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Backup utility for Libvirt / qemu / kvm supporting incremental and differential backups + instant recovery (agentless).


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Backup utility for libvirt, using the latest changed block tracking features. Create online, thin provisioned full and incremental or differential backups of your kvm/qemu virtual machines.

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Existing backup solutions or scripts for libvirt/kvm usually depend on the external snapshot feature to create backups, sometimes even require to shutdown or pause the virtual machine.

Recent additions to both the libvirt and qemu projects have introduced new capabilities that allow to create online (full and incremental) backups, by using so called dirty bitmaps (or changed block tracking).

virtnbdbackup uses these features to create online full and incremental or differential backups.

virtnbdrestore can be used to re-construct the complete image from the thin provisioned backups.

virtnbdmap can be used to map an thin provisioned backup image into a block device on-the-fly, for easy single file restore or even instant boot from an backup image.

For backing up standalone qemu virtual machines not managed by libvirt, see this project: qmpbackup


Obviously you require a libvirt/qemu version that supports the incremental backup features. Since libvirt v7.6.0 and qemu-6.1 the required features are enabled by default and are considered production ready: everything will work out of the box.

Following, you will find a short overview which older libvirt versions may require further adjustments to the virtual machine config.

Libvirt versions <= 7.6.0 (Debian Bullseye, Ubuntu 20.x)

If you are using Debian Bullseye or Ubuntu 20.x, the included libvirt version already has an almost complete support for incremental backup, although it doesn't work properly with migration or some block jobs.

If you don't want to use migration or other blockjobs you can enable the incremental backup feature on these libvirt versions. Change the virtual machine config using virsh edit <vm> like so: (the first line must be changed, too!):

 <domain type='kvm' id='1' xmlns:qemu=''>
   <qemu:add capability='incremental-backup'/>


You must power cycle the virtual machine after enabling the feature! Upstream libvirt strongly discourages enabling the feature on production systems for these libvirt versions.

RHEL/Centos Stream, Alma, Rocky Linux

Version <= 8.5

Up to RHEL/Centos8/Almalinux 8.5, libvirt packages from the advanced virtualization stream support all required features. To install libvirt from the stream use:

yum install centos-release-advanced-virtualization
yum makecache
yum module install virt

and enable the feature by adjusting the virtual machine config.

Version >= 8.6

As of RHEL 8.6, the advanced virtualization stream has been deprecated, and all components supporting the new feature are included in the virt:rhel module, the feature is enabled by default. (Details)

Environment dependencies

  • python libvirt module version >= 6.0.0 (yum install python3-libvirt)
  • python libnbd bindings ( version >= 1.5.5 (yum install python3-libnbd)
  • The virtual machine should use qcow version 3 images to support the full feature set.


There are several ways to install the utility, below you will find an short description for each of them. For Debian and RHEL/SuSE based derivates see releases for pre-built packages.


Please consider to check past issues related to installation if you face any troubles before opening a new issue.

Python package

pip3 install -r requirements.txt
python3 install


Do not install the "nbd" package available on PyPI, it does not provide the required nbd bindings (unfortunately has the same name). You have to additionally install the provided python3-libnbd packages by your distribution, or compile the libnbd bindings by yourself.

RPM package

Packages for RHEL/Fedora and OpenSUSE are available via releases.

To create an RPM package from source by yourself you can follow the steps from the github build workflow.

Debian package

Official packages are available: and are maintained on the Debian salsa codespace.

For the latest packages available check releases.

To create an Debian package from source by yourself you can follow the steps from the github build workflow.


For setup within an virtualenv see venv scripts.

Docker images

You can build an docker image using the existing Dockerfile README

All released versions and master branch are published via github container registry, too. Example:

 docker run -it virtnbdbackup

See packages.

Backup modes and concept

Following backup modes can be used:

  • auto: If the target folder is empty, attempt to execute full backup, otherwise switch to backup mode incremental: allows rotation of backup into monthly folders.

  • full: Full, thin provisioned backup of the virtual machine, a new checkpoint named virtnbdbackup will be created, all existent checkpoints from prior backups matching this name will be removed: a new backup chain is created. The Virtual machine must be online and running for this backup mode to work.

  • copy: Full, thin provisioned backup of the virtual machine disks, no checkpoint is created for further incremental backups, existing checkpoints will be left untouched. This is the default mode and works with qcow images not supporting persistent bitmaps.

  • inc: Perform incremental backup, based on the last full or incremental backup. A checkpoint for each incremental backup is created and saved.

  • diff: Perform differential backup: saves the current delta to the last incremental or full backup.

All required information for restore is stored to the same directory, including the latest virtual machine configuration, checkpoint information, disk data and logfiles.

The target directory must be rotated if a new backup set is created.

If the virtual domain is active and running, a backup job operation via libvirt api is started, which in turn initializes a new nbd server backend listening on a local unix socket. This nbd backend provides consistent access to the virtual machines, disk data and dirty blocks. After the backup process finishes, the job is stopped and the nbd server quits operation.

It is possible to backup multiple virtual machines on the same host system at the same time, using separate calls to the application with a different target directory to store the data.

Supported disk formats / raw disks

libvirt/qemu supports dirty bitmaps, required for incremental backups only with qcow(v3) based disk images. If you are using older image versions, you can only create copy backups, or consider converting the images to a newer format using qemu-img:

qemu-img convert -O qcow2 -o compat=1.1 disk-old.qcow2 disk.qcow2

By default virtnbdbackup will exclude all disks with format raw as well as direct attached (passthrough) disks such as LVM or ZVOL and ISCSI volumes. These type of virtual disks do not support storing checkpoint/bitmap metadata and do not support incremental/differential backup. (more info)

This behavior can be changed if option --raw is specified, raw disks will then be included during a full backup. This of course means that no thin provisioned backup is created for these particular disks.

During restore, these files can be copied "as is" from the backup folder and must not be processed using virtnbdrestore.


The backup data for raw disks will only be crash consistent, be aware that this might result in inconsistent filesystems after restoring!

Backup Examples

Each backup for a virtual machine must be saved to an individual target directory. Once the target directory includes an full backup, it can be used as base for further incremental or differential backups.

Local full/incremental backup

Start full backup of domain vm1, save data to /tmp/backupset/vm1:

virtnbdbackup -d vm1 -l full -o /tmp/backupset/vm1

Start incremental backup for domain vm1, backup only changed blocks to the last full backup, the same directory is used as backup target:

virtnbdbackup -d vm1 -l inc -o /tmp/backupset/vm1

The resulting directory will contain both backups and all other files required to restore the virtual machine. Created logfiles can be used for analyzing backup issues:

├── backup.full.05102021161752.log
├── checkpoints
│   ├── virtnbdbackup.0.xml
│   ├── virtnbdbackup.1.xml
├── vm1.cpt
├── vmconfig.virtnbdbackup.0.xml
├── vmconfig.virtnbdbackup.1.xml

Backing up offline virtual domains

If the virtual domain is not in running state (powered off) virtnbdbackup supports copy and inc/diff backup modes. Incremental and differential backups will then save the changed blocks since last created checkpoint.

Backup mode full is changed to mode copy, because libvirt does not allow to create checkpoints for offline domains.

This behavior can be changed using the -S (--start-domain) option: prior to executing the backup, the virtual domain will then be started in paused state for the time the backup is created: The virtual machines CPU's are halted, but the running QEMU Process will allow all operations required to execute backups.

The option won't alter the virtual domain state if it is already online, thus it can be used for backing up virtual machines whose state is unknown prior to backup.

Application consistent backups

During backup virtnbdbackup attempts to freeze all file systems within the domain using the qemu guest agent filesystem freeze and thaw functions. In case no qemu agent is installed or filesystem freeze fails, a warning is shown during backup:

WARNING [..] Guest agent is not responding: QEMU guest agent is not connected

In case you receive this warning, check if the qemu agent is installed and running in the domain.

It is also possible to specify one or multiple mountpoints used within the virtual machine to freeze only specific filesystems, like so:

virtnbdbackup -d vm1 -l inc -o /tmp/backupset/vm1 -F /mnt,/var

this way only the underlying filesystems on /mnt and /var are frozen and thawed.


It is highly recommended to have an qemu agent running in the virtual domain to ensure file system consistency during backup!

Rotating backups

With backup mode auto it is possible to have a monthly rotation/retention. If the target folder is empty, backup mode auto will create an full backup. On the following executions, it will automatically switch to backup mode incremental, if the target folder already includes an full backup. Example:

virtnbdbackup -d vm1 -l auto -o /tmp/2022-06 -> creates full backup
virtnbdbackup -d vm1 -l auto -o /tmp/2022-06 -> creates inc backup
virtnbdbackup -d vm1 -l auto -o /tmp/2022-06 -> creates inc backup
virtnbdbackup -d vm1 -l auto -o /tmp/2022-07 -> creates full backup
virtnbdbackup -d vm1 -l auto -o /tmp/2022-07 -> creates inc backup

Excluding disks

Option -x can be used to exclude certain disks from the backup. The name of the disk to be excluded must match the disks target device name as configured in the domains xml definition, for example:

virtnbdbackup -d vm1 -l full -o /tmp/backupset/vm1 -x sda

Special devices such as cdrom/floppy or direct attached luns are excluded by default, as they are not supported by the changed block tracking layer.

It is also possible to only backup specific disks using the include option (--include, or -i):

virtnbdbackup -d vm1 -l full -o /tmp/backupset/vm1 -i sdf

Estimating backup size

Sometimes it can be useful to estimate the data size prior to executing the next incremental or differential backup. This can be done by using the option -p which will query the virtual machine checkpoint information for the current size:

virtnbdbackup -d vm1 -l inc -o /tmp/backupset/vm1 -p
[..] INFO virtnbdbackup - handleCheckpoints [MainThread]: Using checkpoint name: [virtnbdbackup.1].
[..] INFO virtnbdbackup - main [MainThread]: Estimated checkpoint backup size: [24248320] Bytes


Not all libvirt versions support the flag required to read the checkpoint size. If the estimated checkpoint size is always 0, your libvirt version might miss the required features.

Backup threshold

If an incremental or differential backup is attempted and the virtual machine is active, it is possible to specify an threshold for executing the backup using the --threshold option. The backup will then only be executed if the amount of data changed meets the specified threshold (in bytes):

virtnbdbackup -d vm1 -l inc -o /tmp/backupset/vm1 --threshold 3311264
[..] INFO virtnbdbackup - handleCheckpoints [MainThread]: Using checkpoint name: [virtnbdbackup.1].
[..] ]virtnbdbackup - main [MainThread]: Backup size [3211264] does not meet required threshold [3311264], skipping backup.

Backup concurrency

If virtnbdbackup saves data to a regular target directory, it starts one thread for each disk it detects to speed up the backup operation.

This behavior can be changed using the --worker option to define an amount of threads to be used for backup. Depending on how many disks your virtual machine has attached, it might make sense to try a different amount of workers to see which amount your hardware can handle best.

If standard output (-) is defined as backup target, the amount of workers is always limited to 1, to ensure a valid Zip file format.


It is possible to enable compression for the stream format via lz4 algorithm by using the --compress option. The saved data is compressed inline and the saveset file is appended with compression trailer including information about the compressed block offsets. By default compression level 2 is set if no parameter is applied. Higher compression levels can be set via:


During the restore, virtnbdrestore will automatically detect such compressed backup streams and attempts to decompress saved blocks accordingly.

Using compression will come with some CPU overhead, both lz4 checksums for block and original data are enabled.

Remote Backup

It is also possible to backup remote libvirt systems. The most convenient way is to use ssh for initiating the libvirt connection (key authentication mandatory).

Before attempting an remote backup, please validate your environment meets the following criteria:

  • DNS resolution (forward and reverse) must work on all involved systems.
  • SSH Login to the remote system via ssh key authentication (using ssh agent or passwordless ssh key) should work without issues.
  • Unique hostnames must be set on all systems involved. (background)
  • Firewall must allow connection on all ports involved.

If the virtual machine has additional files configured, as described in Kernel/initrd and additional files, these files will be copied from the remote system via SSH(SFTP).

QEMU Sessions

In order to backup virtual machines from a remote host, you must specify an libvirt URI to the remote system.

The following example saves the virtual machine vm1 from the remote libvirt host hypervisor to the local directory /tmp/backupset/vm1, it uses the root user for both the libvirt and ssh authentication:

virtnbdbackup -U qemu+ssh://root@hypervisor/system --ssh-user root -d vm1 -o  /tmp/backupset/vm1

See also: Authentication


If you want to run multiple remote backups at the same time you need to pass an unique port for the NBD service used for data transfer via --nbd-port option for each backup session.


By default disk data received from a remote system will be transferred via regular NBD protocol. You can enable TLS for this connection, using the --tls option. Before being able to use TLS, you must configure the required certificates on both sides. See this script.

See the following documentation by the libvirt project for detailed instructions how setup:


You should have installed at least version 1.12.6 of the libnbd library which makes the transfer via NBDS more stable full background

Using a separate network for data transfer

In case you want to use a dedicated network for the data transfer via NBD, you can specify an specific IP address to bind the remote NBD service to via --nbd-ip option.

Piping data to other hosts

If the output target points to standard out (-), virtnbdbackup puts the resulting backup data into an uncompessed zip archive.

A such, it is possible to transfer the backup data to different hosts, or pipe it to other programs.

However, keep in mind that in case you want to perform incremental backups, you must keep the checkpoint files on the host you are executing the backup utility from, until you create another full backup.

If output is set to standard out, virtnbdbackup will create the required checkpoint files in the directory it is executed from.

Here is an example:

 # mkdir backup-weekly; cd backup-weekly
 # virtnbdbackup -d vm1 -l full -o - | ssh root@remotehost 'cat >'
 # [..]
 # INFO outputhelper - __init__: Writing zip file stream to stdout
 # [..]
 # INFO virtnbdbackup - main: Finished
 # INFO virtnbdbackup - main: Adding vm config to zipfile
 # [..]

Any subsequent incremental backup operations must be called from within this directory:

 # cd backup-weekly
 # virtnbdbackup -d vm1 -l inc -o - | ssh root@remotehost 'cat >'

You may consider adding the created checkpoint files to some VCS system, like git, to have some kind of central backup history tracking.

During restore unzip the data from both zip files into a single directory: (use virtnbdrestore to reconstruct the virtual machine images):

 # unzip -o -d restoredata
 # unzip -o -d restoredata

Kernel/initrd and additional files

If an domain has configured custom kernel, initrd, loader or nvram images (usually the case if the domain boots from OVM UEFI BIOS), these files will be saved to the backup folder as well.

Restore examples

For restoring, virtnbdrestore can be used. It reconstructs the streamed backup format back into a usable qemu qcow image.

The restore process will create a qcow image with the original virtual size.

In a second step, the qcow image is then mapped to a ndb server instance where all data blocks are sent to and are applied accordingly. The resulting image can be mounted (using guestmount) or attached to a running virtual machine in order to recover required files.

Dumping backup information

As a first start, the dump parameter can be used to dump the saveset information of an existing backup:

virtnbdrestore -i /tmp/backupset/vm1 -o dump
INFO:root:Dumping saveset meta information
{'checkpointName': 'virtnbdbackup',
 'dataSize': 704643072,
 'date': '2020-11-15T20:50:36.448938',
 'diskName': 'sda',
 'incremental': False,
 'parentCheckpoint': False,
 'streamVersion': 1,
 'virtualSize': 32212254720}

The output includes information about the thick and thin provisioned disk space that is required for recovery, date of the backup and checkpoint chain.

Verifying created backups

As with version >= 1.9.40 virtnbdbackup creates an check sum for each created data file. Using virtnbdrestore you can check the integrity for the created data files without having to restore:

virtnbdrestore -i /tmp/backup/vm1 -o verify
[..] INFO lib common - printVersion [MainThread]: Version: 1.9.39 Arguments: ./virtnbdrestore -i /tmp/backup/vm1 -o verify
[..] INFO root virtnbdrestore - verify [MainThread]: Computing checksum for: /tmp/backup/vm1/
[..] INFO root virtnbdrestore - verify [MainThread]: Checksum result: 541406837
[..] INFO root virtnbdrestore - verify [MainThread]: Comparing checksum with stored information
[..] INFO root virtnbdrestore - verify [MainThread]: OK

this makes it easier to spot corrupted backup files due to storage issues. (background)

Complete restore

To restore all disks within the backupset into a usable qcow image use command:

virtnbdrestore -i /tmp/backupset/vm1 -o /tmp/restore

All incremental backups found will be applied to the target images in the output directory /tmp/restore


The restore utility will copy the latest virtual machine config to the target directory, but won't alter its contents. You have to adjust the config file for the new paths and/or excluded disks to be able to define and run it.

Process only specific disks during restore

A single disk can be restored by using the option -d, the disk name has to match the virtual disks target name, for example:

virtnbdrestore -i /tmp/backupset/vm1 -o /tmp/restore -d sda

Point in time recovery

Option --until allows to perform a point in time restore up to the desired checkpoint. The checkpoint name has to be specified as reported by the dump output (field checkpointName), for example:

virtnbdrestore -i /tmp/backupset/vm1 -o /tmp/restore --until virtnbdbackup.2

It is also possible to specify the source data files specifically used for the rollback via --sequence option, but beware: you must be sure the sequence you apply has the right order, otherwise the restored image might be errnous, example:

virtnbdrestore -i /tmp/backupset/vm1 -o /tmp/restore --sequence,

Restoring with modified virtual machine config

Option -c can be used to adjust the virtual machine configuration during restore accordingly, the following changes are done:

  • UUID of the virtual machine is removed from the config file
  • Name of the virtual machine is prefixed with "restore_" (use option --name to specify desired vm name)
  • The disk paths to the virtual machine are changed to the new target directory.
  • If virtual machine was operating on snapshots/backing store images, the references to the configured backing stores will be removed.
  • Raw devices are removed from VM config if --raw is not specified, as well as floppy or cdrom devices (which aren't part of the backup).


If missing, Kernel, UEFI or NVRAM files are restored to their original location as set in the virtual machine configuration.

A restored virtual machine can then be defined and started right from the restored directory (or use option -D to define automatically):

virtnbdrestore -c -i /tmp/backupset/vm1 -o /tmp/restore
[..] INFO virtnbdrestore - restoreConfig [MainThread]: Adjusted config placed in: [/tmp/restore/vmconfig.virtnbdbackup.0.xml]
[..] INFO virtnbdrestore - restoreConfig [MainThread]: Use 'virsh define /tmp/restore/vmconfig.virtnbdbackup.0.xml' to define VM

Remote Restore

Restoring to a remote host is possible too, same options as during backup apply. The following example will restore the virtual machine from the local directory /tmp/backupset to the remote system "hypervisor", alter its configuration and register the virtual machine:

virtnbdrestore -U qemu+ssh://root@hypervisor/system --ssh-user root -cD -i /tmp/backupset/vm1 -o /remote/target

Post restore steps and considerations

If you restore the virtual machine with its original name on the same hypervisor, you may have to cleanup checkpoint information, otherwise backing up the restored virtual machine may fail, see this discussion

Single file restore and instant recovery

The virtnbdmap utility can be used to map uncompressed backup images from the stream format into an accessible block device on the fly. This way, you can restore single files or even boot from an existing backup image without having to restore the complete dataset.

The utility requires nbdkit with the python plugin to be installed on the system along with required qemu tools (qemu-nbd) and an loaded nbd kernel module. It must be executed with superuser (root) rights or via sudo.

The following example maps an existing backup image to the network block device /dev/nbd0:

 # modprobe nbd max_partitions=15
 # virtnbdmap -f /backupset/vm1/
 [..] INFO virtnbdmap - <module> [MainThread]: Done mapping backup image to [/dev/nbd0]
 [..] INFO virtnbdmap - <module> [MainThread]: Press CTRL+C to disconnect

While the process is running, you can access the backup image like a regular block device:

fdisk -l /dev/nbd0
Disk /dev/nbd0: 2 GiB, 2147483648 bytes, 4194304 sectors

You can also create an mapped "point in time" recovery image by passing a sequence of full and incremental backups as parameter. The changes from the incremental backups will then be replayed to the block device on the fly and the image will represent the latest state:

virtnbdmap -f /backupset/vm1/,/backupset/vm1/,/backupset/vm1/
[..] INFO virtnbdmap - main [MainThread]: Need to replay incremental backups
[..] INFO virtnbdmap - main [MainThread]: Replaying offset 420 from /backup/
[..] INFO virtnbdmap - main [MainThread]: Replaying offset 131534 from /backup/
[..] INFO virtnbdmap - main [MainThread]: Replaying offset 33534 from /backup/
[..] INFO virtnbdmap - <module> [MainThread]: Done mapping backup image to [/dev/nbd0]
[..] INFO virtnbdmap - <module> [MainThread]: Press CTRL+C to disconnect

The original image will be left untouched as nbdkits copy on write filter is used to replay the changes.

Further you can create an overlay image via qemu-img and boot from it right away (or boot directly from the /dev/nbd0 device).

qemu-img create -b /dev/nbd0 -f qcow2 bootme.qcow2
qemu-system-x86_64 -enable-kvm -m 2000 -hda bootme.qcow2

To remove the mappings, stop the utility via "CTRL-C"


If the virtual machine includes volume groups, the system will attempt to set them online as you create the mapping, because the copy on write device is writable by default. If your host system is using the same volume group names this could lead to issues (check dmesg or journalctl then). In case the volume groups are online, it is recommended to change them to offline just before you remove the mapping, to free all references to the mapped nbd device (vgchange -a n <vg_name>)


If you map the image device with the --readonly option you may need to pass certain options to the mount command (-o norecovery,ro) in order to be able to mount the filesystems. This may also be the case if no qemu agent was installed within the virtual machine during backup.

Transient virtual machines: checkpoint persistency on clusters

In case virtual machines are started in transient environments, such as using cluster solutions like pacemaker situations can appear where the checkpoints for the virtual machine defined by libvirt are not in sync with the bitmap information in the qcow files.

In case libvirt creates a checkpoint, the checkpoint information is stored in two places:

  • var/lib/libvirt/qemu/checkpoint/<domain_name>
  • In the bitmap file of the virtual machines qcow image.

Depending on the cluster solution, in case virtual machines are destroyed on host A and are re-defined on host B, libvirt loses the information about those checkpoints. Unfortunately libvirtd scans the checkpoint only once during startup.

This can result in a situation, where the bitmap is still defined in the qcow image, but libvirt doesn't know about the checkpoint, backup then fails with:

Unable to execute QEMU command 'transaction': Bitmap already exists

By default virtnbdbackup attempts to store the checkpoint information in the default backup directory, in situations where it detects a checkpoint is missing, it attempts to redefine them from the prior backups.

In order to store the checkpoint information at some central place the option --checkpointdir can be used, this allows having persistent checkpoints stored across multiple nodes:

As example:

  1. Create backup on host A, store checkpoints in a shared directory between hosts in /mnt/shared/vm1:

virtnbdbackup -d vm1 -l full -o /tmp/backup_hosta --checkpointdir /mnt/shared/vm1

  1. After backup, the virtual machine is relocated to host B and loses its information about checkpoints and bitmaps, thus, the next full backup usually fails with:
virtnbdbackup -d vm1 -l full -o /tmp/backup_hostb
unable to execute QEMU command 'transaction': Bitmap already exists: virtnbdbackup.0
  1. Now pass the checkpoint dir and files written from host A, and virtnbdbackup will redefine missing checkpoints and execute a new full backup. As the new full backup removes all prior checkpoints the bitmap information is in sync after this operation and backup succeeds:
virtnbdbackup -d vm1 -l full -o /tmp/backup_hostb --checkpointdir /mnt/shared/vm1
redefineCheckpoints: Redefine missing checkpoint virtnbdbackup.0

See also: #10

Supported Hypervisors

virtnbdbackup uses the lowest layer on top of libvirt to allow its functionality, you can also use it with more advanced hypervisors solutions such as ovirt, RHEV or OpenNebula, but please bear in mind that it was not developed to target all of those solutions specifically!

Ovirt, RHEV or OLVM

If you are using the ovirt node based hypervisor hosts you should consider creating a virtualenv via the venv scripts and transferring it to the node system.

On regular centos/alma/rhel based nodes, installation via RPM package should be preferred. The incremental backup functionality can be enabled via ovirt management interface.

Usually ovirt restricts access to the libvirt daemon via different authentication methods. Use the -U parameter in order to specify an authentication file, if you chose to run the utility locally on the hypervisor:

virtnbdbackup -U qemu:///system?authfile=/etc/ovirt-hosted-engine/virsh_auth.conf -d vm1 -o /tmp/backupset/vm1

You can also use remote backup functionality:

  • System must be reachable via ssh public key auth as described in the Remote Backup section.
  • Some OVIRT based setups may deny SASL based authentication if the hostname used to connect to does not match the hostname from the libvirt certificate. more info
  • Firewall port for NBD must be open:
 root@hv-node~# firewall-cmd --zone=public --add-port=10809/tcp

and then backup via:

virtnbdbackup -U qemu+ssh://root@hv-node/session -d vm -o /backup --password password --user root --ssh-user root


virtnbdrestore has not been adopted to cope with the ovirt specific domain xml format, so redefining and virtual machine on the node might not work.


See past issues


Both virtnbdbackup and virtnbdrestore commands support authenticating against libvirtd with the usual URIs. Consider using the following options:

-U: Specify an arbitrary connection URI to use against libvirt

--user: Username to use for the specified connection URI

--password: Password to use for the specified connection URI.

It is also possible to specify the credentials stored as authentication file like it would be possible using the virsh -c option:

 -U qemu:///system?authfile=/etc/virsh_auth.conf ..


The default connection URI used is qemu:///system which is usually the case if virtual machines operate as root user. Use the qemu:///session URI to backup virtual machines as regular user.


Backup Format

Currently, there are two output formats implemented:

  • stream: the resulting backup image is saved in a streamlined format, where the backup file consists of metadata about offsets and lengths of zeroed or allocated contents of the virtual machines disk. This is the default. The resulting backup image is thin provisioned.
  • raw: The resulting backup image will be a full provisioned raw image, this should mostly be used for debugging any problems with the extent handler, it won't work with incremental backups.


In order to save only used data from the images, dirty blocks are queried from the NBD server. The behavior can be changed by using the option -q to use common qemu tools (nbdinfo). By default virtnbdbackup uses a custom implemented extent handler.

Backup I/O and performance: scratch files

If virtual domains handle heavy I/O load during backup (such as writing or deleting lots of data while the backup is active) you might consider using the --scratchdir option to change the default scratch file location.

During the backup operation qemu will use the created scratch files for fleecing, thus it is recommended to store these files on storage that meets the same I/O performance requirements as the backup target.

The free space on the default scratch directory (/var/tmp) must be enough to be able to keep all fleecing data while the backup is active.


To get more detailed debug output use --verbose option. To enable NBD specific debugging output export LIBNBD_DEBUG environment variable prior to executing the backup or restore:

virtnbdbackup [..] --verbose


The thin provisioned backups are bigger than the original qcow images

Virtual machines using the qcow format do compress data. During backup, the image contents are exposed as NBD device which is a RAW device. The backup data will be at least as big as the used data within the virtual machine.

You can use the --compress option or other tools to compress the backup images in order to save storage space or consider using a deduplication capable target file system.

Backup fails with "Cannot store dirty bitmaps in qcow2 v2 files"

If the backup fails with error:

ERROR [..] internal error: unable to execute QEMU command dirty bitmaps in qcow2 v2 files

consider migrating your qcow files to version 3 format. QEMU qcow image version 2 does not support storing advanced bitmap information, as such only backup mode copy is supported.

Backup fails with "Timed out during operation: cannot acquire state change lock"

If backups fail with error:

ERROR [..] Timed out during operation: cannot acquire state change lock (held by monitor=remoteDispatchDomainBackupBegin)

there is still some block jobs operation active on the running domain, for example a live migration or another backup job. It may also happen that virtnbdbackup crashes abnormally or is forcibly killed during backup operation, unable to stop its own backup job.

You can use option -k to forcibly kill any running active block jobs for the domain, but use with care. It is better to check which operation is active with the virsh domjobinfo command first.

virtnbdbackup  -d vm2 -l copy -k  -o -
  INFO virtnbdbackup - main: Stopping domain jobs

Backup fails with "Failed to bind socket to /var/tmp/virtnbdbackup.XX: Permission denied"

The issue is most likely an active apparmor profile that prevents the qemu daemon from creating its socket file for the nbd server. Try to disable apparmor using the aa-teardown command for the current session you are executing a backup or restore. You can also add the following lines:

/var/tmp/virtnbdbackup.* rw,
/var/tmp/backup.* rw,

to the configuration files (might not exist by default):


See also: #7

High memory usage during backup

libnbd python implementation has had various memory leaks in older versions which cause such problems.

For centos 8 based distributions these fixes have been backported to libnbd 1.4.0.

The fix itself was released with libnbd 1.5.2, so be sure to use at least this version if using virtnbdbackup on any other distribution.

See also: #8

Test your backups!

The utility is provided "as is", i take no responsibility or warranty if you face any issues recovering your data! The only way to ensure your backups are valid and your backup plan works correctly is to repeatedly test the integrity by restoring them! If you discover any issues, please do not hesitate to report them.


Backup howto for Debian Bullseye:

Short video: