2.5 Summarize the purpose and operation of virtualization.
📘CompTIA Server+ (SK0-005)
In a virtualized environment, physical server resources like CPU, memory, disk space, and network interfaces are shared among multiple virtual machines (VMs). Resource allocation and provisioning is the process of deciding how much of each resource a VM gets and ensuring it works efficiently without overloading the host server.
This section covers:
- CPU
- Memory
- Disk
- NIC (Network Interface Card)
- Overprovisioning
- Scalability
1. CPU (Central Processing Unit) Allocation
- Each VM is assigned a certain amount of CPU resources from the host server.
- This can be done as dedicated CPUs or shared CPUs.
- Dedicated CPU: VM gets full access to certain CPU cores.
- Shared CPU: VM uses CPU cores only when they are free.
- Key terms for exams:
- vCPU (virtual CPU): A CPU core allocated to a VM. A VM can have 1 or multiple vCPUs.
- CPU scheduling: Hypervisor decides which VM gets CPU time when multiple VMs need it.
- Example in IT: A VM running a web server may need 2 vCPUs, while a VM running light monitoring software may only need 1 vCPU.
2. Memory (RAM) Allocation
- Memory is assigned to each VM to allow programs and OS to run efficiently.
- Types of memory allocation:
- Static allocation: A fixed amount of memory is reserved for the VM.
- Dynamic allocation: The hypervisor adjusts memory based on current VM usage.
- Ballooning: A hypervisor technique where unused memory is temporarily “borrowed” from one VM to give to another.
- Key exam points:
- Over-allocation can cause memory contention, slowing down VMs.
- Proper memory management ensures stable performance for all VMs.
3. Disk Allocation
- Virtual disks (vDisks) are created for VMs on the physical storage.
- Methods of disk provisioning:
- Thick provisioning: All disk space is reserved upfront.
- Thin provisioning: Disk space grows as the VM needs it. Saves storage but can risk running out of space if not monitored.
- Key exam points:
- VMs must have enough disk space for OS, applications, and data.
- Virtual disks can be shared or dedicated. Shared disks are common in cluster environments.
4. NIC (Network Interface Card) Allocation
- VMs use virtual NICs (vNICs) to connect to networks.
- vNICs map to physical NICs on the host server, often through virtual switches.
- Key exam points:
- A VM can have multiple vNICs for different networks (e.g., public, private, management).
- Bandwidth can be limited or prioritized to avoid network congestion.
- IT Example: A VM hosting a database may have one NIC for internal traffic and another for backups.
5. Overprovisioning
- Overprovisioning means assigning more virtual resources than physically available on the host.
- Hypervisors rely on the fact that VMs rarely use 100% of all resources simultaneously.
- Example in IT: A host has 16 GB RAM. It might run 6 VMs each configured with 4 GB RAM (24 GB total). It works because not all VMs use full memory at the same time.
- Exam Tip: Overprovisioning improves efficiency but can cause performance issues if too many VMs demand resources at once.
6. Scalability
- Scalability is the ability to increase or decrease resources for VMs as needed.
- Two types:
- Vertical scaling (scale-up): Add more CPU, memory, or storage to a VM.
- Horizontal scaling (scale-out): Add more VMs to handle increased workload.
- Exam points:
- Virtualization makes it easier to scale compared to physical servers.
- Monitoring tools track VM performance to decide when scaling is needed.
Summary Table (Exam Quick Reference)
| Resource | What It Is | Key Points for Exams |
|---|---|---|
| CPU | Virtual CPU cores (vCPU) | Dedicated vs shared, CPU scheduling |
| Memory (RAM) | RAM for VMs | Static vs dynamic allocation, ballooning, contention |
| Disk | Virtual disks (vDisks) | Thick vs thin provisioning, shared vs dedicated |
| NIC | Virtual NICs (vNIC) | Connects VM to network, bandwidth limits, multiple NICs |
| Overprovisioning | Assigning more resources than physical | Efficient use, risk of contention |
| Scalability | Adjusting resources | Vertical (scale-up) vs Horizontal (scale-out) |
Exam Tips
- Understand differences between static and dynamic allocation for memory and CPU.
- Know thin vs thick provisioning for storage.
- Remember overprovisioning is common but risky.
- Be able to explain scalability in virtualization and types (vertical vs horizontal).
- Think in terms of IT usage, not physical examples — how servers, VMs, and hypervisors manage resources.
