This morning, my colleague Joe Hoegler pointed me to a new post on VMware’s Business Critical Applications blog entitled High Availability for Exchange 2010 without DAG. Joe recently achieved his Microsoft Certified Master on Exchange 2010 and has a great deal of experience with Exchange. He and I have worked together on projects where we’ve been successful in virtualizing Exchange 2010 on vSphere. We both read the article and spent some time discussing it and both came to the same conclusions, so we wanted to share some of our collective thoughts.
The basic premise of the article is this – you can achieve high availability with Exchange 2010 without the use of Database Availability Groups (DAG) by using a combination of VMware HA and Symantec’s ApplicationHA. Application HA leverages APIs exposed in vSphere 4.1 that let third party applications interact and communicate with VMware HA to perform various functions such as restarting the guest if the entire application has failed. ApplicationHA can also be used to restart individual guest services, so it can provide a level of application awareness beyond what VMware HA can do alone.
Joe and I agree that the blog is technically correct in that you can get high availability for Exchange 2010 without using DAGs if you use these tools. Where we don’t agree with the author is in the usefulness of this kind of solution compared to the functionality that a DAG actually provides.
More than just HA
DAGs provide high availability of the Exchange environment but they do much more than that. They facilitate DR scenarios by including a native replication technology that lets organizations easily replicate their email data to another site that can be brought online quickly and safely. You can also easily move databases between Mailbox servers within the same site for routine system maintenance, Windows patches, etc. DAGs can also detect a failure and activate another database copy quickly, most times (but not always) quicker than a VM can reboot. Additionally, because a reboot can be required with an ApplicationHA/VMware HA solution, there is technically an outage to the end users and the end user experience associated with a reboot repopulating the Exchange database cache isn’t ideal.
DAGs provide high availability at the storage layer
Since a DAG allows up to 16 copies of any mailbox database, it can provide high availability at the storage layer in addition to just protecting the server and Exchange services. This allows you to put your databases on completely separate storage platforms for a higher degree of availability and protection from many kinds of storage-related failures. Since Exchange 2010 enables and encourages larger mailboxes and databases, recovery time in the event of catastrophic server failure can be significant if some kind of storage high availability or resiliency isn’t in place.
Management separation
In many firms, the engineering team responsible for Exchange may not be the same as the team responsible for the virtual environment. In these cases, it is important to understand that leveraging a DAG would allow the Exchange team to be wholly responsible for high availability of the Exchange application. Leveraging ApplicationHA/VMware HA to protect Exchange would require coordination between the Exchange and virtualization teams for any high availability issues/troubleshooting and that may not be appropriate or optimal in many environments.
Microsoft supported technology
Whenever you go down the road of protecting Microsoft applications with tools that are provided by third party vendors, you run the risk of having issues with Microsoft support. This used to be more of an issue in the earlier days when host based replication tools were used to protect Exchange, SQL, etc., and is likely less of a problem these days. I suspect that Symantec has done the right thing here and ApplicationHA does not cause issues with Microsoft support, but having never used it I can’t say for sure.
Cost
When it comes to protecting Tier-1 applications like Exchange cost is important but is not often the driving factor in making a technology decision. Email has become one of the most important applications in the organization so firms are more willing to pay up to protect it. That said, the cost of Symantec’s ApplicationHA seems high compared to the costs of implementing a DAG. With a DAG, you’re just paying for an extra Windows license (which may not have a capital cost depending on your Windows licensing model) and an additional Exchange license. That may come out cheaper than ApplicationHA, which is licensed in packs of 5 protected servers. Again cost is usually not the most important factor, but paying less and getting more functionality out of a DAG makes it an attractive option.
Final thoughts
I strongly believe that vSphere provides a great platform for virtualizing Tier-1 applications like Exchange. Joe and I have worked with clients that have virtualized Exchange 2010 for thousands of users on top of vSphere with great success. Yet we both feel that the value that DAGs add to Exchange 2010 makes using them on your virtualized Exchange 2010 deployment well worth the added planning, storage requirements, and restrictions.
In my opinion there are some areas where using ApplicationHA and VMware HA guest level monitoring might be useful, such as:
- Small environments where there is no budget for additional storage purchases to accommodate multiple database copies.
- There is a requirement for using vMotion on Mailbox servers (which is not supported in combination with DAGs). I personally cannot think of a use case where I would want to use vMotion over moving databases between nodes in a DAG, but I recognize there may be situations where this may be desirable.
- An organization already has an investment in a host based or storage based replication product for Exchange but still wants server high availability. Again, with the native replication functionality of Exchange 2010 (and the 3rd party replication API for the DAG) there are less reasons to use features other than what the DAG provides natively.
I believe that ApplicationHA and using VMware HA for guest level monitoring is a huge step in the right direction. There are many other use cases for this technology and I think we’ll start to see more people enabling this for nearly all workloads, particularly the native VM monitoring provided by VMware HA. In the case of Exchange 2010, I think the benefits of DAGs outweigh the benefits that using ApplicationHA provide.