One of the benefits of running a virtual machine (VM) environment on a Storage Area Network (SAN) is the ease and speed of backing up hundreds or thousands of VMs. A single click or scheduled task instantaneously backs up every VM in a datastore with a crash-consistent snapshot. In most cases this snapshot can be used to restore a VM to a functioning state, but it does not protect open files or databases well.
The difference between crash-consistent and application-consistent backups is stark. In a nutshell, a crash-consistent backup creates a snapshot that, when restored, creates a virtual machine that resembles one that has had the power abruptly turned off. The snapshot will return the virtual machine’s operating system to its pre-crash state, but do little to preserve the consistency of open files or transactional databases that reside on it. On the other hand, an application-consistent backup will ensure that all database transactions are committed and all disk transactions are completed prior to a snapshot. This ensures the data integrity of open files and databases on every backup. Because of this, recovery from a failure will happen without data loss, and can proceed instantly, as a data consistency check is not required.
Given the advantages that application-consistent backup’s offer, why would they not be the norm? Creating a crash-consistent snapshot is easy and free; every SAN vendor includes this feature with their hardware. Getting application-consistent snapshots requires additional software, and storage vendors charge a premium for this advanced solution. Implementation of this type of solution can be complex, depending on the SAN vendor. Many IT departments lack the skills to implement such a solution.
Another answer is lack of understanding; Either by IT staff, or a cloud services provider. It is critical that the differences between these two backup schemes be well understood by whoever is maintaining virtual machines for the enterprise, and the question of whether application-consistent backups are being provided should be asked up front, rather than after the fact. A cloud services provider should be aware of, and offer to provide application-consistent backups whenever they are required.
If a virtual machine is running services such as Active Directory, Exchange, Oracle, SQL, or SharePoint, it is imperative that an application-consistent backup be performed. This will ensure that a complete recovery without data loss will always be possible, and that it will take less time. The potential benefits arising from a fast response to a failure situation, coupled with the security of knowing that the data is intact, should make an application-consistent backup standard procedure in any organization.
