Domain state capture using Libvirt

In order to aid application developers to choose which operations best suit their needs, this page compares the different means for capturing state related to a domain managed by libvirt.

The information here is primarily geared towards capturing the state of an active domain. Capturing the state of an inactive domain essentially amounts to copying the contents of guest disks, followed by a fresh boot of the same domain configuration with disks restored back to that saved state.

State capture trade-offs

One of the features made possible with virtual machines is live migration -- transferring all state related to the guest from one host to another with minimal interruption to the guest's activity. In this case, state includes domain memory (including register and device contents), and domain storage (whether the guest's view of the disks are backed by local storage on the host, or by the hypervisor accessing shared storage over a network). A clever observer will then note that if all state is available for live migration, then there is nothing stopping a user from saving some or all of that state at a given point of time in order to be able to later rewind guest execution back to the state it previously had. The astute reader will also realize that state capture at any level requires that the data must be stored and managed by some mechanism. This processing might fit in a single file, or more likely require a chain of related files, and may require synchronization with third-party tools built around managing the amount of data resulting from capturing the state of multiple guests that each use multiple disks.

There are several libvirt APIs associated with capturing the state of a guest, which can later be used to rewind that guest to the conditions it was in earlier. The following is a list of trade-offs and differences between the various facets that affect capturing domain state for active domains:

Duration
Capturing state can be a lengthy process, so while the captured state ideally represents an atomic point in time corresponding to something the guest was actually executing, capturing state tends to focus on minimizing guest downtime while performing the rest of the state capture in parallel with guest execution. Some interfaces require up-front preparation (the state captured is not complete until the API ends, which may be some time after the command was first started), while other interfaces track the state when the command was first issued, regardless of the time spent in capturing the rest of the state. Also, time spent in state capture may be longer than the time required for live migration, when state must be duplicated rather than shared.
Amount of state
For an online guest, there is a choice between capturing the guest's memory (all that is needed during live migration when the storage is already shared between source and destination), the guest's disk state (all that is needed if there are no pending guest I/O transactions that would be lost without the corresponding memory state), or both together. Reverting to partial state may still be viable, but typically, booting from captured disk state without corresponding memory is comparable to rebooting a machine that had power cut before I/O could be flushed. Guests may need to use proper journaling methods to avoid problems when booting from partial state.
Quiescing of data
Even if a guest has no pending I/O, capturing disk state may catch the guest at a time when the contents of the disk are inconsistent. Cooperating with the guest to perform data quiescing is an optional step to ensure that captured disk state is fully consistent without requiring additional memory state, rather than just crash-consistent. But guest cooperation may also have time constraints, where the guest can rightfully panic if there is too much downtime while I/O is frozen.
Quantity of files
When capturing state, some approaches store all state within the same file (internal), while others expand a chain of related files that must be used together (external), for more files that a management application must track.
Impact to guest definition
Capturing state may require temporary changes to the guest definition, such as associating new files into the domain definition. While state capture should never impact the running guest, a change to the domain's active XML may have impact on other host operations being performed on the domain.
Third-party integration
When capturing state, there are tradeoffs to how much of the process must be done directly by the hypervisor, and how much can be off-loaded to third-party software. Since capturing state is not instantaneous, it is essential that any third-party integration see consistent data even if the running guest continues to modify that data after the point in time of the capture.
Full vs. incremental
When periodically repeating the action of state capture, it is useful to minimize the amount of state that must be captured by exploiting the relation to a previous capture, such as focusing only on the portions of the disk that the guest has modified in the meantime. Some approaches are able to take advantage of checkpoints to provide an incremental backup, while others are only capable of a full backup even if that means re-capturing unchanged portions of the disk.
Local vs. remote
Domains that completely use remote storage may only need some mechanism to keep track of guest memory state while using external means to manage storage. Still, hypervisor and guest cooperation to ensure points in time when no I/O is in flight across the network can be important for properly capturing disk state.
Network latency
Whether it's domain storage or saving domain state into remote storage, network latency has an impact on snapshot data. Having dedicated network capacity, bandwidth, or quality of service levels may play a role, as well as planning for how much of the backup process needs to be local.

An example of the various facets in action is migration of a running guest. In order for the guest to be able to resume on the destination at the same place it left off at the source, the hypervisor has to get to a point where execution on the source is stopped, the last remaining changes occurring since the migration started are then transferred, and the guest is started on the target. The management software thus must keep track of the starting point and any changes since the starting point. These last changes are often referred to as dirty page tracking or dirty disk block bitmaps. At some point in time during the migration, the management software must freeze the source guest, transfer the dirty data, and then start the guest on the target. This period of time must be minimal. To minimize overall migration time, one is advised to use a dedicated network connection with a high quality of service. Alternatively saving the current state of the running guest can just be a point in time type operation which doesn't require updating the "last vestiges" of state prior to writing out the saved state file. The state file is the point in time of whatever is current and may contain incomplete data which if used to restart the guest could cause confusion or problems because some operation wasn't completed depending upon where in time the operation was commenced.

State capture APIs

With those definitions, the following libvirt APIs related to state capture have these properties:

virDomainManagedSave
This API saves guest memory, with libvirt managing all of the saved state, then stops the guest. While stopped, the disks can be copied by a third party. However, since any subsequent restart of the guest by libvirt API will restore the memory state (which typically only works if the disk state is unchanged in the meantime), and since it is not possible to get at the memory state that libvirt is managing, this is not viable as a means for rolling back to earlier saved states, but is rather more suited to situations such as suspending a guest prior to rebooting the host in order to resume the guest when the host is back up. This API also has a drawback of potentially long guest downtime, and therefore does not lend itself well to live backups.
virDomainSave
This API is similar to virDomainManagedSave(), but moves the burden on managing the stored memory state to the user. As such, the user can now couple saved state with copies of the disks to perform a revert to an arbitrary earlier saved state. However, changing who manages the memory state does not change the drawback of potentially long guest downtime when capturing state.
virDomainSnapshotCreateXML
This API wraps several approaches for capturing guest state, with a general premise of creating a snapshot (where the current guest resources are frozen in time and a new wrapper layer is opened for tracking subsequent guest changes). It can operate on both offline and running guests, can choose whether to capture the state of memory, disk, or both when used on a running guest, and can choose between internal and external storage for captured state. However, it is geared towards post-event captures (when capturing both memory and disk state, the disk state is not captured until all memory state has been collected first). Using QEMU as the hypervisor, internal snapshots currently have lengthy downtime that is incompatible with freezing guest I/O, but external snapshots are quick when memory contents are not also saved. Since creating an external snapshot changes which disk image resource is in use by the guest, this API can be coupled with virDomainBlockCommit() to restore things back to the guest using its original disk image, where a third-party tool can read the backing file prior to the live commit. See also the XML details used with this command.
virDomainFSFreeze, virDomainFSThaw
This pair of APIs does not directly capture guest state, but can be used to coordinate with a trusted live guest that state capture is about to happen, and therefore guest I/O should be quiesced so that the state capture is fully consistent, rather than merely crash consistent. Some APIs are able to automatically perform a freeze and thaw via a flags parameter, rather than having to make separate calls to these functions. Also, note that freezing guest I/O is only possible with trusted guests running a guest agent, and that some guests place maximum time limits on how long I/O can be frozen.
virDomainCheckpointCreateXML
This API does not actually capture guest state, rather it makes it possible to track which portions of guest disks have changed between a checkpoint and the current live execution of the guest. However, while it is possible use this API to create checkpoints in isolation, it is more typical to create a checkpoint as a side-effect of starting a new incremental backup with virDomainBackupBegin() or at the creation of an external snapshot with virDomainSnapshotCreateXML2(), since a second incremental backup is most useful when using the checkpoint created during the first. See also the XML details used with this command.
virDomainBackupBegin, virDomainBackupEnd
This API wraps approaches for capturing the state of disks of a running guest, but does not track accompanying guest memory state. The capture is consistent to the start of the operation, where the captured state is stored independently from the disk image in use with the guest and where it can be easily integrated with a third-party for capturing the disk state. Since the backup operation is stored externally from the guest resources, there is no need to commit data back in at the completion of the operation. When coupled with checkpoints, this can be used to capture incremental backups instead of full.

Examples

The following two sequences both accomplish the task of capturing the disk state of a running guest, then wrapping things up so that the guest is still running with the same file as its disk image as before the sequence of operations began. The difference between the two sequences boils down to the impact of an unexpected interruption made at any point in the middle of the sequence: with such an interruption, the first example leaves the guest tied to a temporary wrapper file rather than the original disk, and requires manual clean up of the domain definition; while the second example has no impact to the domain definition.

Backup via temporary snapshot

virDomainFSFreeze()
virDomainSnapshotCreateXML(VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY)
virDomainFSThaw()
third-party copy the backing file to backup storage # most time spent here
virDomainBlockCommit(VIR_DOMAIN_BLOCK_COMMIT_ACTIVE) per disk
wait for commit ready event per disk
virDomainBlockJobAbort() per disk

Direct backup

virDomainFSFreeze()
virDomainBackupBegin()
virDomainFSThaw()
wait for push mode event, or pull data over NBD # most time spent here
virDomainBackupEnd()