Overview of Storage for the Mailbox Server Role

•  Replicated database copies increase the amount of storage space required. If your organization uses DAGs to replicate mailbox databases for high availability, consider the number of database copies when you calculate how much disk space you need and what it costs.
•  Slower disks cost much less per GB than faster disks. The reduced disk I/O requirements of Exchange Server 2013 mean that large-capacity 7,200-RPM disks are suitable for many organizations. You can obtain 7,200-RPM disks of equal size with the SATA or SAS interface.
SAS disks cost slightly more than SATA disks, but in testing at Microsoft, SAS disks had a 50 percent lower failure rate than SATA disks.
•  Direct attached storage (DAS) is less expensive than a storage area network (SAN). As a result, DAS is preferable if you use DAGs to create multiple replicated copies of data. You can purchase external drive arrays and use them to connect a large number of disks to a single server. The lower reliability of DAS is offset by the multiple database copies in the DAG. If you have a SAN with available space, then you might prefer to use the SAN for the higher reliability it provides.
•  JBOD use when you have three or more replicas of a database in a DAG. JBOD provides no redundancy, but this is acceptable because the DAG has multiple database copies. JBOD is used with
DAS.
•  Can use SANs for all server storage. If you use a SAN, the increased reliability may mean that you choose to implement fewer database copies in a DAG. You also can keep some database copies on a SAN and others on DAS. Even when a SAN is used, we recommend having two database copies.
•  Internet small computer system interface (iSCSI) SAN typically has lower performance than a
Fibre Channel SAN, but it also is much less expensive. If you use a SAN, the lower I/O requirements
in Exchange Server 2013 make iSCSI an option to Fibre Channel in a wide range of scenarios.
•  Use RAID to increase the redundancy of the disk system if there are less than three database copies
in a DAG. A variety of RAID types are available to increase the performance and redundancy of the
disk system. RAID 10 is the best-performing RAID option, because it has the speed of a striped set
and the redundancy of mirroring. However, it is fairly expensive, because 50% of the disk space is
used for redundant data. You can use the Exchange Server Mailbox Server Role Requirements
Calculator to help you plan the storage configuration of Mailbox servers. This spreadsheet contains
many calculations to help you accurately estimate the hardware requirements to support a specific
number of users with a specific storage configuration. You can download this tool, which is updated
regularly from the Microsoft website.