There are two ways to back up a virtual machine. One is to treat it as a traditional machine and back up using a locally installed software package or agent. The second method is to back up the virtual machine as an object from the perspective of the host computer. The traditional method is the most direct and the easiest to understand and implement. The second method requires specialized software that runs on the host and is more complicated. Both have their strengths and weaknesses.
Traditional Backup Approach
Virtual machine operating systems don’t function significantly differently from those of physical machines. To that end, there is no restriction on installing a backup program or agent inside a virtual machine and backing it up as though it were physical. The only limitation here is that a backup device cannot be directly attached to a virtual machine in most cases. Backup run from within a virtual machine will behave exactly as it would if the machine were not virtualized.
- No change or relearning from pre-virtualization methods
- No special requirements
Backup software does not need to be Hyper-V aware or compatible
- Each virtual machine must be licensed for backup as though it were physical
- If tape is the backup target, the tape device will likely need to be attached to another physical host
- A “bare metal” style of backup/restore will not be as complete or as portable as a per-VM backup
- Snapshots and other elements of the virtual machine metadata are not captured
These backups must be treated as though they were run from a standard physical machine. Any issues that would prevent a file from being backed up on a physical machine will also prevent it from being backed up on a virtual machine. For instance, if your backup software cannot process open files, then they will be skipped.
A few software packages exist that can interact with the Hyper-V role to back up virtual machines. This is not a simple process and only a few backup vendors fully support it. However, there are plenty of reasons that a host-level backup is desirable and there are ways to mitigate the problems.
- The entire state of the VM can be captured including all of its metadata and snapshots, so “bare metal” types of restores are fairly simple.
- Licensing is usually per-host or per physical CPU, so costs are almost always less than a traditional approach.
- Virtual machine backups are the most portable; VHDs can go anywhere
- VM-level backups aren’t the same as traditional methods, so some new learning and approaches are required
- Connection and usage of storage devices may present a problem, although usually not worse than in any centralized backup system with distributed agents
- Most VM backup strategies copy entire VHDs, even if they’re mostly empty
It is not always possible to back up a VM without pausing it briefly.
As the last bullet point notes, it is sometimes necessary to pause a VM in order to back it up from the host. Such a pause is typically very brief and may not even result in a service outage for connected clients, but backup administrators must be aware of the possibility. Coverage of live versus paused host-based backups will be covered in a posting entitled “Requirements for Live Backup of Hyper-V Guests”.
You can also read Eric’s take on Hyper-V Backup Strategies for the Hyper-V management OS.