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How do 32-bit OSes on a PowerEdge 2600 utilize more than 4 gigs memory?

Posted on 2009-04-05
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Last Modified: 2012-05-06
I have a PowerEdge 2600.  It's a decent server...  several years old, but quite capable.

I'm currently running ESX v3.5 on it.  I have two VMs - both Windows Server 2008 (32-bit).

I'd like to max out the memory to 12 gigabytes.  My question is this:

Will ESX see the full 12 gigs of memory, and if so, will the guest VMs be able to use more than 4 gigs?
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Question by:Pantz
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by:tigermatt
ID: 24071438

The architecture of a 32-bit Operating System means that it can only ever utilise 4GB of RAM maximum. This is defined by the fact it can address 2^32 bytes of RAM (32, from 32-bit OS) which equates to 4GB.

VMWare will be able to utilise the RAM as it is 64-bit software. If you installed Server 2008 Enterprise Edition 32-bit as the Virtual Machines, you could enable the PAE (Physical Address Extension) which would extend the address space to 36-bits and be able to address a maximum of 2^36 bytes of RAM (64GB). Alternatively, you could reinstall the VMs as 64-bit VMs, which could natively address more than 4GB.

-Matt
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by:Pantz
ID: 24071495
Thanks for the response, Matt.

So here is where the confusion is: the Xeon processors inside of the PowerEdge 2600 are not 64-bit capable processors.  I did try and install a 64-bit OS inside of ESX server to no avail.  So, I guess, according to your comment, assuming the ESX server can use a similar PAE technology, I could enable PAE in the 32-bit guest OSes to use additional resources.

What do you think?
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tigermatt earned 125 total points
ID: 24071983

If the processor in the server is not 64-bit enabled, the only way it can be working is through the use of PAE. VMWare must be using PAE to address the additional RAM (up to 2^36 bytes = 64GB). Presuming the Virtual Machines' OS is Server 2003/2008 Enterprise 32-bit, you can enable PAE to address the additional RAM. However, under VMWare ESX, PAE guest VMs are not supported; they may work, but obviously you'll have to test at your own risk: http://www.vmware.com/support/reference/common/pae_guest.html.

PAE will not work on Server 2003/2008 Standard Edition because the RAM is limited in the 32-bit version of those particular OSs to 32-bit -- this is a software restriction which is imposed by licensing and cannot be worked around, no matter whether you enable PAE or not.

-Matt
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