qmpbackup

qmpbackup is designed to create live full and incremental backups of running qemu virtual machines via QMP protocol. It makes use of the dirty-bitmap feature introduced in later qemu versions. It was mostly created for educational purposes and is by no means complete. It works with standalone qemu processes.

If you want to backup qemu virtual machines which are managed via libvirt, see this project:

https://github.com/abbbi/virtnbdbackup

Prerequisites

The virtual machine must be reachable via QMP protocol on a unix socket, usually this happens by starting the virtual machine via:

 qemu-system-<arch> <options> -qmp unix:/path/socket,server,nowait

qmpbackup makes use of this socket to pass needed commands to the virtual machine.

Usage

In order to create a full backup use the following command:

 qmpbackup --socket /path/socket backup --level full --target /tmp/backup/

the command will create a new dirty bitmap and backup the virtual machines disks to /tmp/backup/<disk-id>/FULL-<timestamp>. It ensures consistency by creating the bitmap and backup within one QMP transaction.

See the following discussion on the qemu-block mailinglist regarding this topic:

https://lists.nongnu.org/archive/html/qemu-block/2016-11/msg00682.html

Bitmaps will be added with persistent option flag, which means they are stored permanently and are available between virtual machine shutdowns.

Second step is to change some data within your virtual machine and let qmpbackup create an incremental backup for you, this works by:

 qmpbackup --socket /path/socket backup --level inc --target /tmp/backup/

the changed delta since your last full (or inc) backup will be dumped to /tmp/backup/<disk-id>INC-<timestamp>, the dirty-bitmap is automatically cleared after this and you can continue creating further incremental backups by re-issuing the command likewise.

There is also the auto backup level which combines the full and inc backup levels. If there's no existing bitmap for the VM, full will run. If a bitmap exists, inc will be used.

Monthly Backups

Using the --monthly flag with the backup command, backups will be placed in monthly folders in a YYYY-MM format. The above combined with the auto backup level, backups will be created in monthly backup chains.

With a VM named myfirstvm and the date being 2021-11, the following command:

/qmpbackup --socket /path/socket backup --level auto --monthly --target /tmp/backup

will place backups in the following backup path: /tmp/backup/myfirstvm/2021-11/

When the date changes to 2021-12 and qmpbackup is run, backups will be placed in /tmp/backup/myfirstvm/2021-12/ and a new full backup will be created.

Filesystem Quisce

In case the virtual machine has an guest agent installed you can set the Qemu Guest Agent socket (--agent-socket) and request filesytem quisce via --quisce option:

  qmpbackup --socket /tmp/vm --agent-socket /tmp/qga.sock backup --level full --target /tmp/ --quisce

Restore

Restoring your data is a matter of rebasing the created qcow images by using standard tools such as qemu-img or qmprebase.

A image backup based on a backup folder containing the following backups:

 /tmp/backup/ide0-hd0
 ├── FULL-1480542683
 ├── INC-1480542701
 └── INC-1480542712

can be rolled back by using qmprebase, it uses common qemu tools to check consistency and does a rollback of your image file:

 qmprebase  rebase --dir /tmp/backup/ide0-hd0

While rebasing the saveset chain is merged into your FULL image which then contains the latest state and can be booted via Qemu again.

Note: It makes sense to copy the existing backup directory to a temporary folder before rebasing, to not alter your existing backups.

Misc commands

In order to remove existing dirty-bitmaps use:

 qmpbackup --socket /tmp/vm cleanup --remove-bitmaps

see

 qmpbackup --help 

for more information and commands.

Limitations

1. Using the QMP protocol it cannot be used together with libvirt as libvirt exclusively uses the virtual machines monitor socket. See virtnbdbackup.

2. Qemus drive-backup function does currently not support dumping data as a stream, it also cannot work with fifo pipes as the blockdriver expects functions as ftruncate and fseek to work on the target file, so the backup target must be a directory.