RAID 0+1, as the name implies, is a combination of both RAID 0 and RAID 1 arrays, where data is both striped and mirrored to provide fault tolerance and performance improvements. RAID 0+1 setups are fairly expensive because of the duplicate disks (they require at least four disks—two striped disks and their mirrored duplicates), but the improved fault tolerance and increased speed is usually worth the additional cost and complexity. 0+1 is the most common RAID level for Exchange databases.
RAID 5 is another fairly common option for use in Exchange Server 2007. RAID 5 is a set of striped disks that relies on parity to protect you from data loss. Data is spread across all the disks in a RAID 5 array, and if one disk fails no data is lost, but no disk is a duplicate of another in the array. The performance of RAID 5 is about a third of that of RAID 1 and 0+1, because each write to the OS requires three writes to disk. The storage capacity of a RAID 5 array is reduced—the total capacity of a RAID 5 array is equal to the capacity of all the disks in the array minus the capacity of one of those disks. That reduced capacity combined with its slower performance may make another configuration preferable to RAID 5.
To determine which RAID level is best for your environment, you should assess what the impact of RAID will be on the overall IOPS capacity of your drive system. RAID 1 and 0+1 don't have any impact on the drives' ability to handle IOPS, but RAID 5 will produce only one third the performance of the same drives in a non-RAID 5 configuration. During the design phase, it's important to evaluate all of the RAID options or work with someone who can guide you through the complexities of storage architecture design. Be sure to take advantage of tools from storage vendors and the tools I mentioned in the introduction to test your storage systems and make sure they're meeting your requirements.
Configurations to Fit Your Business Needs
The most important consideration when selecting an Exchange Server storage configuration is to make sure it will fit your business needs. Each option has its benefits and drawbacks. For example, don't use an unsupported version of NAS. If you have a limited storage budget, don't opt for an FC SAN. Don't forget to test everything before putting your new disk system into a production environment. I recommend using Jetstress to make sure everything works before you take the leap and make your system live.
You're likely to find that one of the options discussed will provide a workable solution to fit the performance, scalability, and budget requirements of your environment, regardless of what version of Exchange you're running. I'd like to hear how you've managed to circumvent tricky environments to improve your Exchange storage infrastructure, so feel free to drop me an email.
Sidebar: Exchange 2010 Considerations
Disk Type
Exchange Server 2010 has major improvements in the way it utilizes the storage system. In Exchange Server 2007, it was common for customers to use high-speed FC or SAS disks because of the IOPS capacity they provided. Exchange 2010 has a significantly lower IOPS profile than previous versions, so you may not require FC or SAS disks for performance any longer.
RAID Types
Databases Availability Group (DAG), a new feature in Exchange 2010, allows for configurations that don't require RAID on the server. A configuration known as Just a Bunch Of Disks (JBOD) has been added to provide for additional customer choice. JBOD is accomplished by application-level database replication as part of DAG's functionality.
IOPS/Memory Calculation
More than ever, it's important when using Exchange 2010 to take the time to estimate the correct amount of memory and IOPS load/capacity needed for deployments. Exchange 2010 has much higher memory requirements for large mailbox support than past versions, and all Exchange 2010 designs should use the Microsoft best practice guidelines and test tools for final validation.
Third-Party Impact
As more and more applications take advantage of Exchange, it's common to see Exchange deployments that are undersized because third-party use hasn't been factored into their designs. When designing an Exchange 2010 system, ensure that all third-party applications are accounted for. Wireless devices, third-party add-ins for Outlook, and others all have an effect on the overall performance of your Exchange server.
Related Reading: