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Creating a backup, whether full or incremental, is done via virDomainBackupBegin(), which takes an XML description of the actions to perform, as well as an optional second XML document describing a checkpoint to create at the same point in time. See also a comparison between the various state capture APIs.
There are two general modes for backups: a push mode (where the hypervisor writes out the data to the destination file, which may be local or remote), and a pull mode (where the hypervisor creates an NBD server that a third-party client can then read as needed, and which requires the use of temporary storage, typically local, until the backup is complete).
The instructions for beginning a backup job are provided as attributes and elements of the top-level domainbackup element. This element includes an optional attribute mode which can be either "push" or "pull" (default push). virDomainBackupGetXMLDesc() can be used to see the actual values selected for elements omitted during creation (for example, learning which port the NBD server is using in the pull model or what file names libvirt generated when none were supplied). The following child elements and attributes are supported:
An optional element giving the name of an existing checkpoint of the domain, which will be used to make this backup an incremental one. In the push model, only changes since the named checkpoint are written to the destination. In the pull model, the NBD server uses the NBD_OPT_SET_META_CONTEXT extension to advertise to the client which portions of the export contain changes since the named checkpoint. If omitted, a full backup is performed.
Present only for a pull mode backup. Contains the same attributes as the `protocol element of a disk <formatdomain.html#hard-drives-floppy-disks-cdroms>`__ attached via NBD in the domain (such as transport, socket, name, port, or tls), necessary to set up an NBD server that exposes the content of each disk at the time the backup is started.
Note that for the QEMU hypervisor the TLS environment in controlled using backup_tls_x509_cert_dir, backup_tls_x509_verify, and backup_tls_x509_secret_uuid properties in /etc/libvirt/qemu.conf.
An optional listing of instructions for disks participating in the backup (if omitted, all disks participate and libvirt attempts to generate filenames by appending the current timestamp as a suffix). If the entire element was omitted on input, then all disks participate in the backup, otherwise, only the disks explicitly listed which do not also use backup='no' will participate. On output, this is the state of each of the domain's disk in relation to the backup operation.
This sub-element describes the backup properties of a specific disk, with the following attributes and child elements:
A mandatory attribute which must match the <target dev='name'/> of one of the disk devices specified for the domain at the time of the checkpoint.
Setting this attribute to yes(default) specifies that the disk should take part in the backup and using no excludes the disk from the backup.
This attribute overrides the implied backup mode inherited from the definition of the backup itself. Value full forces a full backup even if the backup calls for an incremental backup, and incremental coupled with the attribute incremental='CHECKPOINTNAME for the disk forces an incremental backup from CHECKPOINTNAME.
An optional attribute giving the name of an existing checkpoint of the domain which overrides the one set by the <incremental> element.
Allows modification of the NBD export name for the given disk. By default equal to disk target. Valid only for pull mode backups.
Allows modification of the name of the bitmap describing dirty blocks for an incremental backup exported via NBD export name for the given disk. Valid only for pull mode backups.
A mandatory attribute to describe the type of the disk, except when backup='no' is used. Valid values include file, or block. Similar to a disk declaration for a domain, the choice of type controls what additional sub-elements are needed to describe the destination.
Output only. The value can be used to refer to the scratch or output file of the backup in APIs such as virDomainSetBlockThreshold.
Valid only for push mode backups, this is the primary sub-element that describes the file name of the backup destination, similar to the source sub-element of a domain disk. An optional sub-element driver can also be used, with an attribute type to specify a destination format different from qcow2. See documentation for scratch below for additional configuration.
Valid only for pull mode backups, this is the primary sub-element that describes the file name of the local scratch file to be used in facilitating the backup, and is similar to the source sub-element of a domain disk. Currently only file and block scratch storage is supported. The file scratch file is created and deleted by libvirt in the given location. A block scratch device must exist prior to starting the backup and is formatted. The block device must have enough space for the corresponding disk data including format overhead. If VIR_DOMAIN_BACKUP_BEGIN_REUSE_EXTERNAL flag is used the file for a scratch of file type must exist with the correct format and size to hold the copy and is used without modification. The file is not deleted after the backup but the contents of the file don't make sense outside of the backup. The same applies for the block device which must be formatted appropriately. Similarly to the domain `disk <formatdomain.html#hard-drives-floppy-disks-cdroms>`__ definition scratch and target can contain seclabel and/or encryption subelements to configure the corresponding properties.
Use virDomainBackupBegin() to perform a full backup using push mode. The example lets libvirt pick the destination and format for 'vda', fully specifies that we want a raw backup of 'vdb', and omits 'vdc' from the operation.
<domainbackup>
<disks>
<disk name='vda' backup='yes'/>
<disk name='vdb' type='file'>
<target file='/path/to/vdb.backup'/>
<driver type='raw'/>
</disk>
<disk name='vdc' backup='no'/>
</disks>
</domainbackup>
If the previous full backup also passed a parameter describing checkpoint XML that resulted in a checkpoint named 1525889631, we can make another call to virDomainBackupBegin() to perform an incremental backup of just the data changed since that checkpoint, this time using the following XML to start a pull model export of the 'vda' and 'vdb' disks, where a third-party NBD client connecting to '/path/to/server' completes the backup (omitting 'vdc' from the explicit list has the same effect as the backup='no' from the previous example):
<domainbackup mode="pull">
<incremental>1525889631</incremental>
<server transport="unix" socket="/path/to/server"/>
<disks>
<disk name='vda' backup='yes' type='file'>
<scratch file='/path/to/file1.scratch'/>
</disk>
</disks>
</domainbackup>