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Firecracker Snapshotting

Table of Contents

About microVM snapshotting

MicroVM snapshotting is a mechanism through which a running microVM and its resources can be serialized and saved to an external medium in the form of a snapshot. This snapshot can be later used to restore a microVM with its guest workload at that particular point in time.

Snapshotting in Firecracker

Supported platforms

The Firecracker snapshot feature is supported on all CPU micro-architectures listed in README.

Overview

A Firecracker microVM snapshot can be used for loading it later in a different Firecracker process, and the original guest workload is being simply resumed.

The original guest which the snapshot is created from, should see no side effects from this process (other than the latency introduced by the snapshot creation process).

Both network and vsock packet loss can be expected on guests that are resumed from snapshots in another Firecracker process. It is also not guaranteed that the state of the network connections survives the process. Furthermore, vsock connections that are open when the snapshot is taken are closed, but existing vsock listen sockets in the guest still remain active and can accept new connections after resume (see Vsock device reset).

In order to make restoring possible, Firecracker snapshots save the full state of the following resources:

  • the guest memory,
  • the emulated HW state (both KVM and Firecracker emulated HW).

The state of the components listed above is generated independently, which brings flexibility to our snapshotting support. This means that taking a snapshot results in multiple files that are composing the full microVM snapshot:

  • the guest memory file,
  • the microVM state file,
  • zero or more disk files (depending on how many the guest had; these are managed by the users).

The design allows sharing of memory pages and read only disks between multiple microVMs. When loading a snapshot, instead of loading at resume time the full contents from file to memory, Firecracker creates a MAP_PRIVATE mapping of the memory file, resulting in runtime on-demand loading of memory pages. Any subsequent memory writes go to a copy-on-write anonymous memory mapping. This has the advantage of very fast snapshot loading times, but comes with the cost of having to keep the guest memory file around for the entire lifetime of the resumed microVM.

Snapshot files management

The Firecracker snapshot design offers a very simple interface to interact with snapshots but provides no functionality to package or manage them on the host.

The threat containment model states that the host, host/API communication and snapshot files are trusted by Firecracker.

To ensure a secure integration with the snapshot functionality, users need to secure snapshot files by implementing authentication and encryption schemes while managing their lifecycle or moving them across the trust boundary, like for example when provisioning them from a repository to a host over the network.

Firecracker is optimized for fast load/resume, and it's designed to do some very basic sanity checks only on the vm state file. It only verifies integrity using a 64-bit CRC value embedded in the vm state file, but this is only a partial measure to protect against accidental corruption, as the disk files and memory file need to be secured as well. It is important to note that CRC computation is validated before trying to load the snapshot. Should it encounter failure, an error will be shown to the user and the Firecracker process will be terminated.

Performance

The Firecracker snapshot create/resume performance depends on the memory size, vCPU count and emulated devices count. The Firecracker CI runs snapshot tests on all supported platforms.

Developer preview status

Diff snapshots are still in developer preview while we are diving deep into how the feature can be combined with guest_memfd support in Firecracker.

Limitations

  • High snapshot restoration latency when cgroups V1 are in use. We strongly recommend to deploy snapshots on cgroups V2 enabled hosts for the implied kernel versions - related issue.
  • Guest network connectivity is not guaranteed to be preserved after resume. For recommendations related to guest network connectivity for clones please see Network connectivity for clones.
  • Snapshotting on arm64 works for both GICv2 and GICv3 enabled guests. However, restoring between different GIC version is not possible.
  • If a CPU template is not used on x86_64, overwrites of MSR_IA32_TSX_CTRL MSR value will not be preserved after restoring from a snapshot.
  • Resuming from a snapshot that was taken during early stages of the guest kernel boot might lead to crashes upon snapshot resume. We suggest that users take snapshot after the guest microVM kernel has booted. Please see VMGenID device limitation.

Firecracker Snapshotting characteristics

  • Fresh Firecracker microVMs are booted using anonymous memory, while microVMs resumed from snapshot load memory on-demand from the snapshot and copy-on-write to anonymous memory.
  • Resuming from a snapshot is optimized for speed, while taking a snapshot involves some extra CPU cycles for synchronously writing memory pages to the memory snapshot file. Taking a full snapshot of a microVM results in the full contents of guest memory being written to the snapshot, and particularly, in all guest memory being faulted in.
  • The memory file and microVM state file are generated by Firecracker on snapshot creation. The disk contents are not explicitly flushed to their backing files.
  • The API calls exposing the snapshotting functionality have clear Prerequisites that describe the requirements on when/how they should be used.
  • The Firecracker microVM's MMDS config is included in the snapshot. However, the data store is not persisted across snapshots.
  • Configuration information for metrics and logs are not saved to the snapshot. These need to be reconfigured on the restored microVM.
  • On x86_64, if a vCPU has MSR_IA32_TSC_DEADLINE set to 0 when a snapshot is taken, Firecracker replaces it with the MSR_IA32_TSC value from the same vCPU. This is to guarantee that the vCPU will continue receiving TSC interrupts after restoring from the snapshot even if an interrupt is lost when taking a snapshot.

Snapshot versioning

The microVM state snapshot file uses a data format that has a version in the form of MAJOR.MINOR.PATCH. Each Firecracker binary supports a fixed version of the snapshot data format. When creating a snapshot, Firecracker will use the supported data format version. When loading snapshots, Firecracker will check that the snapshot version is compatible with the version it supports. More information about the snapshot data format and details about snapshot data format versions can be found at versioning.

Snapshot API

Firecracker exposes the following APIs for manipulating snapshots: Pause, Resume and CreateSnapshot can be called only after booting the microVM, while LoadSnapshot is allowed only before boot.

Pausing the microVM

To create a snapshot, first you have to pause the running microVM and its vCPUs with the following API command:

curl --unix-socket /tmp/firecracker.socket -i \
    -X PATCH 'http://localhost/vm' \
    -H 'Accept: application/json' \
    -H 'Content-Type: application/json' \
    -d '{
            "state": "Paused"
    }'

Prerequisites: The microVM is booted. Successive calls of this request keep the microVM in the Paused state. Effects:

  • on success: microVM is guaranteed to be Paused.
  • on failure: no side-effects.

Creating snapshots

Warning

Diff snapshot support is in developer preview. See this section for more info.

Now that the microVM is paused, you can create a snapshot, which can be either a fullone or a diff one. Full snapshots always create a complete, resume-able snapshot of the current microVM state and memory. Diff snapshots save at least the current microVM state and the memory accessed since the last snapshot (full or diff) in a sparse file (but they might include more pages than strictly needed due to technical limitation in Firecracker's ability to accurately track accesses). Diff snapshots are generally not resume-able, but must be merged with a base snapshot into a full snapshot. The exception here are diff snapshots of booted VMs, which are immediately resumable. In this context, we will refer to the base as the first memory file created by a /snapshot/create API call and the layer as a memory file created by a subsequent /snapshot/create API call. The order in which the snapshots were created matters and they should be merged in the same order in which they were created. To merge a diff snapshot memory file on top of a base, users should copy its content over the base. This can be done using the rebase-snap (deprecated) or snapshot-editor tools provided with the firecracker release:

snapshot-editor edit-memory rebase \
     --memory-path path/to/base \
     --diff-path path/to/layer

After executing the command above, the base would be a resumable snapshot memory file describing the state of the memory at the moment of creation of the layer. More layers which were created later can be merged on top of this base.

This process needs to be repeated for each layer until the one describing the desired memory state is merged on top of the base, which is constantly updated with information from previously merged layers. Please note that users should not merge state files which resulted from /snapshot/create API calls and they should use the state file created in the same call as the memory file which was merged last on top of the base.

Creating full snapshots

For creating a full snapshot, you can use the following API command:

curl --unix-socket /tmp/firecracker.socket -i \
    -X PUT 'http://localhost/snapshot/create' \
    -H  'Accept: application/json' \
    -H  'Content-Type: application/json' \
    -d '{
            "snapshot_type": "Full",
            "snapshot_path": "./snapshot_file",
            "mem_file_path": "./mem_file"
    }'

Details about the required and optional fields can be found in the swagger definition.

Note: If the files indicated by snapshot_path and mem_file_path don't exist at the specified paths, then they will be created right before generating the snapshot. If they exist, the files will be truncated and overwritten.

Prerequisites: The microVM is Paused.

Effects:

  • on success:

    • The file indicated by snapshot_path (e.g. /path/to/snapshot_file) contains the devices' model state and emulation state. The one indicated by mem_file_path(e.g. /path/to/mem_file) contains a full copy of the guest memory.
    • The generated snapshot files are immediately available to be used (current process releases ownership). At this point, the block devices backing files should be backed up externally by the user. Please note that block device contents are only guaranteed to be committed/flushed to the host FS, but not necessarily to the underlying persistent storage (could still live in host FS cache).
    • If dirty page tracking is enabled, the snapshot creation resets then the dirtied page bitmap and marks all pages clean (from a dirty page tracking point of view).

  • on failure: no side-effects.

Notes:

  • The separate block device file components of the snapshot have to be handled by the user.

Creating diff snapshots

For creating a diff snapshot, you should use the same API command, but with snapshot_type field set to Diff.

Note: If not specified, snapshot_type is by default Full.

curl --unix-socket /tmp/firecracker.socket -i \
    -X PUT 'http://localhost/snapshot/create' \
    -H  'Accept: application/json' \
    -H  'Content-Type: application/json' \
    -d '{
            "snapshot_type": "Diff",
            "snapshot_path": "./snapshot_file",
            "mem_file_path": "./mem_file",
    }'

Prerequisites: The microVM is Paused.

Diff snapshots come in two flavors. If track_dirty_pages was set to true when configuring the /machine-config resource or when restoring from a snapshot via /snapshot/load, Firecracker will use KVM's dirty page log runtime functionality to ensure the diff snapshot only contains exactly pages that were written to since boot / snapshot restoration. If track_dirty_pages is not enabled, Firecracker uses the mincore(2) syscall to determine which pages to include in the snapshot. As such, this mode of snapshot taking will only work if swap is disabled, as mincore does not consider pages written to swap to be "in core". This potentially results in bigger memory files (although they are still sparse), but avoids the runtime overhead of dirty page logging.

Note: Dirty page tracking negates most of the benefits of huge pages.

Effects:

  • on success:
    • The file indicated by snapshot_path contains the devices' model state and emulation state, same as when creating a full snapshot. The one indicated by mem_file_path contains this time a diff copy of the guest memory; the diff consists of the memory pages which have been dirtied since the last snapshot creation or since the creation of the microVM, whichever of these events was the most recent.
    • All the other effects mentioned in the Effects paragraph from Creating full snapshots section apply here.
  • on failure: no side-effects.

Note: This is an example of an API command that enables dirty page tracking:

curl --unix-socket /tmp/firecracker.socket -i  \
    -X PUT 'http://localhost/machine-config' \
    -H 'Accept: application/json'            \
    -H 'Content-Type: application/json'      \
    -d '{
            "vcpu_count": 2,
            "mem_size_mib": 1024,
            "smt": false,
            "track_dirty_pages": true
    }'

Enabling this support enables KVM dirty page tracking, so it comes at a cost (which consists of CPU cycles spent by KVM accounting for dirtied pages); it should only be used when needed.

Creating a snapshot has some minor effects on the currently running microVM:

  • The vsock device is reset, causing the driver to terminate connection on resumption.
  • On x86_64, a notification for KVM-clock is injected to notify the guest about being paused.

Resuming the microVM

You can resume the microVM by sending the following API command:

curl --unix-socket /tmp/firecracker.socket -i \
    -X PATCH 'http://localhost/vm' \
    -H 'Accept: application/json' \
    -H 'Content-Type: application/json' \
    -d '{
            "state": "Resumed"
    }'

Prerequisites: The microVM is Paused. Successive calls of this request are ignored (microVM remains in the running state). Effects:

  • on success: microVM is guaranteed to be Resumed.
  • on failure: no side-effects.

Loading snapshots

If you want to load a snapshot, you can do that only before the microVM is configured (the only resources that can be configured prior are the logger and the metrics systems) by sending the following API command:

curl --unix-socket /tmp/firecracker.socket -i \
    -X PUT 'http://localhost/snapshot/load' \
    -H  'Accept: application/json' \
    -H  'Content-Type: application/json' \
    -d '{
            "snapshot_path": "./snapshot_file",
            "mem_backend": {
                "backend_path": "./mem_file",
                "backend_type": "File"
            },
            "track_dirty_pages": true,
            "resume_vm": false
    }'

The backend_type field represents the memory backend type used for loading the snapshot. Accepted values are:

  • File - rely on the kernel to handle page faults when loading the contents of the guest memory file into memory.
  • Uffd - use a dedicated user space process to handle page faults that occur for the guest memory range. Please refer to this for more details on handling page faults in the user space.

The meaning of backend_path depends on the backend_type chosen:

  • if using File, then backend_path should contain the path to the snapshot's memory file to be loaded.
  • when using Uffd, backend_path refers to the path of the unix domain socket used for communication between Firecracker and the user space process that handles page faults.

When relying on the OS to handle page faults, the command below is also accepted. Note that mem_file_path field is currently under the deprecation policy. mem_file_path and mem_backend are mutually exclusive, therefore specifying them both at the same time will return an error.

curl --unix-socket /tmp/firecracker.socket -i \
    -X PUT 'http://localhost/snapshot/load' \
    -H  'Accept: application/json' \
    -H  'Content-Type: application/json' \
    -d '{
            "snapshot_path": "./snapshot_file",
            "mem_file_path": "./mem_file",
            "track_dirty_pages": true,
            "resume_vm": false
    }'

Details about the required and optional fields can be found in the swagger definition.

Prerequisites: A full memory snapshot and a microVM state file must be provided. The disk backing files, network interfaces backing TAPs and/or vsock backing socket that were used for the original microVM's configuration should be set up and accessible to the new Firecracker process (in which the microVM is resumed). These host-resources need to be accessible at the same relative paths to the new Firecracker process as they were to the original one.

Effects:

  • on success:
    • The complete microVM state is loaded from snapshot into the current Firecracker process.
    • It then resets the dirtied page bitmap and marks all pages clean (from a diff snapshot point of view).
    • The loaded microVM is now in the Paused state, so it needs to be resumed for it to run.
    • The memory file (pointed by backend_path when using File backend type, or pointed by mem_file_path) must be considered immutable from Firecracker and host point of view. It backs the guest OS memory for read access through the page cache. External modification to this file corrupts the guest memory and leads to undefined behavior.
    • The file indicated by snapshot_path, that is used to load from, is released and no longer used by this process.
    • If track_dirty_pages is set, subsequent diff snapshots will be based on KVM dirty page tracking.
    • If resume_vm is set, the vm is automatically resumed if load is successful.
  • on failure: A specific error is reported and then the current Firecracker process is ended (as it might be in an invalid state).

Notes: The track_dirty_pages configuration is not saved when creating a snapshot, so you need to explicitly set track_dirty_pages again when sending the LoadSnapshot command if you want to be able to do dirty page tracking based diff snapshots from a loaded microVM.

It is also worth knowing, a microVM that is restored from snapshot will be resumed with the guest OS wall-clock continuing from the moment of the snapshot creation. For this reason, the wall-clock should be updated to the current time, on the guest-side. More details on how you could do this can be found at a related FAQ.

Provisioning host disk space for snapshots

Depending on VM memory size, snapshots can consume a lot of disk space. Firecracker integrators must ensure that the provisioned disk space is sufficient for normal operation of their service as well as during failure scenarios. If the service exposes the snapshot triggers to customers, integrators must enforce proper disk quotas to avoid any DoS threats that would cause the service to fail or function abnormally.

Ensure continued network connectivity for clones

For recommendations related to continued network connectivity for multiple clones created from a single Firecracker microVM snapshot please see this doc.

Snapshot security and uniqueness

When snapshots are used in a such a manner that a given guest's state is resumed from more than once, guest information assumed to be unique may in fact not be; this information can include identifiers, random numbers and random number seeds, the guest OS entropy pool, as well as cryptographic tokens. Without a strong mechanism that enables users to guarantee that unique things stay unique across snapshot restores, we consider resuming execution from the same state more than once insecure.

For more information please see this doc

Usage examples

Example 1: secure usage

Boot microVM A -> ... -> Create snapshot S -> Terminate
                                           -> Load S in microVM B -> Resume -> ...

Here, microVM A terminates after creating the snapshot without ever resuming work, and a single microVM B resumes execution from snapshot S. In this case, unique identifiers, random numbers, and cryptographic tokens that are meant to be used once are indeed only used once. In this example, we consider microVM B secure.

Example 2: potentially insecure usage

Boot microVM A -> ... -> Create snapshot S -> Resume -> ...
                                           -> Load S in microVM B -> Resume -> ...

Here, both microVM A and B do work starting from the state stored in snapshot S. Unique identifiers, random numbers, and cryptographic tokens that are meant to be used once may be used twice. It doesn't matter if microVM A is terminated before microVM B resumes execution from snapshot S or not. In this example, we consider both microVMs insecure as soon as microVM A resumes execution.

Example 3: potentially insecure usage

Boot microVM A -> ... -> Create snapshot S -> ...
                                           -> Load S in microVM B -> Resume -> ...
                                           -> Load S in microVM C -> Resume -> ...
                                           [...]

Here, both microVM B and C do work starting from the state stored in snapshot S. Unique identifiers, random numbers, and cryptographic tokens that are meant to be used once may be used twice. It doesn't matter at which points in time microVMs B and C resume execution, or if microVM A terminates or not after the snapshot is created. In this example, we consider microVMs B and C insecure, and we also consider microVM A insecure if it resumes execution.

Reusing snapshotted states securely

Virtual Machine Generation Identifier (VMGenID) is a virtual device that allows VM guests to detect when they have resumed from a snapshot. It works by exposing a cryptographically random 16-bytes identifier to the guest. The VMM ensures that the value of the identifier changes every time the VM a time shift happens in the lifecycle of the VM, e.g. when it resumes from a snapshot.

Linux supports VMGenID since version 5.18 for systems with ACPI support. Linux 6.10 added support also for systems that use DeviceTree instead of ACPI. When Linux detects a change in the identifier, it uses its value to reseed its internal PRNG.

Firecracker supports VMGenID device both on x86 and Aarch64 platforms. Firecracker will always enable the device. During snapshot resume, Firecracker will update the 16-byte generation ID and inject a notification in the guest before resuming its vCPUs.

As a result, guests that run Linux versions >= 5.18 will re-seed their in-kernel PRNG upon snapshot resume. User space applications can rely on the guest kernel for randomness. State other than the guest kernel entropy pool, such as unique identifiers, cached random numbers, cryptographic tokens, etc will still be replicated across multiple microVMs resumed from the same snapshot. Users need to implement mechanisms for ensuring de-duplication of such state, where needed.

Vsock device reset

The vsock device is reset across snapshot/restore to avoid inconsistent state between device and driver leading to breakage (#2218). This is done by sending a VIRTIO_VSOCK_EVENT_TRANSPORT_RESET event to the guest driver during SnapshotCreate (#2562). On SnapshotResume, when the VM becomes active again, the vsock driver closes all existing connections. Existing listen sockets still remain active, but their CID is updated to reflect the current guest_cid. More details about this event can be found in the official Virtio document here.

VMGenID device limitation

During snashot resume, Firecracker updates the 16-byte generation ID of the VMGenID device and injects an interrupt in the guest before resuming vCPUs. If the snapshot was taken at the very early stages of the guest kernel boot process proper interrupt handling might not be in place yet. As a result, the kernel might not be able to handle the injected notification and crash. We suggest to users that they take snapshots only after the guest kernel has completed booting, to avoid this issue.

Where can I resume my snapshots?

Snapshots must be resumed on an software and hardware configuration which is identical to what they were generated on. However, in limited cases, snapshots can be resumed on identical hardware instances where they were taken on, but using newer host kernel versions. While we do not provide any guarantees on this setup (and do not recommend doing this in production), we are currently aware of the compatibility table reported below:

.metal instance type taken on host kernel restored on host kernel
{c5n,m5n,m6i,m6a} 5.10 6.1

For example, a snapshot taken on a m6i.metal host running a 5.10 host kernel can be restored on a different m6i.metal host running a 6.1 host kernel (but not vice versa), but could not be restored on a c5n.metal host.