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Without question, the trend is toward VM image backup at the hypervisor level and offloading the backup task to a proxy backup server -- and as the number of virtual servers grows it becomes even more relevant. Mechanisms to enable efficient VM image backup and capabilities vary significantly between Citrix XenServer, Microsoft Hyper-V and VMware vSphere.
VMware vStorage APIs for Data Protection
Data protection has been a sore spot for VMware and it took VMware until vSphere 4 to get it right. Prior to vSphere 4, VMware provided VMware Consolidated Backup (VCB) to offload backups from hypervisors to a proxy server, but it wasn't widely adopted due to some grave shortcomings. With VCB, snapshots of virtual machine disk (VMDK) images were taken and copied in full to a proxy server from which backups were run. Offloading the backup to a proxy server minimized the impact of backups on VMs, but it required additional storage for the snapshots.
"VCB was clumsy and the biggest problem with it was how to size the proxy server to where all snapshots needed to be copied," said Lauren Whitehouse, a senior analyst at Milford, Mass.-based Enterprise Strategy Group (ESG).
The vSphere 4 vStorage APIs for Data Protection (VADP), the successor to VCB, addresses the shortcomings of VCB. To start with, VADP no longer requires copying data to a proxy server; instead, snapshots can now be mounted to a proxy server where they're backed up to disk or tape. While VCB only supported taking full snapshots of a VMDK, regardless of how much it changed, VADP supports efficient snapshots via its change block tracking (CBT) feature. CBT keeps track of changes within a VMDK at a block level and enables efficient snapshots of changes only.
vSphere 4 is fully integrated with VSS to enable application-consistent snapshots of VSS-enabled applications running on virtual machines. To be able to take advantage of VSS, however, VMware Tools needs to be installed on the virtual machine. vSphere communicates with VSS via VMware Tools.
To back up a VM via VADP, a "quiesce" command is sent to vSphere to instruct the VM to flush data in memory to disk and no longer accept writes. If VMware Tools is installed on the VM, VMware Tools can pass on the "quiesce" to VSS-enabled applications on the virtual machine to also "freeze" applications within the VM. A snapshot is then taken; on completion of the snapshot, the "freeze" is removed from the VM and VSS-enabled applications. Finally, the snapshot is mounted to the backup proxy from where it's backed up to disk or tape.
VADP is widely supported by third-party backup applications. In addition to major backup application vendors (Arkeia Software, CA, CommVault Systems Inc., EMC Corp., IBM, Quest Software Inc./BakBone Software Inc. and Symantec Corp.), smaller vendors such as PHD Virtual Technologies and Veeam Software offer virtual server backup applications with VADP support. Additionally, vSphere provides its own backup tool called VMware Data Recovery (VDR). VDR is delivered as a virtual appliance to perform snapshots and deduplication to a backup disk target. VMware has positioned VDR as a lower end backup product.
Microsoft Hyper-V and VSS
Thanks to VSS, Microsoft got data protection for Hyper-V right from the get-go. In many ways, a VMware VADP backup cycle resembles backing up Hyper-V. A backup app dispatches a "quiesce" command to a Hyper-V VM via VSS to flush data in memory to disk; VSS then takes a snapshot and removes the freeze from the VM. Similar to VADP, the snapshot can then be replicated or mapped to a dedicated backup proxy server. The "quiesce" can be extended to VSS-enabled applications within VMs, but requires the so-called backup integration service installed on the VM, akin to VADP requiring VMware Tools.
This was first published in July 2011