I was talking with some folks at Cisco Live 2013 last week in Orlando about, well, backup, of course. We had a pretty good discussion on the topic of agent vs. agentless backup for virtual servers. One of the gentlemen in the discussion had a massive IT environment (petabytes) which was almost completely virtualized on Hyper-V. He’s in the process of migrating everything to Server 2012, but still has most of his hosts running W2K8 R2.
Anyway, we debated the merits of installing agents on guest VM versus running agentless host-level snapshot for backup. We agreed that having agents on guests does allow the flexibility of architecting different backup strategies for application data and server/OS/file-level data. It really comes down to what business requirements are, and what level of recovery is required (recovery-time objectives vs. recovery-point objectives). His environment consists of applications running on MS SQL Server which are highly transactional in nature. So for him, the ability to do point-in-time transaction log-level recovery of his applications is critical. The backup architecture which works best for him include having backup agents specifically targeted at his SQL databases, with transaction log backups running every 15 minutes.
But, other types of environments may not requires such granularity, and thus protecting virtual servers with host-level snapshot backups running incremental forever backup strategy is completely sufficient. One misconception that some of the folks at the show had was an assumption that host-level block-based backups prevented them from being able to restore a single file, folder or data element from the block-based backup. With Unitrends software, item-level point-in-time recovery from a block-based host-level snapshot backup is completely possible. We synthesize the blocks of the backups to the requested point-in-time and present the user with an iSCSI LUN or a network share which is browsable. The user simply locates the granular elements to restore and copies them to their desired location. Of course, full virtual server recovery and instant recovery are possible as well, but some folks don’t realize they can still have granular single-file type restores with agentless host-level backups.
It was a really good discussion, and we thought the show was fantastic. My feet are still hating me for 4 days of standing, but the rest of me really enjoyed it. 
