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I have a 32 bit Windows 7 box with 1.2 TB of disk space and 4 GB of RAM. I have installed Oracle's VirtualBox and running Ubuntu 14 64-bit inside it. While installing Ubuntu I created a partition of 8 GB. My questions are as below:

  1. I understand that my OS is installed in that 8 GB of space and when I will start the guest OS then it will leaded from that space.
    • So, space occupied by OS + space required by OS for persistent storage is 8 GB, or guest OS can take up some space beyond 8GB from my main hard disk?
    • Am I correct in understanding that 8 GB is like a virtual hard disk for my OS.
  2. As I could understand from other posts, that my host OS will occupy some RAM (before start of guest OS) and
    • then guest OS will start then it can get only left over RAM or how it works?
    • When I installed OS I didn't get any option about how much RAM you want to allocate. If there is default RAM allocated then how much RAM was allocated.
    • Can hard disk space we used by guest OS as virtual RAM and if so then only from those 8 GB or main hard disk space as well.
  3. My host OS is 32 bit, but guest OS is 64 bit, now I understand that 32 bit system will have 4 bytes of memory address and 64 bit system will have 8 bytes of memory address. I don't how should I understand it.
  4. Is my guest OS on same hardware or because it is 64 bit OS, some registers or something in hardware is changed?
  5. Will a given application will run fast because it is on 64 bit system which means it can have faster CPU cycles etc.?

Please forgive for novice questions.

P.S.There are many posts there host and guest OS are discussed but in none of them I could find complete information related to resource sharing and other questions I asked.

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  • It is preferred if you can post separate questions instead of combining your questions into one. That way, it helps the people answering your question and also others hunting for atleast one of your questions. Thanks!
    – guntbert
    Jun 30, 2015 at 17:54

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I am unfortunately not sure if I understood all questions the right way, so please feel free to ask again if something is not clear:

VirtualBox is a usual application as e.g. a Word processor is. That means the application asks the underlying OS (because an OS cares about the resources a computer HW has) for RAM. Of course it is a special application because it uses OS-drivers to get special rights e.g. like exclusive accesses over a USB device.

  1. When setting up your virtual hard disk is a file on your host. This file even is NOT 8GB big in the beginning but it is getting larger when you occupy more space. But it will never be bigger than the 8GB you specify. Your guest operating system will notice a "disk full" state when you full use the 8GB. Because it is a file you can even raise the size later on via the "VBoxManage modfiyhd ---resize" command.

    • The setting of your virtual machine also has a constraint how many RAM the machine is allowed to use.
    • When you have a 4GB RAM host machine and your guest is configured as a 16GB RAM machine, then the host OS will do what it always does when applications need more RAM than physical available: it swaps to hard disk (when it is allowed to do so, but this is the usual case on a desktop OS). This means you as a user will recognize that your machine is slow when the virtual machine runs applications that need more RAM.
    • You can change the maximum RAM size of your guest machine in the Settings Dialog -> System -> Motherboard -> Base Memory

  3. Well I not fully understand this question. Virtual box is a virtual machine, that means it is a program that simulates a PC processor and its periphery. It also simulates the 64bit processor behavior on a 32bit machine.

  4. Your guest OS is on different hardware, because your hardware is simulated in the virtual machine. That is also the reason why you need to install the "VirtualBoxAdditions Drivers" in your guest OS to use the underlying host hardware more effectively.

  5. Not at all. The underlying host operating system is on 32bit, 64bit in the guest has to be simulated and this costs more processor instructions that a native application. This also means that a virtual machine can never be faster than the underlying hardware.

I hope I wrote down the answers you were looking for.

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  • Thank you for your response, I will follow up, need some time, +1'ed already ..
    – hagrawal7777
    Jun 25, 2015 at 17:01

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