These scripts allow to attach backups from a Proxmox Backup Server to a
running VM as additional disks (pbs-attach-backup.sh, to be run on the
Proxmox Virtual Environment hypervisor) and mount them inside the VM with a
RAM-backed write buffer (pbs-mount-backup.sh).
If the QEMU Guest Agent is installed (and enabled), the mounting (and
unmounting) is triggered automatically from the hypervisor. For that to work,
the pbs-mount-backup.sh has to be put in the same directory as
pbs-attach-backup.sh.
Proxmox Backup Server is great for backups of VMs in Proxmox Virtual Environment, and great for disaster recovery of whole VMs, too! But while it has a UI for "File Restore", restoring several – possibly huge – files requires one to download them from the hypervisor WebUI to the client and then copy them to the server.
With this method of attaching the backup as an additional disk, we can eliminate the administrator's client computer from the restore operation. Even better, we can provide users of the VM with the full contents of a backup and let them choose what to restore, without copying a single bit that's not needed!
On the hypervisor:
apt install jq socat
The script should determine most variables (like PBS storage ID and credentials) automatically, and ask for the rest:
# pbs-attach-backup.sh
VMID NAME STATUS MEM(MB) BOOTDISK(GB) PID
[...]
174 dev.example.org running 1024 10.00 49455
[...]
Enter VMID to restore: 174
1) vm/174/2022-10-11T01:08:51Z
2) vm/174/2022-10-12T01:08:48Z
3) vm/174/2022-10-13T01:08:26Z
4) vm/174/2022-10-14T01:08:28Z
5) vm/174/2022-10-15T01:08:45Z
6) vm/174/2022-10-16T01:08:33Z
7) vm/174/2022-10-17T01:08:37Z
8) vm/174/2022-10-18T01:08:35Z
9) vm/174/2022-10-19T01:08:50Z
10) vm/174/2022-10-20T01:08:58Z
11) vm/174/2022-10-21T01:08:38Z
12) vm/174/2022-10-22T01:08:25Z
13) vm/174/2022-10-23T01:08:12Z
14) vm/174/2022-10-24T01:08:19Z
Choose snapshot: 14
Locking VM
update VM 174: -lock rollback
Attaching drive drive-scsi0.img.fidx from snapshot vm/174/2022-10-24T01:08:19Z
to VM 174
Where should the backup be mounted to? [/mnt/backup]
How much RAM should be allocated to buffer writes to mounted backup? [128]
Mounting backup in VM...
Mounted /dev/mapper/mounted_backup6 (assumed backup of /dev/sda6 mounted on /) to /mnt/backup/.
Mounted /dev/mapper/mounted_backup5 (assumed backup of /dev/sda5 mounted on /var/tmp) to /mnt/backup/var/tmp.
Mounted /dev/mapper/mounted_backup1 (assumed backup of /dev/sda1 mounted on /boot) to /mnt/backup/boot.
Snapshot attached. Press return when done...
Unmounting backup in VM...
Detaching snapshot from VM
Unlocking VM
#
You can pass only the VM ID or that and the snapshot name on the command line, too:
# pbs-attach-backup.sh 174 vm/174/2022-10-24T01:08:19Z
Locking VM
[...]
As you can see, the VM is locked for the duration of the mount. This is to prevent migrations of the VM, since PVE doesn't know about the additional disk and hence it wouldn't be available on the new hypervisor.
As a side effect, it also prevents new backups from being created! So limit the duration of the attachment to the duration it's actually needed.