Re: [PATCH v4 9/9] migration: introduce snapshot-{save, load, delete} QMP commands

[Markus Armbruster]

Daniel P. Berrangé <berrange@redhat.com> writes:

> savevm, loadvm and delvm are some of the few HMP commands that have never
> been converted to use QMP. The reasons for the lack of conversion are
> that they blocked execution of the event thread, and the semantics
> around choice of disks were ill-defined.
>
> Despite this downside, however, libvirt and applications using libvirt
> have used these commands for as long as QMP has existed, via the
> "human-monitor-command" passthrough command. IOW, while it is clearly
> desirable to be able to fix the problems, they are not a blocker to
> all real world usage.
>
> Meanwhile there is a need for other features which involve adding new
> parameters to the commands. This is possible with HMP passthrough, but
> it provides no reliable way for apps to introspect features, so using
> QAPI modelling is highly desirable.
>
> This patch thus introduces new snapshot-{load,save,delete} commands to
> QMP that are intended to replace the old HMP counterparts. The new
> commands are given different names, because they will be using the new
> QEMU job framework and thus will have diverging behaviour from the HMP
> originals. It would thus be misleading to keep the same name.
>
> While this design uses the generic job framework, the current impl is
> still blocking. The intention that the blocking problem is fixed later.
> None the less applications using these new commands should assume that
> they are asynchronous and thus wait for the job status change event to
> indicate completion.
>
> In addition to using the job framework, the new commands require the
> caller to be explicit about all the block device nodes used in the
> snapshot operations, with no built-in default heuristics in use.
>
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
[...]
> diff --git a/qapi/job.json b/qapi/job.json
> index 280c2f76f1..b2cbb4fead 100644
> --- a/qapi/job.json
> +++ b/qapi/job.json
> @@ -22,10 +22,17 @@
>  #
>  # @amend: image options amend job type, see "x-blockdev-amend" (since 5.1)
>  #
> +# @snapshot-load: snapshot load job type, see "snapshot-load" (since 5.2)
> +#
> +# @snapshot-save: snapshot save job type, see "snapshot-save" (since 5.2)
> +#
> +# @snapshot-delete: snapshot delete job type, see "snapshot-delete" (since 
> 5.2)
> +#
>  # Since: 1.7
>  ##
>  { 'enum': 'JobType',
> -  'data': ['commit', 'stream', 'mirror', 'backup', 'create', 'amend'] }
> +  'data': ['commit', 'stream', 'mirror', 'backup', 'create', 'amend',
> +           'snapshot-load', 'snapshot-save', 'snapshot-delete'] }
>  
>  ##
>  # @JobStatus:
> diff --git a/qapi/migration.json b/qapi/migration.json
> index 675f70bb67..b584c0be31 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -1720,3 +1720,123 @@
>  ##
>  { 'event': 'UNPLUG_PRIMARY',
>    'data': { 'device-id': 'str' } }
> +
> +##
> +# @snapshot-save:
> +#
> +# Save a VM snapshot
> +#
> +# @job-id: identifier for the newly created job
> +# @tag: name of the snapshot to create
> +# @devices: list of block device node names to save a snapshot to

Looks like you dropped the idea to also accept drive IDs.  Is that for
good, or would you like to add it later?

> +# @vmstate: block device node name to save vmstate to
> +#
> +# Applications should not assume that the snapshot save is complete
> +# when this command returns. The job commands / events must be used
> +# to determine completion and to fetch details of any errors that arise.
> +#
> +# Note that the VM CPUs will be paused during the time it takes to
> +# save the snapshot

End the sentence with a period, please.

> +#
> +# It is strongly recommended that @devices contain all writable
> +# block device nodes if a consistent snapshot is required.

If it doesn't, the snapshot is partial, and a consistent restore from a
partial snapshot is generally impossible.  The comment is okay as is.

> +#
> +# If @tag already exists, an error will be reported
> +#
> +# Returns: nothing
> +#
> +# Example:
> +#
> +# -> { "execute": "snapshot-save",
> +#      "data": {
> +#         "job-id": "snapsave0",
> +#         "tag": "my-snap",
> +#         "vmstate": "disk0",
> +#         "devices": ["disk0", "disk1"]
> +#      }
> +#    }
> +# <- { "return": { } }
> +#
> +# Since: 5.2
> +##
> +{ 'command': 'snapshot-save',
> +  'data': { 'job-id': 'str',
> +            'tag': 'str',
> +            'vmstate': 'str',
> +            'devices': ['str'] } }
> +
> +##
> +# @snapshot-load:
> +#
> +# Load a VM snapshot
> +#
> +# @job-id: identifier for the newly created job
> +# @tag: name of the snapshot to load.
> +# @devices: list of block device node names to load a snapshot from
> +# @vmstate: block device node name to load vmstate from
> +#
> +# Applications should not assume that the snapshot save is complete
> +# when this command returns. The job commands / events must be used
> +# to determine completion and to fetch details of any errors that arise.
> +#
> +# Note that the VM CPUs will be paused during the time it takes to
> +# save the snapshot
> +#
> +# It is strongly recommended that @devices contain all writable
> +# block device nodes that can have changed since the original
> +# @snapshot-save command execution.
> +#
> +# Returns: nothing
> +#
> +# Example:
> +#
> +# -> { "execute": "snapshot-load",
> +#      "data": {
> +#         "job-id": "snapload0",
> +#         "tag": "my-snap",
> +#         "vmstate": "disk0",
> +#         "devices": ["disk0", "disk1"]
> +#      }
> +#    }
> +# <- { "return": { } }
> +#
> +# Since: 5.2
> +##
> +{ 'command': 'snapshot-load',
> +  'data': { 'job-id': 'str',
> +            'tag': 'str',
> +            'vmstate': 'str',
> +            'devices': ['str'] } }
> +
> +##
> +# @snapshot-delete:
> +#
> +# Delete a VM snapshot
> +#
> +# @job-id: identifier for the newly created job
> +# @tag: name of the snapshot to delete.
> +# @devices: list of block device node names to delete a snapshot from
> +#
> +# Applications should not assume that the snapshot save is complete
> +# when this command returns. The job commands / events must be used
> +# to determine completion and to fetch details of any errors that arise.
> +#
> +# Returns: nothing
> +#
> +# Example:
> +#
> +# -> { "execute": "snapshot-delete",
> +#      "data": {
> +#         "job-id": "snapdelete0",
> +#         "tag": "my-snap",
> +#         "devices": ["disk0", "disk1"]
> +#      }
> +#    }
> +# <- { "return": { } }
> +#
> +# Since: 5.2
> +##
> +{ 'command': 'snapshot-delete',
> +  'data': { 'job-id': 'str',
> +            'tag': 'str',
> +            'devices': ['str'] } }
[...]