VMware ESXi and vSphere Cluster Management
Managing Virtual Machine Memory in Oracle VirtualBox
Learn how VirtualBox Base Memory works, how to change a VM's RAM allocation, and how to balance guest and host memory safely.
Host RAM and Guest RAM
RAM, also called physical memory, is the working memory installed in the host computer. The host operating system is the operating system running directly on that computer.
A virtual machine (VM) is a software-defined computer running inside the host. Its guest operating system is the operating system installed inside that VM.
Oracle VirtualBox presents part of the host's physical RAM to the guest as virtual RAM. In VirtualBox, this amount is called Base Memory. While a VM is running, its assigned memory is reserved from the host's usable memory. For example, assigning 512 MB to a VM means approximately 512 MB is no longer available for the host and its applications during that VM's operation.
Why Memory Allocation Matters
The practical number of VMs a computer can run is constrained primarily by available memory and storage capacity. Each concurrently running VM consumes its configured memory allocation, so you must plan the allocations across all active VMs.
Allocating too little RAM can make the guest operating system slow or prevent it from meeting its requirements. Allocating too much can slow the host, cause heavy memory pressure or swapping, and leave too little memory for other host applications. A VM's configured memory is not free capacity that the host can comfortably use while the VM is running.
Memory allocation planning examples
| Host RAM | VM RAM allocation | Memory remaining for host while VM runs | Expected consideration |
|---|---|---|---|
| 1 GB | 512 MB | Approximately 512 MB | The host has limited room for its operating system and other applications. |
| 8 GB | 4 GB | Approximately 4 GB | Check whether the host and its active applications can work comfortably with the remaining memory. |
| 16 GB | Two VMs at 4 GB each | Approximately 8 GB | Plan the combined allocation of both VMs, not just the allocation of one VM. |
| 16 GB | One VM at 12 GB | Approximately 4 GB | The guest may have substantial memory, but the host may become slow or unable to run other applications comfortably. |
Choosing an Appropriate Base Memory Value
Choose a value by considering three factors:
- Guest operating system requirements: Check the minimum and recommended RAM for the operating system you plan to install.
- Guest workload: A lightweight command-line system needs less memory than a desktop system running a browser, development tools, databases, or other demanding software.
- Host capacity: Determine the host's total RAM and reserve sufficient memory for the host operating system and all active host applications.
A safe decision process is to determine the host's total RAM, reserve enough for the host and its active applications, and assign only the remaining practical amount to the guest. When planning several VMs, add their configured RAM allocations together before deciding whether the host has enough capacity.
Changing Base Memory in VirtualBox
The VM must be fully powered off before its base memory allocation can be changed. A running VM cannot normally have this hardware setting changed, and a VM in a saved state may also prevent the change.
- Safely shut down the guest operating system and power off the VM. Do not leave it running or merely paused.
- In Oracle VirtualBox, select the target VM.
- Open Settings.
- Open System, then select Motherboard.
- Locate the Base Memory control.
- Enter a suitable value or move the memory slider.
- Select OK to save the setting.
- Start the VM.
The Base Memory control specifies how much RAM VirtualBox presents to the guest operating system. The changed allocation takes effect after you start the VM.
VirtualBox VM > Settings > System > Motherboard > Base MemoryPractical Examples
Changing one VM's allocation
A user shuts down a VM, opens its System > Motherboard settings, changes Base Memory, saves the setting with OK, and starts the VM. The guest uses the new allocation after startup.
Planning two VMs
Suppose a host has 16 GB of RAM and two VMs are configured with 4 GB each. Running both consumes approximately 8 GB for the guests, leaving approximately 8 GB for the host and its applications. The user should also account for other host resource use and avoid starting additional memory-heavy VMs unless sufficient capacity remains.
Troubleshooting Memory Settings
Base Memory cannot be changed
Verify that the VM is powered off rather than running, paused, or in a saved state. Shut it down completely, discard or otherwise exit the saved state when appropriate, and reopen the Motherboard settings.
The host becomes slow after starting a VM
Reduce the VM's memory allocation or close unnecessary host applications. Leave more RAM available to the host operating system. If several VMs are active, stop one or more of them and reassess the combined allocation.
The guest performs poorly
Check whether the guest operating system and its workload require more RAM. Increase the allocation only when the host has adequate spare memory. Giving the guest more RAM is not a safe improvement if it causes the host to run out of usable memory.
Several VMs run slowly
Reduce the number of concurrent VMs or rebalance their individual memory allocations. Add the RAM assigned to every running VM and ensure that enough remains for the host and its active applications.
Exam-Relevant Notes
- Host physical RAM belongs to the host computer; Base Memory is the portion presented to a guest VM.
- A VM must be fully powered off before changing its Base Memory setting.
- The navigation path is select VM > Settings > System > Motherboard.
- Assigned VM memory is reserved from the host while the VM is running.
- Every concurrently running VM consumes its configured memory allocation.
- Changes to the memory setting take effect after the VM is started.