Sizing disk storage to support Exchange Servers is more than just tossing a bunch of disks at the server and hoping they are sufficient. You must not only calculate the number of disks that are required to provide adequate storage but also ensure that they will collectively provide sufficient I/O capacity.

Exchange Server databases must be sized on additional factors other than just how big the mailbox will grow; the total size of Exchange data includes not only the mailbox data but also database white space and deleted items. If full text indexing is enabled, the full text indexing files should also be considered in disk space calculations.

The I/O load that the user community places on the disk subsystem must also be considered when sizing the disk subsystem. The I/O load may actually require more disks than the space consumed by the users' data. This was certainly the case in Exchange 2003 where a small number of disks may be required for the data but a larger number of disks are required to support the total IOPS load that the user community actually generates.