image chain will be (where
live QEMU is writing to [D])::
[A] <-- [C] <-- [D]
QMP invocation for ``block-stream``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For `Case-1`_, to merge contents of all the backing files into the
active layer, where 'node-D' is the current active image (by default
``block-stream`` will flatten the entire chain); ``qmp-shell`` (and its
corresponding JSON output)::
(QEMU) block-stream device=node-D job-id=job0
{
"execute": "block-stream",
"arguments": {
"device": "node-D",
"job-id": "job0"
}
}
For `Case-2`_, merge contents of the images [B] and [C] into [D], where
image [D] ends up referring to image [A] as its backing file::
(QEMU) block-stream device=node-D base-node=node-A job-id=job0
And for `Case-3`_, of "intermediate" streaming", merge contents of
images [B] into [C], where [C] ends up referring to [A] as its backing
image::
(QEMU) block-stream device=node-C base-node=node-A job-id=job0
Progress of a ``block-stream`` operation can be monitored via the QMP
command::
(QEMU) query-block-jobs
{
"execute": "query-block-jobs",
"arguments": {}
}
Once the ``block-stream`` operation has completed, QEMU will emit an
event, ``BLOCK_JOB_COMPLETED``. The intermediate overlays remain valid,
and can now be (optionally) discarded, or retained to create further
overlays based on them. Finally, the ``block-stream`` jobs can be
restarted at anytime.
Live block commit --- ``block-commit``
--------------------------------------
The ``block-commit`` command lets you merge live data from overlay
images into backing file(s). Since QEMU 2.0, this includes "live active
commit" (i.e. it is possible to merge the "active layer", the right-most
image in a disk image chain where live QEMU will be writing to, into the
base image). This is analogous to ``block-stream``, but in the opposite
direction.
Again, starting afresh with our example disk image chain, where live
QEMU is writing to the right-most image in the chain, [D]::
[A] <-- [B] <-- [C] <-- [D]
The disk image chain can be shortened in one of the following ways:
.. _`block-commit_Case-1`:
(1) Commit content from only image [B] into image [A]. The resulting
chain is the following, where image [C] is adjusted to point at [A]
as its new backing file::
[A] <-- [C] <-- [D]
(2) Commit content from images [B] and [C] into image [A]. The
resulting chain, where image [D] is adjusted to point to image [A]
as its new backing file::
[A] <-- [D]
.. _`block-commit_Case-3`:
(3) Commit content from images [B], [C], and the active layer [D] into
image [A]. The resulting chain (in this case, a consolidated single
image)::
[A]
(4) Commit content from image only image [C] into image [B]. The
resulting chain::
[A] <-- [B] <-- [D]
(5) Commit content from image [C] and the active layer [D] into image
[B]. The resulting chain::
[A] <-- [B]
QMP invocation for ``block-commit``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For :ref:`Case-1 <block-commit_Case-1>`, to merge contents only from
image [B] into image [A], the invocation is as follows::
(QEMU) block-commit device=node-D base=a.qcow2 top=b.qcow2 job-id=job0
{
"execute": "block-commit",
"arguments": {
"device": "node-D",
"job-id": "job0",
"top": "b.qcow2",
"base": "a.qcow2"
}
}
Once the above ``block-commit`` operation has completed, a
``BLOCK_JOB_COMPLETED`` event will be issued, and no further action is
required. As the end result, the backing file of image [C] is adjusted
to point to image [A], and the original 4-image chain will end up being
transformed to::
[A] <-- [C] <-- [D]
.. note::
The intermediate image [B] is invalid (as in: no more further
overlays based on it can be created).
Reasoning: An intermediate image after a 'stream' operation still
represents that old point-in-time, and may be valid in that context.
However, an intermediate image after a 'commit' operation no longer
represents any point-in-time, and is invalid in any context.
However, :ref:`Case-3 <block-commit_Case-3>` (also called: "active
``block-commit``") is a *two-phase* operation: In the first phase, the
content from the active overlay, along with the intermediate overlays,
is copied into the backing file (also called the base image). In the
second phase, adjust the said backing file as the current active image
-- possible via issuing the command ``block-job-complete``. Optionally,
the ``block-commit`` operation can be cancelled by issuing the command
``block-job-cancel``, but be careful when doing this.
Once the ``block-commit`` operation has completed, the event
``BLOCK_JOB_READY`` will be emitted, signalling that the synchronization
has finished. Now the job can be gracefully completed by issuing the
command ``block-job-complete`` -- until such a command is issued, the
'commit' operation remains active.
The following is the flow for :ref:`Case-3 <block-commit_Case-3>` to
convert a disk image chain such as this::
[A] <-- [B] <-- [C] <-- [D]
Into::
[A]
Where content from all the subsequent overlays, [B], and [C], including
the active layer, [D], is committed back to [A] -- which is where live
QEMU is performing all its current writes).
Start the "active ``block-commit``" operation::
(QEMU) block-commit device=node-D base=a.qcow2 top=d.qcow2 job-id=job0
{
"execute": "block-commit",
"arguments": {
"device": "node-D",
"job-id": "job0",
"top": "d.qcow2",
"base": "a.qcow2"
}
}
Once the synchronization has completed, the event ``BLOCK_JOB_READY`` will
be emitted.
Then, optionally query for the status of the active block operations.
We can see the 'commit' job is now ready to be completed, as indicated
by the line *"ready": true*::
(QEMU) query-block-jobs
{
"execute": "query-block-jobs",
"arguments": {}
}
{
"return": [
{
"busy": false,
"type": "commit",
"len": 1376256,
"paused": false,
"ready": true,
"io-status": "ok",
"offset": 1376256,
"device": "job0",
"speed": 0
}
]
}
Gracefully complete the 'commit' block device job::
(QEMU) block-job-complete device=job0
{
"execute": "block-job-complete",
"arguments": {
"device": "job0"
}
}
{
"return": {}
}
Finally, once the above job is completed, an event
``BLOCK_JOB_COMPLETED`` will be emitted.
.. note::
The invocation for rest of the cases (2, 4, and 5), discussed in the
previous section, is omitted for brevity.
Live disk synchronization --- ``drive-mirror`` and ``blockdev-mirror``
----------------------------------------------------------------------
Synchronize a running disk image chain (all or part of it) to a target
image.
Again, given our familiar disk image chain::
[A] <-- [B] <-- [C] <-- [D]
The ``drive-mirror`` (and its newer equivalent ``blockdev-mirror``) allows
you to copy data from the entire chain into a single target image (which
can be located on a different host).
Once a 'mirror' job has started, there are two possible actions while a
``drive-mirror`` job is active:
(1) Issuing the command ``block-job-cancel`` after it emits the event
``BLOCK_JOB_CANCELLED``: will (after completing synchronization of
the content from the disk image chain to the target image, [E])
create a point-in-time (which is at the time of *triggering* the
cancel command) copy, contained in image [E], of the the entire disk
image chain (or only the top-most image, depending on the ``sync``
mode).
(2) Issuing the command ``block-job-complete`` after it emits the event
``BLOCK_JOB_COMPLETED``: will, after completing synchronization of
the content, adjust the guest device (i.e. live QEMU) to point to
the target image, and, causing all the new writes from this point on
to happen there. One use case for this is live storage migration.
About synchronization modes: The synchronization mode determines
*which* part of the disk image chain will be copied to the target.
Currently, there are four different kinds:
(1) ``full`` -- Synchronize the content of entire disk image chain to
the target
(2) ``top`` -- Synchronize only the contents of the top-most disk image
in the chain to the target
(3) ``none`` -- Synchronize only the new writes from this point on.
.. note:: In the case of ``drive-backup`` (or ``blockdev-backup``),
the behavior of ``none`` synchronization mode is different.
Normally, a ``backup`` job consists of two parts: Anything
that is overwritten by the guest is first copied out to
the backup, and in the background the whole image is
copied from start to end. With ``sync=none``, it's only
the first part.
(4) ``incremental`` -- Synchronize content that is described by the
dirty bitmap
.. note::
Refer to the :doc:`bitmaps` document in the QEMU source
tree to learn about the detailed workings of the ``incremental``
synchronization mode.
QMP invocation for ``drive-mirror``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To copy the contents of the entire disk image chain, from [A] all the
way to [D], to a new target (``drive-mirror`` will create the destination
file, if it doesn't already exist), call it [E]::
(QEMU) drive-mirror device=node-D target=e.qcow2 sync=full job-id=job0
{
"execute": "drive-mirror",
"arguments": {
"device": "node-D",
"job-id": "job0",
"target": "e.qcow2",
"sync": "full"
}
}
The ``"sync": "full"``, from the above, means: copy the *entire* chain
to the destination.
Following the above, querying for active block jobs will show that a
'mirror' job is "ready" to be completed (and QEMU will also emit an
event, ``BLOCK_JOB_READY``)::
(QEMU) query-block-jobs
{
"execute": "query-block-jobs",
"arguments": {}
}
{
"return": [
{
"busy": false,
"type": "mirror",
"len": 21757952,
"paused": false,
"ready": true,
"io-status": "ok",
"offset": 21757952,
"device": "job0",
"speed": 0
}
]
}
And, as noted in the previous section, there are two possible actions
at this point:
(a) Create a point-in-time snapshot by ending the synchronization. The
point-in-time is at the time of *ending* the sync. (The result of
the following being: the target image, [E], will be populated with
content from the entire chain, [A] to [D])::
(QEMU) block-job-cancel device=job0
{
"execute": "block-job-cancel",
"arguments": {
"device": "job0"
}
}
(b) Or, complete the operation and pivot the live QEMU to the target
copy::
(QEMU) block-job-complete device=job0
In either of the above cases, if you once again run the
`query-block-jobs` command, there should not be any active block
operation.
Comparing 'commit' and 'mirror': In both then cases, the overlay images
can be discarded. However, with 'commit', the *existing* base image
will be modified (by updating it with contents from overlays); while in
the case of 'mirror', a *new* target image is populated with the data
from the disk image chain.
QMP invocation for live storage migration with ``drive-mirror`` + NBD
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Live storage migration (without shared storage setup) is one of the most
common use-cases that takes advantage of the ``drive-mirror`` primitive
and QEMU's built-in Network Block Device (NBD) server. Here's a quick
walk-through of this setup.
Given the disk image chain::
[A] <-- [B] <-- [C] <-- [D]
Instead of copying content from the entire chain, synchronize *only* the
contents of the *top*-most disk image (i.e. the active layer), [D], to a
target, say, [TargetDisk].
.. important::
The destination host must already have the contents of the backing
chain, involving images [A], [B], and [C], visible via other means
-- whether by ``cp``, ``rsync``, or by some storage array-specific
command.)
Sometimes, this is also referred to as "shallow copy" -- because only
the "active layer", and not the rest of the image chain, is copied to
the destination.
.. note::
In this example, for the sake of simplicity, we'll be using the same
``localhost`` as both source and destination.
As noted earlier, on the destination host the contents of the backing
chain -- from images [A] to [C] -- are already expected to exist in some
form (e.g. in a file called, ``Contents-of-A-B-C.qcow2``). Now, on the
destination host, let's create a target overlay image (with the image
``Contents-of-A-B-C.qcow2`` as its backing file), to which the contents
of image [D] (from the source QEMU) will be mirrored to::
$ qemu-img create -f qcow2 -b ./Contents-of-A-B-C.qcow2 \
-F qcow2 ./target-disk.qcow2
And start the destination QEMU (we already have the source QEMU running
-- discussed in the section: `Interacting with a QEMU instance`_)
instance, with the following invocation. (As noted earlier, for
simplicity's sake, the destination QEMU is started on the same host, but
it could be located elsewhere)::
$ ./x86_64-softmmu/qemu-system-x86_64 -display none -nodefconfig \
-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.
docs/live-block-ops.txt 已删除 100644 → 0
浏览文件 @ 20df6c76
LIVE BLOCK OPERATIONS
=====================
High level description of live block operations. Note these are not
supported for use with the raw format at the moment.
Note also that this document is incomplete and it currently only
covers the 'stream' operation. Other operations supported by QEMU such
as 'commit', 'mirror' and 'backup' are not described here yet. Please
refer to the qapi/block-core.json file for an overview of those.
Snapshot live merge
===================
Given a snapshot chain, described in this document in the following
format:
[A] <- [B] <- [C] <- [D] <- [E]
Where the rightmost object ([E] in the example) described is the current
image which the guest OS has write access to. To the left of it is its base
image, and so on accordingly until the leftmost image, which has no
base.
The snapshot live merge operation transforms such a chain into a
smaller one with fewer elements, such as this transformation relative
to the first example:
[A] <- [E]
Data is copied in the right direction with destination being the
rightmost image, but any other intermediate image can be specified
instead. In this example data is copied from [C] into [D], so [D] can
be backed by [B]:
[A] <- [B] <- [D] <- [E]
The operation is implemented in QEMU through image streaming facilities.
The basic idea is to execute 'block_stream virtio0' while the guest is
running. Progress can be monitored using 'info block-jobs'. When the
streaming operation completes it raises a QMP event. 'block_stream'
copies data from the backing file(s) into the active image. When finished,
it adjusts the backing file pointer.
The 'base' parameter specifies an image which data need not be
streamed from. This image will be used as the backing file for the
destination image when the operation is finished.
In the first example above, the command would be:
(qemu) block_stream virtio0 file-A.img
In order to specify a destination image different from the active
(rightmost) one we can use its node name instead.
In the second example above, the command would be:
(qemu) block_stream node-D file-B.img
Live block copy
===============
To copy an in use image to another destination in the filesystem, one
should create a live snapshot in the desired destination, then stream
into that image. Example:
(qemu) snapshot_blkdev ide0-hd0 /new-path/disk.img qcow2
(qemu) block_stream ide0-hd0
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