Choosing the right storage system is critical for a successful Exchange Server 2007 deployment. Exchange Server supports three primary types of storage technologies: DAS, SAN, and the iSCSI protocol. There are advantages and disadvantages to each of these storage management options, but all three options are based on simple spinning disks.
Microsoft offers several tools that can help you determine your storage needs, such as the Exchange Storage Calculator and Jetstress to help you quantify your needs. Other free tools are also available, such as Iometer. Properly identifying how you'll use your storage before you commit to an option can save you time and money.
DAS has been around for decades and is a common choice for Exchange Server storage in small-to-midsized businesses, and is increasingly common in enterprises as well. When considering DAS for your Exchange environment, you should investigate the various RAID array options and the importance of multiple disks.
SANs are dependable and scalable centralized data storage resources. SANs operate on their own independent networks and generally use connections based on Fibre Channel (FC) to communicate between various disks and connected hosts. Their drawback is that they're more expensive and more complicated than other options. Software for SANs tends to cost more than that for DAS arrays and is usually packaged separately from the hardware, while DAS arrays often include a set of utilities.
iSCSI is a storage protocol used to connect to a network device that moves storage-related data. It allows clients to send SCSI commands to remote, consolidated storage targets (or disk arrays) in the same way the client can interact with a locally attached disk. A common misconception is that you can connect iSCSI over your existing LAN infrastructure. Although this is technically possible, it isn't recommended. iSCSI devices are less expensive than FC devices, but you should still use dedicated hardware and cables. At the very least, you should have a dedicated virtual LAN and keep your devices relatively close together—running iSCSI over a WAN isn't a good idea.
NAS vs. iSCSI
iSCSI is sometimes incorrectly referred to as NAS. Although iSCSI storage systems are connected to a TCP/IP network like NAS, iSCSI isn't the same as traditional NAS. NAS is a type of device rather than a protocol. NAS uses standard network protocols, such as Server Message Block (SMB) and the Microsoft Windows Network, to emulate a storage device. iSCSI is a true storage protocol that is supported for Exchange deployments.
Traditional NAS is no longer supported in Exchange 2007, as stated in the Exchange team's blog. Even in Exchange Server 2003 installations, Microsoft supports only the use of Microsoft Windows Hardware Quality Labs qualified NAS storage devices. I haven't personally had good experiences with any NAS products; if you're determined to choose NAS, check with your vendor to ensure the end-to-end solution is designed for use with Exchange 2003.
DAS: Cheap and Easy
There's a good reason DAS is so common for Exchange Server storage—it's the cheapest and offers the best performance of the three approaches presented here. DAS uses a single host and is best for small to mid-sized companies, but poses significant challenges when scaling to large numbers of users. If you need to add spindles for extra space and performance, DAS solutions may not expand as easily as SAN solutions.
How the DAS solution will be managed long term should be another key part of your decision process. Keep in mind that there are several factors involved in calculating the cost of storage, and capital expenditure is just one—DAS may seem cheap up front, but remember to take into account the cost of managing it long term.
In a DAS environment, more spindles equals higher I/O operations per second (IOPS) capacity and better IOPS performance. The size of the individual disks affects data storage, but your I/O rate will be significantly better with a RAID array of ten 180GB drives than with an array of six 300GB drives. Capacity and I/O throughput don't increase at the same rate.
Using a RAID array with fault tolerance can increase your financial overhead costs and decrease performance, but you can add additional disks to an array to speed up your I/O on top of the added tolerance. Using such an array is well worth the investment if you want to avoid permanent data loss and potential downtime to the end user.