One of the challenges, however, of protecting virtual machines, versus physical servers, is the storage method used within virtual machines – namely virtual disks and raw disks.
Where a group of related virtual disk files exists on the server, it presents itself as storage devices to the virtual machine operating system. This causes encapsulation of the entire file set, which provides benefits in its autonomy, but also complexity in its protection. Alternatively, the raw disk method allows for virtual machines to access a physical partition on the host server directly.
The most common customer need is to protect unique and sometimes conflicting applications on multiple servers to a single recovery server.
While the solution must support a ‘many-to-one’ model for data replication, certain applications cannot co-exist on the same server due to conflicts or operational requirements.
By leveraging virtual machines on the target platform, each failed source can be protected and made available by an independent virtual machine target.
Some leading solutions enable replication within a guest OS or on the host OS. Replicating for a guest OS is just like replicating from a traditional physical server, so no special considerations are necessary.
This offers the flexibility of providing file level selection for replication of individual files with the virtual disks belonging to a virtual machine.
Replicating from the guest means that disk-writes can be captured at the host’s file system layer, while all other applications operate above the actual file system layer.
This allows transparent and reliable data replication to a separate target server running another copy, sending only real-time byte-level changes across any IP connection.
With the addition of virtual servers such as VMware GSX, users have the choice of replicating data from within the guest operating systems or from the host operating system.
Used in conjunction with byte-level data replication technology (as above), this offers the capability to replicate non-Windows OS virtual machines as well as providing complete failover capability of the guest OS.