Features/ImageStreamingAPI: Difference between revisions
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=Changelog= | =Changelog= | ||
v5: | |||
* Change non-existent 'NotSupported' error to 'Unsupported' | |||
v4: | v4: | ||
* Rename block_stream_set_speed to block_job_set_speed, slight semantics change | * Rename block_stream_set_speed to block_job_set_speed, slight semantics change | ||
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DeviceInUse: streaming is already active on this device | DeviceInUse: streaming is already active on this device | ||
DeviceNotFound: device name is invalid | DeviceNotFound: device name is invalid | ||
Unsupported: image streaming is not supported by this device | |||
Events: | Events: | ||
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Errors: | Errors: | ||
Unsupported: job type does not support speed setting | |||
DeviceNotActive: streaming is not active on this device | DeviceNotActive: streaming is not active on this device | ||
Revision as of 12:12, 3 August 2011
Changelog
v5:
- Change non-existent 'NotSupported' error to 'Unsupported'
v4:
- Rename block_stream_set_speed to block_job_set_speed, slight semantics change
- Add 'speed' value to stream info object
- Clarify BLOCK_JOB_COMPLETED 'error' field semantics
v3:
- Rename cancel and info operations to make a reusable block job API
- Rename offset/size to position/end and make their units undefined for block job API
- Add block_stream_set_speed, similar to migrate_set_speed
v2:
- Remove iteration interface where management tool drives individual copy iterations
- Add block_stream_cancel command (like migrate_cancel)
- Add 'base' common backing file argument to block_stream
- Replace QError object in BLOCK_STREAM_COMPLETED with an error string
- Add error documentation
Image streaming API
The stream commands populate an image file by streaming data from its backing file. Once all blocks have been streamed, the dependency on the original backing image is removed. The stream commands can be used to implement post-copy live block migration and rapid deployment.
The block_stream command starts streaming the image file. Streaming is performed in the background while the guest is running.
When streaming completes successfully or with an error, the BLOCK_JOB_COMPLETED event is raised.
The progress of a streaming operation can be polled using query-block-jobs. This returns information regarding how much of the image has been streamed for each active streaming operation.
The block_job_cancel command stops streaming the image file. The image file retains its backing file. A new streaming operation can be started at a later time.
The block_job_set_speed command adjusts the rate limit for a streaming operation. This can be used to limit the impact on storage performance during streaming.
The command synopses are as follows:
block_stream ------------ Copy data from a backing file into a block device. The block streaming operation is performed in the background until the entire backing file has been copied. This command returns immediately once streaming has started. The status of ongoing block streaming operations can be checked with query-block-jobs. The operation can be stopped before it has completed using the block_job_cancel command. If a base file is specified then sectors are not copied from that base file and its backing chain. When streaming completes the image file will have the base file as its backing file. This can be used to stream a subset of the backing file chain instead of flattening the entire image. On successful completion the image file is updated to drop the backing file. Arguments: - device: device name (json-string) - base: common backing file (json-string, optional) Errors: DeviceInUse: streaming is already active on this device DeviceNotFound: device name is invalid Unsupported: image streaming is not supported by this device Events: On completion the BLOCK_JOB_COMPLETED event is raised with the following fields: - type: job type ("stream" for image streaming, json-string) - device: device name (json-string) - end: maximum progress value (json-int) - position: current progress value (json-int) - speed: rate limit, bytes per second (json-int) - error: error message (json-string, only on error) The completion event is raised both on success and on failure. On success position is equal to end. On failure position and end can be used to indicate at which point the operation failed. On failure the error field contains a human-readable error message. There are no semantics other than that streaming has failed and clients should not try to interpret the error string. Examples: -> { "execute": "block_stream", "arguments": { "device": "virtio0" } } <- { "return": {} } block_job_set_speed ------------------- Set maximum speed for a background block operation. This command can only be issued when there is an active block job. Throttling can be disabled by setting the speed to 0. Arguments: - device: device name (json-string) - value: maximum speed, in bytes per second (json-int) Errors: Unsupported: job type does not support speed setting DeviceNotActive: streaming is not active on this device Example: -> { "execute": "block_job_set_speed", "arguments": { "device": "virtio0", "value": 1024 } } block_job_cancel ---------------- Stop an active block streaming operation. This command returns once the active block streaming operation has been stopped. It is an error to call this command if no operation is in progress. The image file retains its backing file unless the streaming operation happens to complete just as it is being cancelled. A new block streaming operation can be started at a later time to finish copying all data from the backing file. Arguments: - device: device name (json-string) Errors: DeviceNotActive: streaming is not active on this device DeviceInUse: cancellation already in progress Examples: -> { "execute": "block_job_cancel", "arguments": { "device": "virtio0" } } <- { "return": {} } query-block-jobs ---------------- Show progress of ongoing block device operations. Return a json-array of all block device operations. If no operation is active then return an empty array. Each operation is a json-object with the following data: - type: job type ("stream" for image streaming, json-string) - device: device name (json-string) - end: maximum progress value (json-int) - position: current progress value (json-int) - speed: rate limit, bytes per second (json-int) Progress can be observed as position increases and it reaches end when the operation completes. Position and end have undefined units but can be used to calculate a percentage indicating the progress that has been made. Example: -> { "execute": "query-block-jobs" } <- { "return":[ { "type": "stream", "device": "virtio0", "end": 10737418240, "position": 709632, "speed": 0 } ] }
How live block copy works
Live block copy does the following:
- Create and switch to the destination file: snapshot_blkdev virtio-blk0 destination.$fmt $fmt
- Stream the base into the image file: block_stream -a virtio-blk0