As you plan your Exchange storage strategy, you must balance three criteria: capacity, availability, and performance. The choices you make as you plan and implement your storage solution affect the cost associated with administration and maintenance of your Exchange organization.
Capacity - In Exchange, your total capacity is approximately equal to the number of mailboxes multiplied by the amount of storage allocated to each mailbox. If your organization is supporting public folders, you must add the appropriate amount of disk space to accommodate public folder storage.
Availability - The level of e-mail availability your messaging system requires depends on your business needs. For some companies, e-mail usage is light and considered non-essential. However, for many companies today, e-mail is a mission-critical service. The priority that your company places on e-mail determines the level of investment and resources allocated to a consistently available e-mail solution. Overall availability is increased by redundancy. Redundancy can mean that you should cluster applications to provide CPU redundancy or implement a redundant array of independent disks (RAID) solution to provide data redundancy.
Performance - Performance requirements are also unique to each organization. This chapter refers to performance as it relates to throughput and latency. With regard to storage technology, throughput is measured by how many reads and writes per second a storage device can perform. Latency is measured as the time in milliseconds that a transaction takes to complete a read or write operation.
THIRD-PARTY SIZING TOOLS
Dell Sizing Tools
HP Sizing Tools
ADDITIONAL ONLINE RESOURCES
Exchange Server on MSDN
Exchange Server on TechNet
Exchange Product Group BLOG