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Using the /3GB switch on Windows 2003 Enterprise servers running AD and Exchange 2003

Posted on 2009-04-14
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Last Modified: 2012-05-06
Microsoft recommends not using the /3GB switch on a server running both Exchange and AD if it isn't Small Business 2003 because when you put a /3GB switch into the Boot.ini file of the operating system, you reallocate the virtual address space distribution to give User mode programs 3 GB of space and limit the kernel to 1 GB.

Here's my scenario - We have a one server environment running Windows 2003 Server (Enterprise) and Windows 2003 Exchange (Enterprise) with AD on the same server.  The Server is new, a pair of quad core Xeon processors, 16GB of RAM, 15K hard drives.  It's a fast box.  We only have about 20 users.  We use the server as a file server and for Exchange - not much else.

Is there a way to utilize the extra RAM and not shoot myself in the foot?  Can we get away with 1GB of  RAM for kernet if we allocate the /3GB switch?  These users like their large e-mail boxes.  Is there a way to tell Windows to keep the 2GB for the Kernel but is X amount for Exchange?  
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Question by:leadeaters
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dolomiti earned 375 total points
ID: 24142638
hi,
I have heard different opnions about /3GB, also directly from Microsoft Support.
I concluded that /3GB is a flag to use in XP Workstation with heavy application (CAD),
and in little server(standard edition), and you see just 4GB physical memory.

In a Enterpise Edition, with 16GB RAM, I'ld leave 2GB to the kernel:
in a similar situation, EntEdt, 16GB RAM MSCS, Microsoft support suggested to not use /3GB flag.

However, create 2 entries in boot.ini: one with /3GB flag and anothe without:
test them and decide by yourself.

vic
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by:Mestha
Mestha earned 375 total points
ID: 24143533
16gb of RAM on an Exchange 2003 server is going to confuse Exchange. I would have put VMWARE ESXi on to the machine and given Exchange what it really needs (2gb of RAM). I have seen performance issues with Exchange having over 4gb of RAM which went away when I removed the RAM, so who knows what would happen with 16gb of RAM.

Exchange will never use all of that RAM, it isn't RAM heavy. It likes storage.

Simon.
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by:Raghuv
Raghuv earned 375 total points
ID: 24145721
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by:bdesmond
bdesmond earned 375 total points
ID: 24175714
So as others have pointed out, 12GB of the 16GB of RAM in this box is effectively useless.

You should go ahead with /3GB here AND /USERVA=3030. You may need to tweak the USERVA setting later if you have resource issues.

If you'd like to limit the box to only making the 4GB visible, the /MAXMEM boot.ini switch is your friend.

Using /3GB is a science not an art. You will get different answers depending on the inputs - alot of things can weigh in on this. This particular scenario it makes sense.  

Thanks,
Brian Desmond
Active Directory MVP
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Author Closing Comment

by:leadeaters
ID: 31570150
I ran a test and it worked fine...until Monday morning when the users started logging in and the whole thing went to crap.  Took out the Mem switch and it was back to normal.  The 16GB is wasted.
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