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DR for virtualized servers

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As part of this process, VCB quiesces the file system in the VM to ensure that the entire state of the VM is captured at the point the snapshot is created. VCB currently falls short when it comes to backing up apps like Exchange running within VMs. Although the latest version of VCB has added a VSS requester to make application-consistent snapshots, it hasn't addressed the restore aspect, as the only way to restore an application to a consistent state is to restore the entire virtual machine.

VMware's Site Recovery Manager (SRM) enables companies to automate the failover process of their virtual server infrastructure. In the absence of a tool like SRM, the failover process needs to be done manually or via custom scripts. The integration with replication software is an instrumental aspect of automating the failover process and through Site Recovery Adapter (SRA) plug-ins, third-party replication software can integrate with SRM.

All major array and replication software vendors have developed or are in the process of offering SRA adapters, which are available directly from VMware. Released in late 2007, SRM is still a pretty new tool with shortcomings that are likely to be addressed in future revisions. "We aren't currently using SRM to bring up virtual machines in the DR site because it doesn't support NFS, but only works for FC LUNs," says Nixon Peabody's Allen. Not having a tool like

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SRM for Hyper-V or XenServer isn't as much of a fundamental problem as it is an inconvenience. "There's nothing in VMware's Site Recovery Manager that couldn't be done via scripts; in fact, SRM provides a GUI to define the recovery process and generates the script for you," explains ESG's Bowker.

VMware was the first hypervisor vendor to perfect live migration of virtual machines through VMotion, which moves an entire running VM instantaneously from one server to another with zero downtime to apps. The entire state of a VM is encapsulated by a set of files stored on shared storage, and VMware's VMFS cluster file system allows the source and target VMware ESX server to access these VM files concurrently. The active memory and precise execution state of a virtual machine can then be rapidly transmitted over a high-speed network.

Although VMotion is used mostly for operational reasons, users are taking advantage of it as part of their DR strategies to distribute the workload to other ESX servers. Jim Yarber is senior manager of network operations at HeritageBank of the South in Albany, GA, a regional bank serving southwest Georgia and north central Florida. VMotion is an instrumental part of his DR plan. "In case of a failover, virtual machines are brought up on three ESX servers in the DR site," says Yarber. "To accommodate the increased workload, we then use VMotion to transparently migrate virtual servers to other ESX servers."

Yarber also takes advantage of VMware HA, which monitors virtual server availability and automatically moves and restarts failed virtual servers on other ESX servers. "VMware HA enables us to have high availability without the need for expensive standby hardware and additional software required by traditional HA cluster solutions," he says.


Microsoft Hyper-V
In Hyper-V and its predecessor Virtual Server, virtual disks are stored as VHD files on the Windows file system and are no different from other files. As a result, existing Windows data protection tools and methods can be used unchanged to back up and restore VHD files. "Double-Take can replicate Hyper-V file-level changes just like we replicate Exchange DB files," says Bob Roudebush, director of solutions engineering at Double-Take Software.

Unlike VMware ESX Server, Hyper-V leverages the NTFS file system, not a clustered file system, which makes live migration more challenging. In Microsoft's quick migration, which is based on traditional clustering, all VMs on a LUN are migrated at the same time. As Hyper-V-based HA is still dependent on cluster software, Hyper-V users can opt for a next-generation cluster server like the latest version of Symantec's VCS, which supports up to 256 heterogeneous nodes and is very lightweight. "Unlike VMware HA, which provides DR for VM containers without regard to the apps within the virtual server, VCS focuses on HA for apps," says Symantec's Nadeau.

Even though Microsoft Hyper-V is a huge step forward from Virtual Server, especially regarding management, reliability and scalability, it isn't quite on par with VMware ESX Server yet. It also relies to a larger degree on third-party tools and custom scripts for DR than does ESX Server.

This was first published in December 2008

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