Sizing Recommendations for Exchange 2010

There are some changes in architecture in Exchange 2010. All clients, even Outlook, connect via the Client Access Servers (CAS). There are some changes in recommendations around the ratio of resource for CAS/Hub and the mailbox servers.

Microsoft provides detailed guidelines on this Technet page and this one.

The basic guidelines for server role ratios are as follows. They're based on the number of mailbox server cores.

• Hub Transport Server — 7:1 (no antivirus on Hub) or 5:1 (antivirus on hub). 1GB of memory for each core.
• Client Access Server (CAS) — 4:3 (note this is much higher than with previous Exchange installations). 2GB of memory for each core.
• Domain Controller — 1:4 (32-bit Domain Controller (DC)) or 1:8 (64-bit DC, and the DC has enough memory to cache the entire NTDS.DIT file).
• Edge Transport Server — Based on peak connections and messages per second and average message size. 1GB of memory for each core.

Each mailbox server should have 4GB of memory plus 2-10MB per mailbox, based on if the mailbox is in light, average, or heavy use.

Thanks to John Savill for the info.