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I have upgraded some machines to 4GB from 2GB to run VirtualBox guest images.

When I tested VirtualBox at 2GB, I had a 50/50 split in memory, which ran slowly (Windows 7 x64 as host and Win7 x32 as guest) due, I am guessing to the memory requirements of Win7.

Now I have 4GB on these machines (a motherboard limit, apparently), I was hoping to run 3GB guests with 1GB for the host, but I am getting big warnings from VirtualBoxnot to overallocate above 50%, and am not allowed over 75%.

My question is that if I could run a 1GB host on a 2GB machine, why can't I run a 1GB host on a 4GB machine running a 3GB guest?

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From your question, I understand your first scenario is this:

1GB host, 1GB guest

And your second scenario is this:

1GB host, 3GB guest

Is this correct?

In scenario 1, you are using 50% of your available RAM for a guest OS (1/2 = .5). Virtual Box will get angry when you do this, but it won't stop working.

In scenario 2, you are using 75% of your available RAM for a guest OS (3/4 = .75). Virtual Box will get very angry when you do this, and I believe it may not even let you do this (though, I've locked up a few host systems by accidentally assigning too much system RAM).

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