With the introduction of Exchange 2010 DAS has become a truly viable option for Exchange mailbox storage. Specifically, Exchange high availability improvements make DAS a viable option. In Exchange 2010 Microsoft introduced Database Availability Groups (DAG) as an improvement over Local Continuous Replication (LCR) Continuous Cluster Replication (CCR) and Standby Continuous Replication (SCR). This improvement meant that each individual mailbox database could have up to sixteen copies. As a result the need for protection against disk failure takes on less importance, hence the viability of DAS as a storage option for Exchange 2010.
As with any option there are advantages and disadvantages. The primary advantage of DAS initially is cost, for smaller deployments DAS combined with Database Availability Groups provides an effective reliable high availability solution that is relatively simple to manage. SAN however has always provided the greatest level of availability and performance, so which is the best for you Exchange 2010 deployment?
With Exchange 2003 and 2007 high availability was a little bit more complicated and in many cases didn’t solve immediate continuity concerns. The technology was evolving but still relied heavily on independent disk systems. For example, In Exchange 2003 high availability was limited to Single Copy Clusters. For larger environments where E-mail was mission critical SAN was the only real option, but you were still vulnerable to corruption issues.
In Exchange 2007 Microsoft introduced LCR, CCR and SCR. This allowed administrators a little more flexibility in disk options however it wasn’t completely autonomous. As we know LCR is limited to local separate disk replication, drawback in hardware failure; CCR replicates to a separate server instance with separate disk allowing for HA failover but still potentially vulnerable to corruption, and, SCR Allows for multiple replication target with delay, which can protect against corruption. This may be up for debate but to receive the best level of HA a combination of CCR and SCR could address the majority of concerns. In this case DAS could be considered viable however a SAN deployment would be the preferred solution. On to Exchange 2010,
As mentioned previously in Exchange 2010 we had the introduction of the DAG, the creation of a DAG brings with it many improvement over the previous HA models and methods. Any mailbox server, located anywhere (as long as some basic requirements are met) can be part of a DAG, this allows administrators a great deal of flexibility when designing their architecture and disk layout. Many of the constraints given by previous methods are relaxed. Points of failure are reduced by the overall flexibility making DAS a potential option for deployment. SAN certainly has its advantages, however many of these needs been addressed within the architecture of Exchange 2010 making the case for DAS.
At this point choosing a disk architecture may come down to the size of your environment and cost. Either choice is capable of providing the performance and availability required. There’s a really good article that can be found here that compares the choices and performs a cost analysis that may be helpful in your decision. Let me know what think, what has your experience been?