Virtualization Backup Technologies: VMware, Hyper‑V, and Citrix Backup Interfaces and Principles

Virtualization backup technology was first introduced by VMware; as virtualization became widespread, mainstream backup software now supports VMware, Hyper‑V, FusionSphere, Citrix, and Xen/KVM‑derived platforms, with specialized solutions like Veeam, eBackup, Avamar, and NetVault vRanger for virtual applications and desktops.

Similar to VM snapshots, backup is a fundamental data‑protection function in virtualization, but it can affect performance and lacks flexible networking, so enterprises typically use dedicated backup software. VMware’s Consolidation Backup tool (VCB) provides drivers and scripts for backup agents installed on Windows‑based backup proxy servers.

VMware Backup Interface and Principles

Since ESXi 4, VMware supports the VADP API for VMFS and NFS disks, enabling backup software to perform non‑intrusive backups and restores. The CBT (Changed Block Tracking) feature lets the VMkernel track modified blocks since the last snapshot, allowing efficient incremental backups.

The backup process starts with the backup management server connecting to the ESX host, requesting the VSA to create a snapshot of the target VM. The VM continues running while the snapshot captures a static view of its data.

The backup server then reads the virtual disks and configuration files from the snapshot and writes them to the backup medium before instructing the ESX host to delete the snapshot.

VADP offers three data‑transfer modes: SAN, Hot‑add, and NBD, each differing in how the VSA retrieves the VM list and data.

SAN backup mode reads data directly from the storage over a SAN (FC/iSCSI) network, bypassing the LAN. The VSA and Media Agent (MA) can be deployed on physical servers; the MA may connect to tape libraries and perform deduplication/compression.

If the production storage supports hardware snapshots (IntelliSnap), the ESXi proxy can mount the snapshot as a temporary datastore, allowing the VSA and MA to extract backup lists and data directly from the SAN (Server‑Free topology).

When hardware snapshots are unavailable, the VSA still accesses the LUN via SAN, obtains the VM list and changed blocks (using CBT), and backs up the data over the SAN.

Hot‑add backup mode installs the VSA on the ESX host, allowing direct reading of VM disks. The VSA can be virtualized; the MA may be virtual or physical. Data is transferred over the LAN to the backup medium.

If the storage supports IntelliSnap, the ESX proxy can mount the snapshot, and the VSA/MA retrieve data over LAN while the production ESXi remains unaffected (Server‑Free topology).

NBD backup mode uses TCP/IP to read backup data. The VSA and MA may be virtual or physical, and data is sent to the backup medium over a LAN‑based connection.

Not all scenarios can use VADP; for example, raw device mapping (RDM) requires treating the VM as a physical machine and installing an iDA agent. Also, VADP backs up the VM, applications, and data together, so backing up only applications may also need a physical‑machine approach.

Hyper‑V Backup Interface and Principles

Hyper‑V relies on Windows VSS (Volume Shadow Copy Service) for backup, with supported products like NetBackup (NBU), Simpana, and Veeam. Storage can be CIFS or CSV (Cluster Shared Volume), the latter built on NTFS and Windows Failover Clustering.

VSS provides snapshot and data‑consistency protection. Simpana can manage multi‑node Hyper‑V clusters, creating protection policies per business needs.

Each Hyper‑V host must install a VSA backup agent to collect VM information. Under Simpana’s scheduler, the backup server uses a Media Agent to store VM data. For Windows applications inside VMs, Simpana leverages VSS to back up both VM and application data; non‑Windows workloads require an iDA agent.

Hyper‑V also offers hardware VSS integration with storage arrays (e.g., NetApp FAS) to achieve Server‑Free backup, though support is still limited.

Windows VSS is broader than VMware’s VADP, providing data‑consistency for many applications (SQL, Exchange, etc.) and is integrated into many storage management solutions (HP Recovery Manager, NetApp SnapManager).

Citrix Backup Interface and Principles

Citrix XenServer offers a backup‑optimizing API, but its capabilities are weaker compared to VMware and Hyper‑V.

XenServer runs on underlying hardware, forming clusters (Xen Pools) that share storage.

For Simpana, the VSA agent is installed on a VM within the Xen Pool, while the Media Agent can be on a separate physical machine or a VM. The VSA uses the Xen API to retrieve VM lists and backs up data via the Media Agent.

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