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When creating a new dynamic VDI why is the suggested default 8GB? This is the maximum and not the actual correct? So why should not be very large so that you don't run into the limit?

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8Gb is the typical minimal size for a modern OS. Even if the disk is dynamically growing, you still don't want it too large, especially because at some point you might end up over committing your storage space, and that's much worse. With VMs it's best practice to allocate just enough space for a VM for it's OS and it's purposes.
But of course, you still have the choice, and can do whatever you want

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What do you mean by "over committing your storage space"? I still don't understand why you wouldn't set a large maximum size since that's only the max and won't be used unless you need it. –  benmccann Feb 11 '12 at 17:23
    
ok, you have 100Gb on your hdd. 10 are taken bu the OS, so you create a VM with 90. Now the VM can grow up to 90Gb, but you installed some software in the host, which took 50. now you have the host OS take 60, and the VM can grow up to 90, but it will not have only 40 left to grow to. When it tries to grow further, it will get an IO error. This is storage overcommitting. –  dyasny Feb 11 '12 at 18:40
    
Ok, thanks for explaining. That's what I was wondering. I still think 8GB is a stupidly low default and the messaging makes it appear more dangerous to change it than it really is, but it's at least nice to understand the logic behind it. –  benmccann Feb 11 '12 at 18:54
    
Starting at the minimum is one of the normal UI design patterns, the number doesn't matter really. After you spend a few years working with VMs, you'll see overcommitting is dangerous, and shouldn't be done whenever possible –  dyasny Feb 11 '12 at 21:54
    
I suppose it depends on the context. I'm just trying to run Ubuntu on my Windows machine. I have hundreds of gigs of free space and am at no risk of ever overcomitting. It is however very likely that I'll hit the 8GB limit in the course of normal usage. Overcommitting would be more dangerous in a server environment where you're running many VMs on one machine like they do on Linode or EC2 (though I'd be very surprised if anyone is using VirtualBox for this purpose). –  benmccann Feb 12 '12 at 1:38

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