This document describes how Firecracker persists microVM state into Firecracker snapshots. It describes the snapshot format, encoding, compatibility and limitations.
Firecracker uses the serde crate 1 along with the bincode 2 format to serialize its state into Firecracker snapshots. Firecracker snapshots have versions that are independent of Firecracker versions. Each Firecracker version declares support for a specific snapshot data format version. When creating a snapshot, Firecracker will use the supported snapshot format version. When loading a snapshot, Firecracker will check that format of the snapshot file is compatible with the snapshot version Firecracker supports.
Firecracker persists the microVM state as 2 separate objects:
- a guest memory file
- a microVM state file.
The block devices attached to the microVM are not considered part of the state and need to be managed separately.
The guest memory file contains the microVM memory saved as a dump of all pages.
In the VM state file, Firecracker stores the internal state of the VMM (device emulation, KVM and vCPUs) with 2 exceptions - serial emulation and vsock backend.
While we continuously improve and extend Firecracker's features by adding new capabilities, devices or enhancements, the microVM state file may change both structurally and semantically with each new release.
A Firecracker snapshot has the following format:
| Field | Bits | Description |
|---|---|---|
| magic_id | 64 | Firecracker snapshot and architecture (x86_64/aarch64). |
| version | M | The snapshot data format version (MAJOR.MINOR.PATCH) |
| state | N | Bincode blob containing the microVM state. |
| crc | 64 | Optional CRC64 sum of magic_id, version and state fields. |
The snapshot format has its own version encoded in the snapshot file itself
after the snapshot's magic_id. The snapshot format version is independent of
the Firecracker version and it is of the form MAJOR.MINOR.PATCH.
Currently, Firecracker uses the
Serde bincode encoder for serializing the
microVM state. The encoding format that bincode uses does not allow backwards
compatible changes in the state, so essentially every change in the microVM
state description will result in bump of the format's MAJOR version. If the
needs arises, we will look into alternative formats that allow more flexibility
with regards to backwards compatibility. If/when this happens, we will define
how changes in the snapshot format reflect to changes in its MAJOR.MINOR.PATCH
version.
During research and prototyping we considered multiple storage formats. The criteria used for comparing these are: performance, size, rust support, specification, versioning support, community and tooling. Performance, size and Rust support are hard requirements while all others can be the subject of trade offs. More info about this comparison can be found here.
Key benefits of using bincode:
- Minimal snapshot size overhead
- Minimal CPU overhead
- Simple implementation
The current implementation relies on the Serde bincode encoder.
Snapshots can be saved and restored on the same kernel version without any issues. There might be issues when restoring snapshots created on different host kernel version even when using the same Firecracker version.
SnapshotCreate and SnapshotLoad operations across different host kernels is considered unstable in Firecracker as the saved KVM state might have different semantics on different kernels.
The current Firecracker devices are backwards compatible up to the version that introduces them. Ideally this property would be kept over time, but there are situations when a new version of a device exposes new features to the guest that do not exist in an older version. In such cases restoring a snapshot at an older version becomes impossible without breaking the guest workload.
The microVM state file links some resources that are external to the snapshot:
- tap devices by device name,
- block devices by block file path,
- vsock backing Unix domain socket by socket name.
To successfully restore a microVM one should check that:
- tap devices are available, their names match their original names since these are the values saved in the microVM state file, and they are accessible to the Firecracker process where the microVM is being restored,
- block devices are set up at their original relative or absolute paths with the proper permissions, as the Firecracker process with the restored microVM will attempt to access them exactly as they were accessed in the original Firecracker process,
- the vsock backing Unix domain socket is available, its name matches the original name, and it is accessible to the new Firecracker process.
Firecracker microVMs snapshot functionality is available for Intel/AMD/ARM64 CPU models that support the hardware virtualizations extensions, more details are available here. Snapshots are not compatible across CPU architectures and even across CPU models of the same architecture. They are only compatible if the CPU features exposed to the guest are an invariant when saving and restoring the snapshot. The trivial scenario is creating and restoring snapshots on hosts that have the same CPU model.
Restoring from an Intel snapshot on AMD (or vice-versa) is not supported.
It is important to note that guest workloads can still execute instructions that are being masked by CPUID and restoring and saving of such workloads will lead to undefined result. Firecracker retrieves the state of a discrete list of MSRs from KVM, more specifically, the MSRs corresponding to the guest exposed features.
The microVM state file format is implemented in the snapshot crate in the Firecracker repository. All Firecracker devices implement the Persist trait which exposes an interface that enables creating from and saving to the microVM state.