Virtual Machine Backup Technologies: VMware VADP, Hyper‑V VSS, and Citrix XenServer Interfaces and Principles

Virtualization backup technology originated with VMware and has expanded as virtualization became widespread; mainstream backup software such as CommVault, Veeam, eBackup, Avamar, and NetVault vRanger now support VMware, Hyper‑V, FusionSphere, Citrix, Xen, and KVM environments.

Backup functions for virtual machines, similar to snapshots, are basic data‑protection features in many virtualization platforms, but they can impact performance and lack flexibility, so dedicated backup software is commonly used.

VMware introduced the VMware Consolidation Backup (VCB) tool in Infrastructure 3, providing drivers and script interfaces for backup software; the tool runs on a Windows‑based backup proxy server and backs up supported VMs or virtual desktops.

VMware Backup Interface and Principles

Since ESXi 4, VMware supports the VADP API for VMFS and NFS datastores, enabling backup software to perform non‑intrusive VM backups and restores; the CBT (Changed Block Tracking) feature allows native incremental backups by tracking changed blocks since the last snapshot.

The backup workflow starts with the backup management server contacting the ESX host, creating a snapshot of the target VM, and then the backup server reads the virtual disks and configuration files from the snapshot to store them on backup media before deleting the snapshot.

VADP offers three data‑transfer modes: SAN, Hot‑add, and NBD. SAN mode reads snapshot data directly over the SAN network (LAN‑free). Hot‑add installs the backup proxy (VSA) inside the ESXi host to read data locally. NBD uses TCP/IP (LAN‑based) to transfer data.

In SAN mode, the backup server accesses VM disks via FC/iSCSI without traversing the LAN; the VSA and media server (MA) can be deployed together or separately, and the MA can attach tape libraries and perform deduplication and compression.

If the storage supports hardware snapshots (IntelliSnap), an ESXi proxy can trigger a hardware snapshot, mount it as a temporary datastore for the VSA, and the MA extracts data over SAN, achieving a server‑free backup topology.

When hardware snapshots are unavailable, the physical MA accesses the datastore LUN via SAN, obtains the VM list and changed data using CBT, and backs up over SAN.

In Hot‑add mode, the VSA runs as a VM on the ESXi host, reading snapshots directly; the VSA can be virtualized while the MA may be virtual or physical, and data is transferred over LAN to the backup medium.

If the storage supports hardware snapshots, an ESXi proxy can create a hardware snapshot and mount it for the VSA, with the MA pulling data over LAN, still a server‑free topology.

In NBD mode, the VSA and MA communicate over TCP/IP (LAN‑based) to transfer data, and both components can be virtualized or physical.

Not all scenarios can use VADP; for example, raw device mapping (RDM) VMs require installing an iDataAgent (iDA) backup client inside the VM.

Hyper‑V Backup Interface and Principles

Hyper‑V relies on Microsoft’s VSS (Volume Shadow Copy Service) for backup, with supported products like NetBackup, Simpana, and Veeam; Simpana is used as an example. Hyper‑V storage can be CIFS or CSV (Cluster Shared Volume), the latter being a NTFS‑based clustered file system common in SAN environments.

VSS provides snapshot and data‑consistency protection; Simpana can manage multiple Hyper‑V nodes, creating protection policies per business needs.

Backup agents (VSA) are installed on each Hyper‑V server to collect VM information; the backup server schedules jobs, and the media server (MA) stores the data. For Windows applications inside VMs, VSS combined with the VSA enables VM and application data backup; non‑Windows workloads require iDA agents.

Hyper‑V also offers hardware‑assisted VSS integration with storage arrays (e.g., NetApp FAS) to perform server‑free backups, though support is still limited.

Citrix XenServer Backup Interface and Principles

Citrix XenServer provides a backup‑optimized API, which is less capable than VMware’s VADP or Hyper‑V’s VSS. XenServer clusters (Xen Pools) share storage.

For Simpana, the VSA must be installed on a VM that has access to the Xen Pool; the MA can be physical or virtual. The VSA uses the Xen API to retrieve VM lists and backs up data via the MA.

Overall, the article outlines the architectures, interfaces, and backup modes for VMware, Hyper‑V, and Citrix virtual environments, highlighting the strengths and limitations of each approach.