VDAP vs VSS

Host Operating System Level Backup is simple so as virtual backup makes data protection easier for the user.

VMware's VADP (vStorage APIs for Data Protection) is technically superior to Microsoft's Hyper-V VSS(Volume Copy Shadow Service) because VADP supports Changed Block Tracking (CBT). On many file systems, CBT identifies the disk sectors altered between two change set IDs. On VMFS partitions, CBT can also identify all the disk sectors that are in use.

Virtual disk block changes are tracked from outside virtual machines, in the virtualization layer. When software performs a backup, it can request transmission of only the blocks that changed since the last backup, or the blocks in use. The CBT feature can be accessed by third-party applications as part of the vSphere APIs for Data Protection (VADP). Applications call VADP to request that the VMkernel return blocks of data that have changed on a virtual disk since the last backup snapshot.

VADP enables centralized, off-host LAN free backup of vSphere virtual machines, reduces ESX host resources to do backup processing, and enables flexible backup windows. A backup product using VADP can backup vSphere virtual machines from a central backup server or virtual machine without requiring backup agents or requiring backup processing to be done inside each guest virtual machine on the ESX host. This offloads backup processing from ESX hosts and reduces costs by allowing each ESX host to run more virtual machines.

VADP leverages the snapshot capabilities of VMware vStorage VMFS to enable backup across SAN without requiring downtime for virtual machines. As a result, backups can be performed non-disruptively at any time of the day without requiring extended backup windows and the downtime to applications and users associated with backup windows.

VSS, which operates at the block level of the file system, provides a backup infrastructure for Microsoft operating systems. The VSS requestor announces that it needs to create a server snapshot. Prior to creating that snapshot, it queries the server to determine which VSS writers have been installed. (It needs this list so it can later instruct each writer to quiesce its associated application).  Then, the VSS requestor instructs each VSS writer to accomplish whichever task is needed for data quiescence. After each VSS writer reports that it has completed pre-backup tasks, the VSS requestor instructs the VSS provider to create a snapshot. The provider tells the requestor where to go to locate the data it needs and the backup begins. When the VM backup is complete, the VSS requestor announces that it has completed its activities. This announcement instructs each VSS writer to perform any post-backup tasks necessary so the computer and its applications can return to regular operation.

VSS writer -  Each VSS-aware application installs its own VSS writer to a computer during the initial installation.

VSS requestor -  Any application that needs to quiesce data for capture can play the role of VSS requestor.

VSS provider - The provider creates and manage the shadow copies of data on the system.