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.
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:
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.)
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.
block-stream
: Live copy of data from backing files into overlay files.Note
Once the ‘stream’ operation has finished, three things to note:
QEMU rewrites the backing chain to remove reference to the now-streamed and redundant backing file;
the streamed file itself won’t be removed by QEMU, and must be explicitly discarded by the user;
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.
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 “activeblock-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:
QEMU rewrites the backing chain to remove reference to now-redundant overlay images that have been committed into a backing file;
the committed file itself won’t be removed by QEMU – it ought to be manually removed;
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 theblock-commit
section further below for more details.
drive-mirror
(andblockdev-mirror
): Synchronize a running disk to another image.drive-backup
(andblockdev-backup
): Point-in-time (live) copy of a block device to a destination.
Interacting with a QEMU instance¶
To show some example invocations of command-line, we will use the following invocation of QEMU, with a QMP server running over UNIX socket:
$ ./x86_64-softmmu/qemu-system-x86_64 -display none -no-user-config \
-M q35 -nodefaults -m 512 \
-blockdev node-name=node-A,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./a.qcow2 \
-device virtio-blk,drive=node-A,id=virtio0 \
-monitor stdio -qmp unix:/tmp/qmp-sock,server,nowait
The -blockdev
command-line option, used above, is available from
QEMU 2.9 onwards. In the above invocation, notice the node-name
parameter that is used to refer to the disk image a.qcow2 (‘node-A’) –
this is a cleaner way to refer to a disk image (as opposed to referring
to it by spelling out file paths). So, we will continue to designate a
node-name
to each further disk image created (either via
blockdev-snapshot-sync
, or blockdev-add
) as part of the disk
image chain, and continue to refer to the disks using their
node-name
(where possible, because block-commit
does not yet, as
of QEMU 2.9, accept node-name
parameter) when performing various
block operations.
To interact with the QEMU instance launched above, we will use the
qmp-shell
utility (located at: qemu/scripts/qmp
, as part of the
QEMU source directory), which takes key-value pairs for QMP commands.
Invoke it as below (which will also print out the complete raw JSON
syntax for reference – examples in the following sections):
$ ./qmp-shell -v -p /tmp/qmp-sock
(QEMU)
Note
In the event we have to repeat a certain QMP command, we will: for
the first occurrence of it, show the qmp-shell
invocation, and
the corresponding raw JSON QMP syntax; but for subsequent
invocations, present just the qmp-shell
syntax, and omit the
equivalent JSON output.
Example disk image chain¶
We will use the below disk image chain (and occasionally spelling it out where appropriate) when discussing various primitives:
[A] <-- [B] <-- [C] <-- [D]
Where [A] is the original base image; [B] and [C] are intermediate overlay images; image [D] is the active layer – i.e. live QEMU is writing to it. (The rule of thumb is: live QEMU will always be pointing to the rightmost image in a disk image chain.)
The above image chain can be created by invoking
blockdev-snapshot-sync
commands as following (which shows the
creation of overlay image [B]) using the qmp-shell
(our invocation
also prints the raw JSON invocation of it):
(QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2
{
"execute": "blockdev-snapshot-sync",
"arguments": {
"node-name": "node-A",
"snapshot-file": "b.qcow2",
"format": "qcow2",
"snapshot-node-name": "node-B"
}
}
Here, “node-A” is the name QEMU internally uses to refer to the base image [A] – it is the backing file, based on which the overlay image, [B], is created.
To create the rest of the overlay images, [C], and [D] (omitting the raw JSON output for brevity):
(QEMU) blockdev-snapshot-sync node-name=node-B snapshot-file=c.qcow2 snapshot-node-name=node-C format=qcow2
(QEMU) blockdev-snapshot-sync node-name=node-C snapshot-file=d.qcow2 snapshot-node-name=node-D format=qcow2
A note on points-in-time vs file names¶
In our disk image chain:
[A] <-- [B] <-- [C] <-- [D]
We have three points in time and an active layer:
Point 1: Guest state when [B] was created is contained in file [A]
Point 2: Guest state when [C] was created is contained in [A] + [B]
Point 3: Guest state when [D] was created is contained in [A] + [B] + [C]
Active layer: Current guest state is contained in [A] + [B] + [C] + [D]
Therefore, be aware with naming choices:
Naming a file after the time it is created is misleading – the guest data for that point in time is not contained in that file (as explained earlier)
Rather, think of files as a delta from the backing file
Live block streaming — block-stream
¶
The block-stream
command allows you to do live copy data from backing
files into overlay images.
Given our original example disk image chain from earlier:
[A] <-- [B] <-- [C] <-- [D]
The disk image chain can be shortened in one of the following different ways (not an exhaustive list).
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]
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]
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:
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]
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]
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]
Commit content from image only image [C] into image [B]. The resulting chain:
[A] <-- [B] <-- [D]
Commit content from image [C] and the active layer [D] into image [B]. The resulting chain:
[A] <-- [B]
QMP invocation for block-commit
¶
For Case-1, to merge contents only from image [B] into image [A], the invocation is as follows:
(QEMU) block-commit device=node-D base=a.qcow2 top=b.qcow2 job-id=job0
{
"execute": "block-commit",
"arguments": {
"device": "node-D",
"job-id": "job0",
"top": "b.qcow2",
"base": "a.qcow2"
}
}
Once the above block-commit
operation has completed, a
BLOCK_JOB_COMPLETED
event will be issued, and no further action is
required. As the end result, the backing file of image [C] is adjusted
to point to image [A], and the original 4-image chain will end up being
transformed to:
[A] <-- [C] <-- [D]
Note
The intermediate image [B] is invalid (as in: no more further overlays based on it can be created).
Reasoning: An intermediate image after a ‘stream’ operation still represents that old point-in-time, and may be valid in that context. However, an intermediate image after a ‘commit’ operation no longer represents any point-in-time, and is invalid in any context.
However, Case-3 (also called: “active
block-commit
”) is a two-phase operation: In the first phase, the
content from the active overlay, along with the intermediate overlays,
is copied into the backing file (also called the base image). In the
second phase, adjust the said backing file as the current active image
– possible via issuing the command block-job-complete
. Optionally,
the block-commit
operation can be cancelled by issuing the command
block-job-cancel
, but be careful when doing this.
Once the block-commit
operation has completed, the event
BLOCK_JOB_READY
will be emitted, signalling that the synchronization
has finished. Now the job can be gracefully completed by issuing the
command block-job-complete
– until such a command is issued, the
‘commit’ operation remains active.
The following is the flow for Case-3 to convert a disk image chain such as this:
[A] <-- [B] <-- [C] <-- [D]
Into:
[A]
Where content from all the subsequent overlays, [B], and [C], including the active layer, [D], is committed back to [A] – which is where live QEMU is performing all its current writes).
Start the “active block-commit
” operation:
(QEMU) block-commit device=node-D base=a.qcow2 top=d.qcow2 job-id=job0
{
"execute": "block-commit",
"arguments": {
"device": "node-D",
"job-id": "job0",
"top": "d.qcow2",
"base": "a.qcow2"
}
}
Once the synchronization has completed, the event BLOCK_JOB_READY
will
be emitted.
Then, optionally query for the status of the active block operations. We can see the ‘commit’ job is now ready to be completed, as indicated by the line “ready”: true:
(QEMU) query-block-jobs
{
"execute": "query-block-jobs",
"arguments": {}
}
{
"return": [
{
"busy": false,
"type": "commit",
"len": 1376256,
"paused": false,
"ready": true,
"io-status": "ok",
"offset": 1376256,
"device": "job0",
"speed": 0
}
]
}
Gracefully complete the ‘commit’ block device job:
(QEMU) block-job-complete device=job0
{
"execute": "block-job-complete",
"arguments": {
"device": "job0"
}
}
{
"return": {}
}
Finally, once the above job is completed, an event
BLOCK_JOB_COMPLETED
will be emitted.
Note
The invocation for rest of the cases (2, 4, and 5), discussed in the previous section, is omitted for brevity.
Live disk synchronization — drive-mirror
and blockdev-mirror
¶
Synchronize a running disk image chain (all or part of it) to a target image.
Again, given our familiar disk image chain:
[A] <-- [B] <-- [C] <-- [D]
The drive-mirror
(and its newer equivalent blockdev-mirror
)
allows you to copy data from the entire chain into a single target image
(which can be located on a different host), [E].
Note
When you cancel an in-progress ‘mirror’ job before the source and
target are synchronized, block-job-cancel
will emit the event
BLOCK_JOB_CANCELLED
. However, note that if you cancel a
‘mirror’ job after it has indicated (via the event
BLOCK_JOB_READY
) that the source and target have reached
synchronization, then the event emitted by block-job-cancel
changes to BLOCK_JOB_COMPLETED
.
Besides the ‘mirror’ job, the “active block-commit
” is the only
other block device job that emits the event BLOCK_JOB_READY
.
The rest of the block device jobs (‘stream’, “non-active
block-commit
”, and ‘backup’) end automatically.
So there are two possible actions to take, after a ‘mirror’ job has
emitted the event BLOCK_JOB_READY
, indicating that the source and
target have reached synchronization:
Issuing the command
block-job-cancel
(after it emits the eventBLOCK_JOB_COMPLETED
) will create a point-in-time (which is at the time of triggering the cancel command) copy of the entire disk image chain (or only the top-most image, depending on thesync
mode), contained in the target image [E]. One use case for this is live VM migration with non-shared storage.Issuing the command
block-job-complete
(after it emits the eventBLOCK_JOB_COMPLETED
) will adjust the guest device (i.e. live QEMU) to point to the target image, [E], causing all the new writes from this point on to happen there.
About synchronization modes: The synchronization mode determines which part of the disk image chain will be copied to the target. Currently, there are four different kinds:
full
– Synchronize the content of entire disk image chain to the targettop
– Synchronize only the contents of the top-most disk image in the chain to the targetnone
– Synchronize only the new writes from this point on.Note
In the case of
drive-backup
(orblockdev-backup
), the behavior ofnone
synchronization mode is different. Normally, abackup
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. Withsync=none
, it’s only the first part.incremental
– Synchronize content that is described by the dirty bitmap
Note
Refer to the Dirty Bitmaps and Incremental Backup document in the QEMU source
tree to learn about the detailed workings of the incremental
synchronization mode.
QMP invocation for drive-mirror
¶
To copy the contents of the entire disk image chain, from [A] all the
way to [D], to a new target (drive-mirror
will create the destination
file, if it doesn’t already exist), call it [E]:
(QEMU) drive-mirror device=node-D target=e.qcow2 sync=full job-id=job0
{
"execute": "drive-mirror",
"arguments": {
"device": "node-D",
"job-id": "job0",
"target": "e.qcow2",
"sync": "full"
}
}
The "sync": "full"
, from the above, means: copy the entire chain
to the destination.
Following the above, querying for active block jobs will show that a
‘mirror’ job is “ready” to be completed (and QEMU will also emit an
event, BLOCK_JOB_READY
):
(QEMU) query-block-jobs
{
"execute": "query-block-jobs",
"arguments": {}
}
{
"return": [
{
"busy": false,
"type": "mirror",
"len": 21757952,
"paused": false,
"ready": true,
"io-status": "ok",
"offset": 21757952,
"device": "job0",
"speed": 0
}
]
}
And, as noted in the previous section, there are two possible actions at this point:
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" } }
Or, complete the operation and pivot the live QEMU to the target copy:
(QEMU) block-job-complete device=job0
In either of the above cases, if you once again run the query-block-jobs command, there should not be any active block operation.
Comparing ‘commit’ and ‘mirror’: In both then cases, the overlay images can be discarded. However, with ‘commit’, the existing base image will be modified (by updating it with contents from overlays); while in the case of ‘mirror’, a new target image is populated with the data from the disk image chain.
QMP invocation for live storage migration with drive-mirror
+ NBD¶
Live storage migration (without shared storage setup) is one of the most
common use-cases that takes advantage of the drive-mirror
primitive
and QEMU’s built-in Network Block Device (NBD) server. Here’s a quick
walk-through of this setup.
Given the disk image chain:
[A] <-- [B] <-- [C] <-- [D]
Instead of copying content from the entire chain, synchronize only the contents of the top-most disk image (i.e. the active layer), [D], to a target, say, [TargetDisk].
Important
The destination host must already have the contents of the backing
chain, involving images [A], [B], and [C], visible via other means
– whether by cp
, rsync
, or by some storage array-specific
command.)
Sometimes, this is also referred to as “shallow copy” – because only the “active layer”, and not the rest of the image chain, is copied to the destination.
Note
In this example, for the sake of simplicity, we’ll be using the same
localhost
as both source and destination.
As noted earlier, on the destination host the contents of the backing
chain – from images [A] to [C] – are already expected to exist in some
form (e.g. in a file called, Contents-of-A-B-C.qcow2
). Now, on the
destination host, let’s create a target overlay image (with the image
Contents-of-A-B-C.qcow2
as its backing file), to which the contents
of image [D] (from the source QEMU) will be mirrored to:
$ qemu-img create -f qcow2 -b ./Contents-of-A-B-C.qcow2 \
-F qcow2 ./target-disk.qcow2
And start the destination QEMU (we already have the source QEMU running – discussed in the section: Interacting with a QEMU instance) instance, with the following invocation. (As noted earlier, for simplicity’s sake, the destination QEMU is started on the same host, but it could be located elsewhere):
$ ./x86_64-softmmu/qemu-system-x86_64 -display none -no-user-config \
-M q35 -nodefaults -m 512 \
-blockdev node-name=node-TargetDisk,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./target-disk.qcow2 \
-device virtio-blk,drive=node-TargetDisk,id=virtio0 \
-S -monitor stdio -qmp unix:./qmp-sock2,server,nowait \
-incoming tcp:localhost:6666
Given the disk image chain on source QEMU:
[A] <-- [B] <-- [C] <-- [D]
On the destination host, it is expected that the contents of the chain
[A] <-- [B] <-- [C]
are already present, and therefore copy only
the content of image [D].
[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" } } }
[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" } }
[On source QEMU] Then, invoke
drive-mirror
(NB: since we’re runningdrive-mirror
withmode=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" } }
[On source QEMU] Once
drive-mirror
copies the entire data, and the eventBLOCK_JOB_READY
is emitted, issueblock-job-cancel
to gracefully end the synchronization, from source QEMU:(QEMU) block-job-cancel device=job0 { "execute": "block-job-cancel", "arguments": { "device": "job0" } }
[On destination QEMU] Then, stop the NBD server:
(QEMU) nbd-server-stop { "execute": "nbd-server-stop", "arguments": {} }
[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:
Create the QCOW2 overlays, to arrive at a backing chain of desired depth
Create the target image (using
qemu-img
), say,e.qcow2
Attach the above created file (
e.qcow2
), run-time, usingblockdev-add
to QEMUPerform
blockdev-mirror
(use"sync": "full"
to copy the entire chain to the target). And notice the eventBLOCK_JOB_READY
Optionally, query for active block jobs, there should be a ‘mirror’ job ready to be completed
Gracefully complete the ‘mirror’ block device job, and notice the the event
BLOCK_JOB_COMPLETED
Shutdown the guest by issuing the QMP
quit
command so that caches are flushedThen, 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:
Create the QCOW2 overlays, to arrive at a backing chain of desired depth
Create the target image (using
qemu-img
), say,e.qcow2
Attach the above created file (
e.qcow2
), run-time, usingblockdev-add
to QEMUPerform
blockdev-backup
(use"sync": "full"
to copy the entire chain to the target). And notice the eventBLOCK_JOB_COMPLETED
Shutdown the guest, by issuing the QMP
quit
command, so that caches are flushedThen, 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.