High Availability and Disaster Recovery for Virtual Environments


Virtual servers are used to reduce operational costs and to improve system efficiency. The growth in virtual servers has created challenges for IT departments regarding high availability and data protection. It is not enough to protect physical servers but also virtual servers as they contain business critical data and information. Virtual servers offer the flexibility, but at the same time if a single physical server containing multiple virtual servers fails, then the impact of data loss is enormous.

Virtualization Benefits

Companies are adopting virtualization at a rapid speed because of the tremendous benefit it offers and some of them include:

  • Server Consolidation: Virtualization helps to consolidate multiple servers into one single physical server thus offering improved operational performance.
  • Reduced Hardware Costs: As the number of physical servers goes down, the cost of servers and associated costs like IT infrastructure, space, etc. will also decrease.
  • Improved Application Security: By having a separate application in each virtual machine, any vulnerability is segregated and it does not affect other applications.
  • Reduced Maintenance: Since virtual servers can easily be relocated and migrated, maintenance of hardware and software can be done with minimal downtime.
  • Enhanced Scalability – The ease with which virtual servers can be deployed will result in improved scalability of IT implementation.

File or Block Level Replication

Different kinds of replication techniques can be used to replicate data between two servers both locally and remotely. In block level, replication is performed by the storage controllers or by mirroring the software. In file-system level (replication of file system changes), the host software performs the replication. In both block and file level replication, it does not matter what type of applications are getting replicated. They are basically application agnostic, but some vendors do offer solutions with some kind of application specificity. But these solutions cannot provide the automation, granularity and other advantages that come with application-specific solution. Also, one needs to be concerned about the following:

  • Replicated server is always in a passive mode – cannot be accessed for reporting/monitoring purposes.
  • Possibility of virus/corruption getting propagated from production server to replicated server.

Application Specific Replication Approach

In this approach, the replication is done at a mailbox or database level and it is very application specific. One can pick and choose the mailboxes or databases that need to be replicated. In the case of Exchange Server, one can set up a granular plan for key executives, sales and IT people, in which the replication occurs more frequently to achieve the required Recovery Point Objective (RPO) and Recovery Time Objective (RTO). For everyone else in the company, another plan can be set up where the replication intervals are not that frequent.

Another advantage of this approach is that the replicated or failover server is in an Active mode. The failover server can be accessed for reporting and monitoring purposes. With other replication approaches, the failover server is in a Passive mode and cannot be used for maintenance, monitoring or reporting purposes.

Backup and Replication

Some solutions offer both backup and replication as part of a single solution. In this case, the backup is integrated with replication and the users get a two-in-one solution. Considered two-tier architecture, these solutions consists of an application and agent environment.  The application server also hosts the network share that stores all the backup files.  The files are stored on this network share and not on any particular target server so as to prevent loss of backup files.  If the target server goes down, users would like to continue to access their backup files in order to rebuild the target server with as little downtime as possible. 

The mailboxes and databases will be backed to the backup server and then replicated to the remote failover server. The full back and restore is done first and then only the changes will be applied through incremental. For restoring emails, mailboxes and databases, the local backup data can be used and for disaster recovery purposes, the remote failover server can be utilized.

Virtual Environments

Many high availability solutions protect data that reside on virtual servers. Customers can have multiple physical servers at the primary location and at the offsite disaster recovery location they can have one physical server with multiple virtual servers. Also, multiple virtual servers from the primary site can be easily backed up and replicated to the disaster recovery site.

With some disaster recovery solutions, both on physical and virtual servers, the appropriate agents are installed and these agents have very small footprint. Because of the limited footprint, the impact on these servers is minimal from a performance perspective.  With other replication solutions, one has to install the entire application on the virtual servers and this will take a huge toll on performance.

Physical to Virtual Servers

In this scenario, the production environment has physical servers and the disaster recovery site is deployed in a virtual environment. Both the physical and virtual servers are controlled by the Application and it can be located either at the production site or at the remote site.

Figure 1

Virtual to Virtual Environments

In order to achieve significant cost savings, some companies not only virtualize their disaster recovery site but also use virtual servers in the production environment. One can have one or more physical servers housing many virtual servers both at production and remote sites.

Figure 2


When a disaster strikes the primary site, then all the users will be failed over to the remote site. Once the primary is rebuilt, one can go through the failback process to the original primary servers very easily. Also, only a particular virtual server containing Exchange or SQL server can be failed over without affecting other physical or virtual servers. 

The only way to make sure that your disaster recovery solution works is to test it periodically. Unfortunately, to do that one has to failover the entire Exchange or SQL server. Administrators will be leery about doing this for fear of crashing the production Exchange or SQL server. Some solutions can create a test mailbox or database and use it for failover/failback testing periodically. Through this approach, customers can be fully assured that their disaster recovery solution will work when it is badly needed and have peace of mind.


Virtual servers in conjunction with certain disaster recovery solutions can be used as a migration tool. If a physical server goes bad, then one can failover to the remote failover virtual server. Once the primary site is rebuilt, then the failback can be easily achieved. With some applications, there is no need to have identical versions of Exchange on primary and failover servers. In fact, one can run Exchange 2003 on primary server and Exchange 2007 on failover server. This feature can be used as a migration tool. For example, you can failover to the failover server which runs Exchange 2007. Upgrade the original primary to Exchange 2007 and failback again. This scenario is applicable to SQL 2000, SQL 2005 and SQL 2008 servers also.


Companies are increasingly adopting virtual servers as virtualization offers many compelling benefits. This increase in virtualization poses tremendous disaster recovery and data protection challenges to IT Administrators. There is a greater need to implement the appropriate high availability and failover solutions to protect these servers.

About Sonasoft

Sonasoft Corp. automates the disk-to-disk backup and recovery process for Microsoft Exchange, SQL and Windows Servers with its groundbreaking SonaSafe Point-Click Recovery solutions. SonaSafe is the only product that provides an integrated backup/recovery and replication solution for Exchange and SQL servers. Designed to simplify and eliminate human error in the backup and recovery process, SonaSafe solutions also centralize the management of multiple servers and provide a cost-effective turnkey disaster recovery strategy for companies of all sizes.
For more information, please visit www.sonasoft.com.

About The Author

Leave a Comment

Your email address will not be published. Required fields are marked *

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.

Scroll to Top