rewrite backup mode explaination for qemu

* try to stress the data consistency implications before going to the
 technical implementation
 * reoderder chapter levels to be consistent with the rest of the page

vzdump.adoc

@@ -33,54 +33,60 @@ system. Proxmox VE backups are always full backups - containing the VM/CT
 configuration and all data.
 Backups can be started via the GUI or via the `vzdump` command line tool.
 
-Backup Storage
---------------
+.Backup Storage
+
 Before a backup can run, a backup storage must be defined.
 Refer to the Storage documentation on how to add a storage. A backup storage
 must be a file level storage, as backups are stored as regular files.
 In most situations, using a NFS server is a good way to store backups.
 You can save those backups later to a tape drive, for off-site archive.
 
-Scheduled Backup
-----------------
+.Scheduled Backup
+
 Backup jobs can be scheduled so that they are executed automatically on
  specific days and times, for selectable nodes and guest systems.
 Configuration of scheduled backups is done at the Datacenter level in the GUI,
 which will generate a cron entry in /etc/cron.d/vzdump.
 
+Backup modes
+------------
 There are several ways to provide consistency (option `mode`), 
 depending on the guest type.
 
-.Backup `mode` for VMs:
+.Backup modes for VMs:
 
 `stop` mode::
-
-This first performns a clean shutdown of the VM to make sure it is
-stopped. It then starts the VM in suspended mode and uses the qemu
-backup feature to dump all data. If the VM was running, we start
-(resume) it immediately after starting the qemu backup task. This
-keeps the downtime as low as possible.
+This mode provides the highest consistency of the backup, at the
+cost of a 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 complete, the 
+Qemu process resumes the VM to full operation mode if it was previously running.
 
 `suspend` mode::
 
-This mode does not really make sense for qemu. Please use snapshot
-mode instead.
+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 simply starts a qemu live backup task. If the guest agent
-is enabled (`agent: 1`) and running, it calls 'guest-fsfreeze-freeze'
-and 'guest-fsfreeze-thaw' to improve consistency.
+This mode provides the lowest operation downtime, at the cost of a small
+inconstancy risk.
+It works by performing a Proxmox VE live backup, in which data blocks are copy
+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
 https://git.proxmox.com/?p=pve-qemu-kvm.git;a=blob;f=backup.txt[here].
 
-NOTE: Qemu backup provides snapshots on any storage type. It does
-not require that the underlying storage supports snapshots.
+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 `mode` for Containers:
+.Backup modes for Containers:
 
 `stop` mode::