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VSS depends on several main components (see "Microsoft Volume Shadow Copy (VSS) component," below): VSS-enabled applications need to implement a so-called VSS-writer that coordinates various components to create consistent shadow copies of one or more volumes. Applications, such as a backup application, need to implement a VSS-requestor to request a volume shadow copy. The key component, though, is the VSS-provider, which creates and maintains shadow copies (snapshots). While VSS-providers are included with the latest Windows OSes, software and storage hardware vendors can provide their own VSS-providers. Noticeably, hardware-based VSS-providers of arrays enable high performance and highly scalable data protection of Hyper-V environments. While it took VMware to implement change block tracking to get to efficient snapshots, in VSS it's a capability of the VSS-provider. For instance, the VSS-provider that's part of Microsoft's operating systems does incremental snapshots via a copy-on-write method; that is, when a change to the original volume occurs but before it's written to disk, the block to be modified is read and stored away.
MICROSOFT VOLUME SHADOW COPY (VSS) COMPONENTS
Enlarge MICROSOFT VOLUME SHADOW COPY (VSS) COMPONENTS diagram.
The support for VM image-level backup of Hyper-V isn't as extensive among third-party backup applications as is support for VADP. For instance, IBM Tivoli Storage Manager (TSM) and PHD Virtual Backup don't have support for it at present. Similar to VMware, Microsoft provides its own backup solution for Hyper-V environments with System Center Data Protection Manager (DPM). DPM provides near-continuous data protection for virtual machines hosted on servers running Hyper-V. With advanced features like disk-to-disk and disk-to-tape support, the ability to recover sub-VM objects like files, protection of virtual machines while live migration is in progress, integration into Microsoft System Center, and support for clustered and standalone Hyper-V systems, DPM provides a state-of-the-art product rather than the entry-level product VMware provides with VDR.
Citrix XenServer backup
Third-party backup applications can initiate full or incremental disk image snapshots of Citrix XenServer VMs through XenAPI (XAPI). These snapshots are usually crash-consistent and depend on applications to regain a consistent state after a restore. Recovering after restoring a crash-consistent backup is analogous to powering up a virtual machine after a power failure.
With XenServer 5.6, Citrix added live memory snapshots to capture the state of a virtual machine when a snapshot is taken and allows reverting to a previous state on restore. Citrix memory snapshot leverages Microsoft VSS, so it's available for VSS-enabled VMs (Microsoft operating systems) but not for Linux virtual machines.
In addition to XenServer snapshots, XenServer supports shared storage snapshots for arrays supported by XenServer. Leveraging snapshot capabilities of arrays is the fastest and most scalable method to protect a XenServer environment, but it's only an option if the storage infrastructure is supported by XenServer.
Like vSphere and Hyper-V, Citrix provides its own VM image backup application with VM Protection and Recovery (VMPR). A scaled-down version of VMPR that lacks features like scheduling is included with all versions of XenServer. A more advanced version that supports scheduling and automation is available as a paid option. Third-party backup application support for XenServer image-level backup is more tenuous than for vSphere and Hyper-V. Among the backup application vendors that support it are Arkeia Software, CommVault, PHD Virtual Technologies and Veeam Software.
VM backup bottom line
Block-based VM image backup on the hypervisor host, ideally via a backup proxy server, is becoming the preferred way of backing up virtual servers. Maturing backup APIs in vSphere, Hyper-V and XenServer, as well as increasing support by backup applications for these APIs combined with performance and scalability merits, are among the main reasons for its adoption. Because most organizations run more than a single hypervisor (more than 70% of companies according to ESG's Whitehouse) and a mix of physical and virtual servers, multihypervisor support and the ability to support both physical and virtual server backups are important considerations when choosing a virtual server backup application.
BIO: Jacob Gsoedl is a freelance writer and a corporate director for business systems.
This was first published in July 2011