This is the third in a series of posts about the various options to achieve HA and DR with Exchange 2010. In the first, I broke the DAG into its basic components (Active Manager and DAG replication). In the second, I gave a quick overview of Native DAG. In this post, I’m going to cover a pretty popular option for folks who’ve already made the decision to virtualize their Exchange environment.
I call it “Virtualized Local DAG” because it uses DAG replication and Active Manager for local HA, but third party technologies for remote replication and failover.
This configuration utilizes a hypervisor such as Hyper-V or VMware’s. A two member DAG group is created, and then both copies are replicated from one site to the other. It’s important to note that because HA is dealt with by Exchange natively, any HA or workload migration features offered by the hypervisor (such as VMHA, DRS, Live Migration and VMotion) should be disabled for the mailbox VMs (other Exchange roles can utilize those features).
As for the network cost, it’s going to be more expensive operationally – you’re replicating two copies of the databases and two copies of the logs, and the storage cost is identical to Native DAG Replication. However, this can be mitigated through the use of interesting compression and data reduction techniques made available by products like RecoverPoint.
- Storage: 4
- Network: 4 (SRDF, MirrorView), 1 (RecoverPoint)
Why would anyone choose this option? Well first, the network cost isn’t much more than Native DAG replication when you need to make sure you can reseed your databases in a reasonable amount of time. And if you use a replication appliance like RecoverPoint, you can end up using even less bandwidth than Native DAG replication, owing to the very good compression and data reduction an appliance like that offers.
Basically, for customers who've decided to virtualize Exchange, and already have working replication technologies for other applications, this option offers a lot of flexibility, integration with their data center strategies, and costs nothing in terms of storage footprint when compared to traditional DAG.
Ultimately, people may choose this option when there’s a need for or concern about:
- Uptime while patching
- Simplified and coordinated failover of all Exchange roles and services
- The ability of the WAN to absorb re-seeding operations that will occur more frequently with log-based replication
- Better control of failover operations than what native Exchange’s Best Copy Selection can provide
- Full site failover of multiple applications (such as provided by PowerShell scripting with Hyper-V or VMware Site Recovery Manager)
- Consistency with other line of business applications
- Controllable RPO
- Synchronous replication (loss-less cross-site failover)
- Multiple recovery points are desired at each site
- Controllable bandwidth utilization
- Compliance and inclusion in a DR plan that includes other applications
- Business requirements that can’t be met by DAG replication or Active Manager
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