Added

• #4145: Added support for per net device metrics. In addition to aggregate metrics net, each individual net device will emit metrics under the label "net_{iface_id}". E.g. the associated metrics for the endpoint "/network-interfaces/eth0" will be available under "net_eth0" in the metrics json object.
• #4202: Added support for per block device metrics. In addition to aggregate metrics block, each individual block device will emit metrics under the label "block_{drive_id}". E.g. the associated metrics for the endpoint "/drives/{drive_id}" will be available under "block_drive_id" in the metrics json object.
• #4205: Added a new vm-state subcommand to info-vmstate command in the snapshot-editor tool to print MicrovmState of vmstate snapshot file in a readable format. Also made the vcpu-states subcommand available on x86_64.
• #4063: Added source-level instrumentation based tracing. See tracing for more details.
• #4138, #4170, #4223, #4247, #4226: Added developer preview only (NOT for production use) support for vhost-user block devices. Firecracker implements a vhost-user frontend. Users are free to choose from existing open source backend solutions or their own implementation. Known limitation: snapshotting is not currently supported for microVMs containing vhost-user block devices. See the related doc page for details. The device emits metrics under the label "vhost_user_{device}_{drive_id}".

Changed

• #4309: The jailer’s option --parent-cgroup will move the process to that cgroup if no cgroup options are provided.
• Simplified and clarified the removal policy of deprecated API elements to follow semantic versioning 2.0.0. For more information, please refer to this GitHub discussion.
• #4180: Refactored error propagation to avoid logging and printing an error on exits with a zero exit code. Now, on successful exit “Firecracker exited successfully” is logged.
• #4194: Removed support for creating Firecracker snapshots targeting older versions of Firecracker. With this change, running ‘firecracker –version’ will not print the supported snapshot versions.
• #4301: Allow merging of diff snapshots into base snapshots by directly writing the diff snapshot on top of the base snapshot’s memory file. This can be done by setting the mem_file_path to the path of the pre-existing full snapshot.

Deprecated

• #4209: rebase-snap tool is now deprecated. Users should use snapshot-editor for rebasing diff snapshots.

Fixed

• #4171: Fixed a bug that ignored the --show-log-origin option, preventing it from printing the source code file of the log messages.
• #4178: Fixed a bug reporting a non-zero exit code on successful shutdown when starting Firecracker with --no-api.
• #4261: Fixed a bug where Firecracker would log “RunWithApiError error: MicroVMStopped without an error: GenericError” when exiting after encountering an emulation error. It now correctly prints “RunWithApiError error: MicroVMStopped with an error: GenericError”.
• #4242: Fixed a bug introduced in #4047 that limited the --level option of logger to Pascal-cased values (e.g. accepting “Info”, but not “info”). It now ignores case again.
• #4286: Fixed a bug in the asynchronous virtio-block engine that rendered the device non-functional after a PATCH request was issued to Firecracker for updating the path to the host-side backing file of the device.
• #4301: Fixed a bug where if Firecracker was instructed to take a snapshot of a microvm which itself was restored from a snapshot, specifying mem_file_path to be the path of the memory file from which the microvm was restored would result in both the microvm and the snapshot being corrupted. It now instead performs a “write-back” of all memory that was updated since the snapshot was originally loaded.