Volume Shadow Copy Service (VSS) is an integral part of the equation to capture crash-consistent and application-consistent backups. VSS produces a point-in-time consistent snapshot by coordinating with various components of the system, namely applications, file-system and storage. To create consistent backups of the virtual machines in the Hyper-V environment, the host based snapshots are required to interface with the Hyper-V VSS writer, which in turn coordinates with the Integration Services installed in the guest operating system to create a crash consistent and application consistent backup of the data of the virtual machines. Microsoft has truly done a great job in providing this consistent interface to quiesce applications and flush data in memory to the disk to create consistent backups.
Like VSS for Microsoft, the vStorage API for Data Protection infrastructure provides an infrastructure to backup the virtual machines in a VMware environment in a crash-consistent and application-consistent manner. In the case of VMware, the snapshot creation of the virtual machines using the VADP infrastructure interfaces with the VMware tools in the guest operating system to create application-consistent backups of the virtual machines.
However, both the VADP infrastructure for VMware and the VSS (and Integration Services) for Hyper-V have some limitations.
In the case of VMware, VADP does not support certain configurations, namely:

  • Virtual machines configured to Raw Device Mapping in physical compatibility mode. This configuration is typically used when the guest virtual machine interfaces with the storage directly.
  • VADP backups cannot capture configurations where the guest virtual machine is configured with a direct attached iSCSI disk.

In the case of Hyper-V, the host-based VSS backups have certain limitations as well, namely:

  • The guest operating system must be formatted with NTFS and configured as basic disks to perform an online backup, otherwise the virtual machine is placed in a “Saved state” for a brief period during the backup, causing loss of access to the virtual machine.
  • The host-based backups cannot capture pass-through or iSCSI direct attached disks.

Backup products that rely on providing only agentless approach to data protection fall short in protecting environments like the onesdescribed above. It is very important that the backup product not be fanatic of a technology, but rather be fanatic about protecting the data in the best way possible. While an agentless approach is recommended and ideal for most Hyper-V and VMware environments, configurations like the ones mentioned above require the backup product to leverage agents to provide a more comprehensive data protection solution. Unitrends believes in a unified agent / agentless approach for data protection. Use one or the other or both to best protect the data you have.

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