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One method of capturing domain disk backups is via the use of incremental backups. Right now, incremental backups are only supported for the QEMU hypervisor when using qcow2 disks at the active layer; if other disk formats are in use, capturing disk backups requires different libvirt APIs (see domain state capture for a comparison between APIs).
Libvirt is able to facilitate incremental backups by tracking disk checkpoints, which are points in time against which it is easy to compute which portion of the disk has changed. Given a full backup (a backup created from the creation of the disk to a given point in time), coupled with the creation of a disk checkpoint at that time, and an incremental backup (a backup created from just the dirty portion of the disk between the first checkpoint and the second backup operation), it is possible to do an offline reconstruction of the state of the disk at the time of the second backup without having to copy as much data as a second full backup would require. Most disk checkpoints are created in conjunction with a backup via virDomainBackupBegin(), although a future API addition of virDomainSnapshotCreateXML2() will also make this possible when creating external snapshots; however, libvirt also exposes enough support to create disk checkpoints independently from a backup operation via virDomainCheckpointCreateXML() since 5.6.0. Likewise, the creation of checkpoints when external snapshots exist is currently forbidden, although future work will make it possible to integrate these two concepts.
Attributes of libvirt checkpoints are stored as child elements of the domaincheckpoint element. At checkpoint creation time, normally only the name, description, and disks elements are settable. The rest of the fields are ignored on creation and will be filled in by libvirt in for informational purposes by virDomainCheckpointGetXMLDesc(). However, when redefining a checkpoint, with the VIR_DOMAIN_CHECKPOINT_CREATE_REDEFINE flag of virDomainCheckpointCreateXML(), all of the XML fields described here are relevant on input, even the fields that are normally described as readonly for output.
The top-level domaincheckpoint element may contain the following elements:
The optional name for this checkpoint. If the name is omitted, libvirt will create a name based on the time of the creation.
An optional human-readable description of the checkpoint. If the description is omitted when initially creating the checkpoint, then this field will be empty.
On input, this is an optional listing of specific instructions for disk checkpoints; it is needed when making a checkpoint on only a subset of the disks associated with a domain. In particular, since QEMU checkpoints require qcow2 disks, this element may be needed on input for excluding guest disks that are not in qcow2 format. If the entire element was omitted on input, then all disks participate in the checkpoint, otherwise, only the disks explicitly listed which do not also use checkpoint='no' will participate. On output, this is the checkpoint state of each of the domain's disks.
This sub-element describes the checkpoint properties of a specific disk with the following attributes:
A mandatory attribute which must match either the <target dev='name'/> or an unambiguous <source file='name'/> of one of the disk devices specified for the domain at the time of the checkpoint.
An optional attribute; possible values are no when the disk does not participate in this checkpoint; or bitmap if the disk will track all changes since the creation of this checkpoint via a bitmap.
The attribute bitmap is only valid if checkpoint='bitmap'; it describes the name of the tracking bitmap (defaulting to the checkpoint name).
The attribute size is ignored on input; on output, it is only present if the VIR_DOMAIN_CHECKPOINT_XML_SIZE flag was used to perform a dynamic query of the estimated size in bytes of the changes made since the checkpoint was created.
Note that updating the backup size may be expensive and the actual required size may increase if the guest OS is actively writing to the disk.
A readonly representation of the time this checkpoint was created. The time is specified in seconds since the Epoch, UTC (i.e. Unix time).
Readonly, present if this checkpoint has a parent. The parent name is given by the sub-element name. The parent relationship allows tracking a list of related checkpoints.
A readonly representation of the inactive domain configuration at the time the checkpoint was created. This element may be omitted for output brevity by supplying the VIR_DOMAIN_CHECKPOINT_XML_NO_DOMAIN flag. The domain will have security-sensitive information omitted unless the flag VIR_DOMAIN_CHECKPOINT_XML_SECURE is provided on a read-write connection.
virDomainCheckpointCreateXML() requires that the <domain> is present when used with VIR_DOMAIN_CHECKPOINT_CREATE_REDEFINE. Since 7.0.0 the <domain> element can be omitted when redefining a checkpoint, but hypervisors may not support certain operations if it's missing.
Using this XML to create a checkpoint of just vda on a qemu domain with two disks and a prior checkpoint:
<domaincheckpoint>
<description>Completion of updates after OS install</description>
<disks>
<disk name='vda' checkpoint='bitmap'/>
<disk name='vdb' checkpoint='no'/>
</disks>
</domaincheckpoint>
will result in XML similar to this from virDomainCheckpointGetXMLDesc():
<domaincheckpoint>
<name>1525889631</name>
<description>Completion of updates after OS install</description>
<parent>
<name>1525111885</name>
</parent>
<creationTime>1525889631</creationTime>
<disks>
<disk name='vda' checkpoint='bitmap' bitmap='1525889631'/>
<disk name='vdb' checkpoint='no'/>
</disks>
<domain type='qemu'>
<name>fedora</name>
<uuid>93a5c045-6457-2c09-e56c-927cdf34e178</uuid>
<memory>1048576</memory>
...
<devices>
<disk type='file' device='disk'>
<driver name='qemu' type='qcow2'/>
<source file='/path/to/file1'/>
<target dev='vda' bus='virtio'/>
</disk>
<disk type='file' device='disk' snapshot='external'>
<driver name='qemu' type='raw'/>
<source file='/path/to/file2'/>
<target dev='vdb' bus='virtio'/>
</disk>
...
</devices>
</domain>
</domaincheckpoint>
With that checkpoint created, the qcow2 image is now tracking all changes that occur in the image since the checkpoint via the persistent bitmap named 1525889631.