Host based backup or agentless backup, as the name indicates, is a system that does not require installation of agents on each and every machine that connects to the network. The agent is installed on a single machine, which in turn identifies all the machines that will be connecting to the network using a single management interface.  The controlling machine may inject small codes into the connecting machines and integrate with VADP or VSS to deliver high performance backups to the remote backup server.  It works very well with physical and virtual machines.

The agentless backup system has been adopted by the second wave of Cloud services such as Asigra.  The software interacts with the VSS or VADP and initiates backup from the connecting machines on the network in a step by step manner.  The data to be backed up is made ready for backup from the controlling system.  The controlling system then communicates with VSS or VADP and requests them to create a snapshot of the volume and feed the same to the remote backup server via the controlling machine interface.  Full virtual machines and physical machines can be recovered in the same manner, but the process can be very complex and long drawn.  This is because the machine looks for metadata and uses the metadata to orchestrate the recovery correctly to the correct machine.  This process cannot be wished away for a variety of reasons, including possible corruption of data that is being recovered.  In short, the transactional post-processing that happens in agentless backup is what distinguishes it from agent based backup.

The advantages of agentless backup are:

  1. Both offline and online virtual machines get backed up
  2. The consumption of CPU, memory and I/o resources is minimal
  3. The update of the agent is not a requirement
  4. Purchase of a single license is sufficient
  5. Any number of machines can connect to a single account

The difficulties that are experienced by computer managers and applications administrators of individual machines on the network have them protesting the use of agentless backup systems.   The administrators have no say on what is to be backed up and what is not.  It is centrally decided and orchestrated.  Critics of this system point out that granular recovery of information is not possible for virtual machines from the backup.  The entire virtual machine may have to be recovered before a single file or folder can be restored.  The tradition demands traditional login operations for access to the backup server.  If the central control fails all access to the backup server, data may be lost.