When the Client Access server (CAS) was first
introduced in Exchange Server 2007, the hardware requirements were more
demanding than its predecessor, the front-end server in Exchange Server
2000/2003. The CAS was given a lot of responsibility because it was
doing things that Mailbox servers used to do. In Exchange Server 2010,
the CAS has been given even more responsibility, and as a result, the
system requirements have again increased.
1. Client Access Server Operating System Requirements
When we talk to clients, they are sometimes confused
about which version of Exchange is required for a particular task.
Therefore, we think it is worth clarifying the software requirements of
the Client Access server role. The Client Access server role is
absolutely fine running on the Standard Edition of Exchange Server
2010, Windows Server 2008 SP2, and Windows Server 2008 R2. Also, like
every other role in Exchange Server 2010, the CAS requires a 64-bit
processor and 64-bit operating system.
In Exchange Server 2010, there is great benefit to
using network load balancing (NLB) to increase the availability of your
Client Access servers. There is typically some confusion around whether
NLB can be used with Windows Server standard editions, since failover
clustering is only available in Enterprise editions.
2. Client Access Server Hardware Recommendations
One of the first considerations when planning for
your installation of the Client Access server role is what hardware to
use. As you can see from the introduction, the CAS has a lot of work to
deal with. Since the CAS now processes client MAPI RPC and address book
traffic, there is additional load that needs to be accounted for in
hardware.
In addition to RPC client access, all of the
rendering for Outlook Web App is done on the CAS, as well as all
message conversion for non-MAPI clients. These factors remove the load
from the Mailbox server (assuming you are running the roles on separate
boxes) and help enable Exchange to scale better.
So what does this mean in terms of hardware? This
means that the CAS makes heavy use of the processor, memory, and
network resources. Disk access is typically not heavy. The
recommendations and requirements for processors are as follows:
As with all versions of Exchange Server, x64-capable processors are required.
Two
processor cores are required for your implementation to be supported by
Microsoft. However, in most implementations, Microsoft recommends that
eight processor cores be used for a CAS.
Microsoft
recommends a maximum of 12 processor cores when a CAS stands alone.
However, when the CAS is combined with the Hub Transport and Mailbox
roles on a single server, Microsoft recommends up to 24 processor cores.
For
every four Mailbox server processor cores, there should be at least
three Client Access server processor cores. So for example, if in one
site you have four Mailbox servers with four processor cores in each
(with a total of sixteen processor cores), then you should have three
Client Access servers with four processor cores (a total of twelve
processor cores) as well.
Memory is the second part of your hardware considerations:
The minimum recommendation is 4 GB of RAM
when a CAS is the only role on the server, or 8 GB of RAM when it is
combined with other roles.
In general, servers should be sized with 2 GB for each CPU core up to a maximum of 16 GB.
The
processor and memory used by the Client Access server role has a linear
relationship. If you add processors later, remember to increase the
amount of memory as well.
When
purchasing memory, you want to pack as much memory on a single chip as
possible. These chips are generally more expensive, but it gives you
the option to scale up later if the load turns out to be heavier than
you expected.
The last of the three areas is networking. The
Client Access server role puts heavy use on the network, and
eliminating network bottlenecks is especially important if you're
servicing clients from multiple sites. This is particularly important
in Exchange Server 2010, since Client Access servers now handle all RPC
client traffic. Therefore, we recommend providing Gigabit Ethernet
where possible.
