Exchange 2010 CAS/HT Memory Configuration

We are in the process of building out our combined CAS/HT servers.  We will have 3 combined CAS/HT servers and each have 2 6GB cores.  The servers came with 64GB of memory.  The discussion we are having internally is whether or not to reduce that amount of memory from 64 to 32GB.  Microsoft states that the recommended max memory for a combined CAS/HT is 2 x number of cores which would put our spec at 24GB.  Personally I feel 64GB is too much and could cause performance problems.  Also we would need to install Enterprise on these servers which will result in a higher cost.  We will also be using a netscaler to load balance the CAS/HT and use SSL offloading.
 

Can you take a brief second to provide your recommendation to the above situation.  It would be much appreciated.
mkllpitAsked:
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Will SzymkowskiSenior Solution ArchitectCommented:
Current setup we have is 2 physical CPU's, 8 cores per CPU we have 32GB allocated to CAS. We are also using Exchange Enterprise. With 32GB of RAM allocated Exchange runs optimally for us and we have 25,000+ mailboxes.


Thanks

Will.
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mkllpitAuthor Commented:
Will, are your CAS/HT combined or are you running separate?
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Will SzymkowskiSenior Solution ArchitectCommented:
CAS/HUB is on the same server. Mailbox server role is installed on their own server.
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Simon Butler (Sembee)ConsultantCommented:
Have to say it is no longer considered best practise to seperate out the roles. Best practise is to have all three roles on the same server, and have lots of identical servers.

Unless you have Enterprise edition of Windows I don't think you will be able to use all of that memory anyway. I would reduce it though, use the memory elsewhere.

Simon.
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mkllpitAuthor Commented:
Interesting Simon.  When did this come about?  What if you were looking to do a cas array with wnlb and also implement a DAG?  I don't think in this case you could have all 3 roles on the same server.
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Simon Butler (Sembee)ConsultantCommented:
I don't use WNLB, period.
The Exchange product team don't recommend its use either.
http://www.stevieg.org/2010/11/exchange-team-no-longer-recommend-windows-nlb-for-client-access-server-load-balancing/

My preferred design is multiple pairs in a DAG with a hardware load balancer in front.

The best practise has pretty much emerged over the last 18 months or so. I haven't split the roles out for over two years now. With the increases in processor power I simply fail to see the point. The only people splitting them out are those that want to use WNLB, can the additional hardware and software licences and that pays for a Kemp load balancer.

Simon.
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Will SzymkowskiSenior Solution ArchitectCommented:
Typically due to the new re-design of Exchange 2013 the Exchange Team at Microsoft have stated that it is more realistic to have the both roles (CAS/Mailbox) on the same server in Exchange 2013. Hub role is no longer a role in Ex2013 and is shared between both CAS and Mailbox roles.

In Exchange 2013 the mailbox database are controlled individually with processes so that if a single database has issues it does not affect other database that reside on the same server. There are other benefits as well like hardware cost, but this is why they suggest keeping all of the roles on the same server.

IMO, it does simplify the Exchange deployment but personally i like to break out CAS/HUB together with Mailbox separate in Ex 2007/2010. In Exchange 2010 the architecture is different than 2013 and I beleive it is also easier to troubleshoot core issues if they are only associating with the mailbox role and nothing else.

Having all of the roles will work regardless, but as you mentioned you cannot then use WNLB, but then again it is not a best practice to use this in production anyways, it should be a Hardware Load Balancer to get best performance.

Will.
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mkllpitAuthor Commented:
Is there any definitive information that 64 GB in the configuration listed above will negatively impact performance versus 24 GB or 32 GB?
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Will SzymkowskiSenior Solution ArchitectCommented:
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