What are the Advantages of Agent vs Agentless Backup?

There are strong feelings in both camps on what the best backup solution is, so I'm going to make an effort to provide you with information so you can decide for yourself. First, let me offer my opinion. Some people prefer the convenience agent-less backups offer because you don't have to restart an operating system when setting up a backup. This would seem like a big plus in my mind. With an agent-based backup you will need to reboot a system after an install because the snapshot driver resides at the operating system kernel level and hence requires the OS to be restarted. On the other hand, because an agent-based backup is loaded in the OS stack it offers increased control and visibility of the host system that is not immediately available to an agent-less backup.  For example, agent-less backups typically need to traverse the file system to determine changes for incremental/differential backups.  This traversal can take longer and be more complex than the agent-based backup which has access to Change Block Tracking (CBT) at the kernel level. Agent-less backups actually use a technique called Dispatch Table Hooking which injects code into a running Windows OS system without requiring a reboot. This technique is sometimes employed by malware and has been explicitly discouraged by Microsoft's kernel engineers as it can easily cause instability in the system resulting in system crashes and unrecoverable data corruption. Agent-based backups rely on local resources to pre-process and compress data before transmitting data across the network to the storage device (NAS/SAN, local USB drive, or remote host).  By contrast, agent-less backups rely more on network resources to transmit application commands across the network as well as data between the target and storage device.  If your local network bandwidth is just barely enough to meet your needs, then the additional network traffic from agent-less backups could impact local network performance. Here are some more reference articles written from both perspectives: Dell AppAssure published the "5 Misperceptions of Agentless VMware Backup" Asigra (a competitor of StorageCraft) published "Agentless Backup is Not a Myth" StorageCraft published "Agent vs. Agentless Backup" as well as an extensive white paper "They Myth of Agentless Backup" Kate at Symantec published "Agent and Agentless VM Backup and Recovery" which describes why you want to consider using agentless backups for VMs. Greg Shields (Microsoft MVP) advocates smart agents in his paper, "VMware and Hyper-V Backups Agents vs Agentless Approach" I'm trying to be open and honest here to point out the differences between agent-less and agent-based backup applications because there are strong arguments for either side. In my opinion, the single most powerful reason for running an agent-based system is that Microsoft seems to indicate that this is the way they intended the operating system to allow backups. Microsoft's own backup software is an agent-based backup system. And I have to believe that since Microsoft authored the OS it makes sense that they would select the best way of implementing a backup application to run on that OS. On the other hand, Microsoft doesn't prohibit agent-less backups and there are many products out there that are agent-less. In the end it may simply be a matter of understanding your specific environment and the pros/cons of an agent-less and agent-based system and then deciding which one works best for you. Personally, I think a well-informed decision is the best type of decision. So if I've helped to clear the waters (or muddy them?) at least you're thinking and examining the options. Cheers!