One of the great benefits for us in IT is that as hardware and storage prices have come down, and performance has increased, we are more able to offer services that in the past was prohibitively expensive to deliver. Rapid deployment and expansion of service, redundancy, and very high availability are all possible now for a fraction of the cost of a few years ago. Granted, it still costs more to provide such high quality service. Let’s take a look at how virtualization, replication and high availability, impact storage requirements and costs.

Virtualization allows us to deploy servers without tying resources to a single specific hardware system. The images can be moved from one system to another, cloned, made redundant and thereby easily allow expansion of particular applications and services. Virtual servers are a foundation for simple, rapid, consistent scalability. Having several or many identical instances allows us to deliver high availability far more easily. Virtual images do take space, and must run on a base platform, so clearly a single VM takes more space and resources that the same service running on dedicated hardware.

High Availability (HA) is the IT goal of having continuously available service for a particular application, connection or resource. Sometimes this is done via fail-over from a primary to a secondary connection or resource. It is also possible via load balancing. The load balancing can be accomplished at the application layer, at a gateway layer, or via an appliance. Load balancing is also possible at the name lookup level. For the purposes of this discussion we are considering application, gateway, and appliance types of load balancing and fail-over. Application layer mail gateway routing is often built-in to the system, whereby the gateway has alternate choices to try if its primary gateway is unavailable. This may be implemented in different ways depending on the vendor and the service. For SMTP there are underlying standards and requirements for gateway and routing behavior.

Continue reading Virtualization, Replication, Storage and High Availability

In my last posts I discussed Exchange Server 2007 Single Copy Clusters, which is one of two clustering features available in Exchange.  In this post I will discuss the other type of Exchange Server 2007 clustering, Cluster Continuous Replication

What is Cluster Continuous Replication?

Cluster Continuous Replication (CCR) for Exchange Server 2007 would not be familiar to anyone who only has clustering experience with previous versions of Exchange.  In a CCR cluster two cluster server nodes connect to non-shared resources in an active/passive configuration.  Exchange storage group and mailbox database information is replicated between the active and passive node using asynchronous log shipping.  I explained asynchronous log shipping in the first part of this series on the basic concepts of Exchange Server 2007 high availability.  The same log shipping occurs in Local Continuous Replication.

The two CCR cluster nodes appear to other computers to be a single system, and when one server node fails the clustered resources are able to fail over to the other node and continue operation.

CCR provides high availability for the Mailbox Server by protecting it from the failure of either cluster server node, as well as protecting it from storage failure.  Because each cluster node is connected to its own non-shared storage, the failure of one does not impact the other.  This also means that CCR disk storage can consist of much cheaper hardware alternatives than an enterprise-grade SAN would cost, making Exchange high availability a reality for businesses with smaller budgets. Continue reading Exchange Server 2007 High Availability Part 4 – Cluster Continuous Replication

In my recent posts I discussed the fundamentals of Exchange Server 2007 high availability and how to use Local Continuous Replication.  In this post I will demonstrate one of the two Exchange Server 2007 clustering methods, Single Copy Clusters.

What are Single Copy Clusters?

Single Copy Clusters (SCC) for Exchange Server 2007 is basically the same as clustering in previous versions of Exchange Server.  Two cluster server nodes connect to shared resources in an active/passive configuration.  The two servers appear to other computers to be a single system, and when one server node fails the clustered resources are able to fail over to the other node and continue operation.

SCC provides high availability for the Mailbox Server by protecting it from the failure of either cluster server node.  Typically the cluster will also consist of redundant networking (e.g. teamed network interfaces, multiple switches) and storage components (e.g. a SAN that is in itself a highly available system through redundant components).  The servers are also usually located in high quality data centers with redundant power and cooling.

SCC is available in the Enterprise edition of Exchange Server 2007.  Because it uses an underlying Windows Server failover cluster, the servers that form part of the cluster must also run the Enterprise edition of Windows Server (either 2003 or 2008).

Unlike LCR an SCC cluster offers high availability benefits but no performance benefits.  Because there is a single copy of each mailbox database within the cluster there is no opportunity to use a passive copy for backup operations.

How to Install a Single Copy Cluster

In this example two Windows Server 2003 Enterprise edition servers have been configured with the basic requirements of a failover cluster:

• A public network interface for normal network operations
• A private network interface for the cluster “heartbeat”
• Shared disk storage for the database, log files, and cluster quorum disk

Once the failover cluster has been configured we can install Exchange Server 2007 on the first node.  Run setup as normal, and when selecting the roles to install choose “Active Clustered Mailbox Role” as the server role.  You will notice that a clustered mailbox server cannot co-exist with any other server role. Continue reading Exchange Server 2007 High Availability Part 3 – Single Copy Clusters

Microsoft Exchange Server 2007 has several different Mailbox Server high availability features included with the product.  Each of the features is similar to the others in some ways but also very different.

In this post I will explain each of the high availability features and which types of scenarios they are suitable for.

What is High Availability?

High availability is a term used to describe the avoidance of unplanned downtime for a computer system through the implementation of hardware and/or software solutions.  Generally speaking a high availability solution will involve the elimination of and single points of failure in the system, often by duplicating or replicating components of the system so that if one fails the other is able to continue performing the role.

An example of downtime would be an email server that has suffered a hard disk crash and is unavailable to users who are then unable to send or receive email.  An example of a high availability solution in this case would be the use of a RAID volume to protect from single disk failures. Continue reading Exchange Server 2007 High Availability Part 1 – HA Fundamentals