Architecting a Hybrid Messaging Strategy with Microsoft Exchange 2013 : Designing the Solution : 7.2 Server Sizing
   
7.2 Server Sizing
Microsoft recommends hyper-threading be disabled on all production Microsoft Exchange servers. As with any virtualized business critical application, when sizing for a new virtualized production deployment, you do not want to oversubscribe processors. Maintain a 1:1 ratio of logical cores to virtual processors on the host. In addition:
• Format the guest operating system database and log volumes at a 64Kb allocation unit size, as recommended by Microsoft.
• Isolate Microsoft Exchange database and log files from other disk intensive application workloads to avoid performance conflicts. Sharing the storage with other applications might negatively impact Microsoft Exchange I/O performance.
• Isolate Microsoft Exchange database and log disk I/O on separate physical disk arrays. Although Microsoft Exchange Server 2013 DAG configuration allows logs and databases on the same LUN, this isolation enables separate log and database disk tuning with backend RAID levels.
• For most Microsoft Exchange Server 2013 environments, Microsoft recommends hardware level RAID 5 for databases and RAID 10 for the logs. RAID 10 can be used for both the transaction logs and database LUNS, as the performance and resilience that it offers is suitable. However, a RAID 10 array makes it quite an expensive proposition for large amounts of data. Therefore, RAID 5 for databases offers a better space verses performance balance when providing users with larger mailboxes (1 GB plus), provided there are adequate disks for the I/O requirements.