16.1. Backup modes

There are several ways to provide consistency (option mode), depending on the guest type.

Backup modes for VMs:

stop mode
This mode provides the highest consistency of the backup, at the cost of a short downtime in the VM operation. It works by executing an orderly shutdown of the VM, and then runs a background Qemu process to backup the VM data. After the backup is started, the VM goes to full operation mode if it was previously running. Consistency is guaranteed by using the live backup feature.
suspend mode
This mode is provided for compatibility reason, and suspends the VM before calling the snapshot mode. Since suspending the VM results in a longer downtime and does not necessarily improve the data consistency, the use of the snapshot mode is recommended instead.
snapshot mode
This mode provides the lowest operation downtime, at the cost of a small inconsistency risk. It works by performing a Proxmox VE live backup, in which data blocks are copied while the VM is running. If the guest agent is enabled (agent: 1) and running, it calls guest-fsfreeze-freeze and guest-fsfreeze-thaw to improve consistency.

A technical overview of the Proxmox VE live backup for QemuServer can be found online here.

Note

Proxmox VE live backup provides snapshot-like semantics on any storage type. It does not require that the underlying storage supports snapshots. Also please note that since the backups are done via a background Qemu process, a stopped VM will appear as running for a short amount of time while the VM disks are being read by Qemu. However the VM itself is not booted, only its disk(s) are read.

Backup modes for Containers:

stop mode
Stop the container for the duration of the backup. This potentially results in a very long downtime.
suspend mode

This mode uses rsync to copy the container data to a temporary location (see option --tmpdir). Then the container is suspended and a second rsync copies changed files. After that, the container is started (resumed) again. This results in minimal downtime, but needs additional space to hold the container copy.

When the container is on a local file system and the target storage of the backup is an NFS/CIFS server, you should set --tmpdir to reside on a local file system too, as this will result in a many fold performance improvement. Use of a local tmpdir is also required if you want to backup a local container using ACLs in suspend mode if the backup storage is an NFS server.

snapshot mode
This mode uses the snapshotting facilities of the underlying storage. First, the container will be suspended to ensure data consistency. A temporary snapshot of the container’s volumes will be made and the snapshot content will be archived in a tar file. Finally, the temporary snapshot is deleted again.

Note

snapshot mode requires that all backed up volumes are on a storage that supports snapshots. Using the backup=no mount point option individual volumes can be excluded from the backup (and thus this requirement).

Note

By default additional mount points besides the Root Disk mount point are not included in backups. For volume mount points you can set the Backup option to include the mount point in the backup. Device and bind mounts are never backed up as their content is managed outside the Proxmox VE storage library.