Libvirt incremental backups use named checkpoints to track which guest-disk blocks have changed since an earlier backup. The workflow is supported only for QEMU with qcow2 disks at the active layer, according to libvirt’s Checkpoint XML documentation. Confirm that your installed host supports the required features before relying on it.
How libvirt incremental backups work
A checkpoint marks a point from which libvirt can track later disk changes. The QEMU driver implements this tracking with block dirty bitmaps. When a later backup names an existing checkpoint, it can transfer changed portions rather than treating the job as a new full backup. A checkpoint is metadata for tracking changes—not a full backup, a VM snapshot, or a copy of the guest’s memory. See libvirt’s QEMU incremental-backup internals.
For a usable recovery chain, retain the backup data and the checkpoint lineage needed to identify subsequent changes. Removing checkpoint metadata can eliminate the ability to continue incremental backups from that point; checkpoints alone cannot restore lost disk contents.
Check host and storage support first
Libvirt’s documented support statement is deliberately narrow: incremental backups are supported for the QEMU hypervisor using qcow2 disks at the active layer. Do not assume this applies to other hypervisors, disk formats, or backing layers. The official pages do not provide a complete version matrix, so validate against the actual installed libvirt and QEMU versions and domain configuration.
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- Confirm that the domain uses QEMU and that each disk you intend to back up is qcow2 at the active layer.
- Inspect the domain’s capabilities XML for reported backup and checkpoint support. Libvirt documents capability reporting in its Domain capabilities XML format.
- Check the installed host’s documentation and configuration for the exact command and XML behavior available there; the online documentation may not match every release.
- Plan where backup data will be written, which disks belong in the backup, and how the resulting data and checkpoint chain will be retained and restored.
Choose push or pull mode
| Mode | Data flow | Destination handling |
|---|---|---|
| Push | The hypervisor writes backup data to the configured destination. | Configure the destination in the backup XML. Push is the default mode. |
| Pull | Libvirt exposes an NBD server for a third-party client to read. | Configure the NBD endpoint and provide temporary scratch storage, typically local, until the backup completes. |
These modes describe data flow, not a guaranteed performance, consistency, or security advantage. Those outcomes depend on the host, client, destination, and deployment; the cited libvirt documentation does not establish a general comparison on those measures. The Backup XML documentation describes the available elements and examples.
Create a backup and its next checkpoint
The documented command pattern is virsh backup-begin DOMAIN BACKUP.xml CHECKPOINT.xml. The backup XML defines the job; the optional checkpoint XML requests a new checkpoint at the time of the backup. For an incremental job, the backup XML must identify an existing checkpoint. Adapt the XML to the domain’s real disks, destinations, access controls, and recovery requirements rather than copying an example unchanged.
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- Create backup XML with the desired top-level mode, disk selection, and target configuration. Use
pushorpull; if mode is omitted, push is the default. - For an incremental backup, add an
<incremental>element naming the existing checkpoint from which changed blocks should be tracked. Disk-level settings can specify or exclude disks and may override the overall mode or checkpoint. - For pull mode, configure the NBD server endpoint and scratch storage, and arrange for the third-party client to read the export.
- Optionally prepare checkpoint XML for the new checkpoint that should be created with the backup.
- Start the asynchronous job with
virsh backup-begin DOMAIN BACKUP.xml CHECKPOINT.xml, replacing the placeholders with the domain name and applicable file paths.
If you omit the backup XML, virsh backup-begin starts a default full push backup. That default is not an incremental job: it does not name a prior checkpoint in backup XML.
Monitor and finish the asynchronous job
The backup runs asynchronously. The virsh manual documents checking push-job progress with domjobinfo or waiting for an event. If you need to stop a job, the manual documents domjobabort. Use the exact domain and job context for your installed virsh version, and verify the job’s outcome before treating its destination as a completed backup.
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- This USB drive provides plug and play simplicity with the included 18 inch USB 3.0 cable
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Manage checkpoint lifetime and recovery
Checkpoint metadata is part of the operational chain, not a substitute for retaining backup data. Keep a record of which checkpoint each incremental job uses and which backup output it produces; preserve the required chain according to the restore procedure for your environment. If checkpoint metadata is deleted, incremental continuation from that point may no longer be possible.
There is a specific lifecycle hazard for transient domains: the virsh manual says their checkpoint metadata is silently lost when the domain quits. Administrators who need checkpoints to survive domain shutdown should account for that behavior when choosing domain lifecycle and retention practices.
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- This USB drive provides plug and play simplicity with the included 18 inch USB 3.0 cable
- The available storage capacity may vary.
Before relying on any chain, confirm that your recovery procedure can reconstruct the intended disk state from the retained backup data and checkpoint history. A checkpoint does not retain the previous contents of changed blocks, and libvirt’s documented checkpoint mechanism does not capture VM memory.
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