Depending on the hypervisor in use, the IT infrastructure in place and the required recovery time objective (RTO) and recovery point objective (RPO), there are a variety of disaster recovery (DR) methods for server virtualization.
Traditional backup and recovery
SMBs with a small number of virtual servers frequently back up virtual machines (VMs) and restore them to the DR site. As the restore can be done on almost any hardware that runs the hypervisor software, hardware requirements for restoring physical servers become a non-issue. Being able to restore multiple VMs to a single host further reduces hardware requirements for the secondary site and significantly lowers the overall cost of disaster recovery.
In its simplest form, virtual servers can be backed up by installing backup agents in the virtual machines. But this adds overhead and will likely impact server performance while backups are running. Companies that run VMware Inc. have the option to deploy VMware's Consolidated Backup (VCB), which removes the backup load from virtual machines. In addition to backing up each virtual machine, backups can be taken at the hypervisor level. This eliminates the need to install agents on each virtual machine, but only allows restoring at the VM level.
Backup software vendors like CommVault have extended their backup suites to accommodate the backup needs of virtual servers. The majority of backup software vendors' products are integrated with VCB and some have added extra features.
"We use VMware VCB and CommVault Galaxy to back up about 70 VMware ESX guests," said Peter Kovaleski, network Unix administrator at Oral Roberts University in Tulsa, Okla. "The CommVault restore agent allows us to directly restore files back to ESX hosts, eliminating the manual copy process of files from the proxy server to the VM."
Synchronous or asynchronous replication
Storage system-based snapshots and replication are the prevailing methods in the enterprise space to get VM images and data from a primary to a secondary site. From a storage and replication perspective, the requirements to protect physical and virtual servers are very similar.
Snapshots are scheduled to capture all changes since the last snapshot. The frequency of snapshots varies and depends on the acceptable RPO. A key requirement during snapshots is quiescing virtual servers to ensure that the entire state of VMs is captured at the point the snapshots are created. Snapshots are then replicated to the secondary site via synchronous or asynchronous replication.
Among all hypervisors, Microsoft Corp. faces the fewest integration issues with storage systems because it uses NTFS and VSS protocols that are widely supported by storage vendors. Similarly, because of its 70% market share, VMware enjoys widespread integration support, especially for its Site Recovery Manager, which is also supported by most major storage vendors.
Storage system-based snapshots and replication are favored by enterprise customers because they're likely to already have storage systems that support these technologies and are hesitant to sign up for less-proven alternatives like continuous data protection (CDP).
"We decided to use NetApp Inc. and NetApp's SnapManager for [our] virtual infrastructure because it allowed us to automate what was previously scripted, from quiescing the virtual machines and taking snapshots to replicating them to the secondary site," explained Peter Allen, director of IT operations at Nixon Peabody LLP in Rochester, NY.
Continuous data protection
CDP products such Virtual Systems Inc.'s Double-Take, FalconStor Software Inc.'s CDP Virtual Appliance for VMware Infrastructure and Network Storage Server (which enables automated, application-consistent failover) and InMage Systems Inc.'s DR-Scout are viable DR alternatives to storage-based snapshots and replication.
These products are less expensive, especially for customers who don't have matching storage systems in the primary and secondary data center. Because changes are captured and replicated as they occur, they add very little overhead to virtual machines. Finally, CDP products not only provide for failing over to the latest replica, but allow users to easily roll back to previous points in time.
"We chose DR-Scout over array-based replication because of its minimal bandwidth use," said Jim Yarber, senior manager of network operations at HeritageBank of the South. "I have a 100 Mb Ethernet connection between our two data centers, and DR-Scout barely scratches it."
While disaster recovery has always been expensive and many plans only cover mission-critical apps, server virtualization is revolutionizing this process. A high level of mobility and the relative hardware independence of virtual servers greatly reduce the cost and complexity of putting a plan in place, enabling companies to expand DR to a larger number of servers and applications.
This article originally appeared in Storage Magazine.
About this author: Jacob Gsoedl is a frequent contributor to "Storage" Magazine.
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