Designing the Right Data Storage Structure for Exchange Server 2010 (part 2) - Choosing the Right Type of Disks

Choosing the Right Type of Disks

When researching SAN and NAS devices, you discover that you have several types of disks available to you. These disks vary by architecture (SCSI versus SATA versus SAS versus Fibre Channel) and by size. Current disks are available in sizes ranging from 72GB to 1.5TB.

In terms of size, your decisions will be based on three factors:

• Price

• Capacity

• Performance

Generally speaking, the larger the disk, the more you pay for it. Capacity refers to the total amount of space you plan to deploy. If, for example, you need to deploy 2TB of space, you can use eight 250GB disks or 32 72GB disks. Why would you pick one configuration over the other?

If you opt to use 8 250GB disks, you use less capacity on your SAN or NAS device. If you expect to expand capacity in the future, you can expand further before needing to purchase additional disk shelves or chassis. The potential downside to this approach is that 8 250GB disks might be more expensive than 32 72GB disks. The other more noticeable impact is in the area of I/O performance. Assuming the spindle speeds were the same for both disks, you would get four times more I/O out of the 32 72GB disks than you would from the 8 250GB disks. Depending on whether your application needed the additional I/O, this might be a deciding factor.

Useful to note is that with sufficient memory in an Exchange Server 2010 server, disk I/O requirements are roughly one-fourth what they were in an Exchange 2003 server with the same number of users. This behavior was specifically engineered into Exchange Server 2010 to take advantage of the ever-increasing capacity of hard disks. Hard disk capacity is increasing drastically every year with nearly no improvements in I/O performance. According to Seagate, although disk capacity increased 15,000 times from 1987 to 2004, the random I/O performance increased only 11 times during the same period.

In addition to choosing the size of the disks you deploy, you also have a choice in terms of the disk architecture. Your most common choices are as follows:

• Serial ATA (SATA)

• SCSI

• Serial attached SCSI (SAS)

• Fibre Channel

SATA is generally the least-expensive option. SATA disks provide excellent throughput, nearly equal to SCSI, at a much better price. High-capacity disks are usually available as SATA first because it is a more common market for disks. Newer implementations of SATA include high-performance functions such as command queuing, which give them performance that approaches that of SCSI.

SCSI disks have been around for decades. It’s a well-proven technology and is known for having high performance and high reliability. SCSI disks are less expensive than Fibre Channel disks but offer lower throughput through the bus. This results in needing more controllers to manage the disks themselves and lower performance than Fibre Channel disks.

SAS disks are growing in popularity due to their reduced form factor (typically 2.5″) and their high performance. By virtue of their form factor, they actually achieve a higher level of I/O than a 3.5″ disk of a similar rotational speed. This is because the read/write heads don’t need to travel as far to get from one side of the disk to the other. They do suffer a bit in the area of overall throughput because the liner speed at the outer portions of the platter are lower than that would be on a 3.5″ disk at the same rotational speed.

Fibre Channel disks are the highest-performance drives available today. They are also the most expensive and generally trail a full generation behind other formats in terms of capacity. If performance is your number-one concern, the Fibre Channel disk can’t be beat.