+https://git.proxmox.com/?p=pve-qemu-kvm.git;a=blob;f=backup.txt[here].
+
+NOTE: For VMs, this _snapshot_ mode relates to the fact that the backup is
+performed while the VM is running. This has here nothing to do with file systems
+snapshots and can be operated on any kind of storage.
+
+.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 filesystem and the target storage of the backup
+is an NFS server, you should set `--tmpdir` to reside on a local filesystem 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` mountpoint option individual volumes
+can be excluded from the backup (and thus this requirement).
+
+NOTE: bind and device mountpoints are skipped during backup operations, like
+volume mountpoints with the backup option disabled.
+