A virtual hard disk is a file that acts like a physical hard drive but exists only in software. Virtual hard disks provide a number of benefits compared to physical drives, which is why many users and organizations utilize them. Some quick answers to common questions about virtual disks include:
- Virtual disks take up less physical space – They exist as files on an existing hard drive rather than requiring an entire physical disk.
- Virtual disks are more flexible – Their size can be changed dynamically and they are easier to move between locations.
- Virtual disks aid disaster recovery – Their files can be backed up and replicated like any other data.
Now let’s explore these benefits and the reasons to use a virtual hard disk in more detail.
More Efficient Use of Physical Storage Space
One of the biggest advantages of using a virtual hard disk is the improved efficiency in physical storage use. A virtual disk exists as a single file or set of files on a physical disk. This allows the virtual disk to occupy only the amount of space it needs at any given time, rather than taking up an entire physical drive.
For example, if you have a 500GB physical hard drive and create a 20GB virtual hard disk, that virtual disk will occupy just 20GB as a file on the physical drive. The remaining 480GB on the physical drive can be used for other files and virtual disks.
If that 20GB virtual drive later needs to be expanded to 40GB, the size of the file grows dynamically to accommodate it. The virtual disk only uses the physical space it needs.
This contrasts with using a physical 20GB hard drive for this purpose. That physical drive would occupy the full space of the disk regardless of how much data is stored on it. Expanding to 40GB would require an entirely new physical disk.
By using virtual disks, you avoid dedicating entire physical drives to individual storage needs. This lets you make the most efficient use of your available physical disk space.
Example of Virtual Disk Storage Efficiency
| Physical Disk | Virtual Disk Size | Physical Space Used |
|---|---|---|
| 500GB Disk | 20GB Virtual Disk | 20GB |
| 500GB Disk | 100GB Virtual Disk | 100GB |
| 500GB Disk | 20GB + 100GB Virtual Disks | 120GB |
As this example illustrates, virtual disks only use the space they need rather than entire physical drives. This allows more efficient storage use overall.
Flexibility in Disk Sizing and Management
Another key advantage of virtual disks is the added flexibility they provide for sizing and management.
With a physical disk, you are limited to the size of the initial volume. If you need more capacity, you have to add an entire new physical drive.
Virtual disks, on the other hand, can be expanded dynamically as needed. If you create a 20GB virtual disk and later need 40GB, you can simply expand the virtual disk file to the larger capacity. This avoids the need to manage entirely separate drives to increase storage.
In addition, virtual disks are much easier to move between locations than physical drives. Moving a physical drive requires shutting down the system, unplugging cables, physically moving the drive, and recabling in the new location.
With a virtual disk, you simply copy the virtual disk file to the new location. No cabling or hardware changes are required. This makes it easy to migrate virtual disks as needed for load balancing, system upgrades, or other purposes.
The dynamic sizing and ease of movement provide much more flexibility in storage management compared to physical drives.
Virtual Disk Flexibility Examples
| Action | Physical Disk Process | Virtual Disk Process |
|---|---|---|
| Expand storage capacity | Add new physical disk | Expand capacity of existing virtual disk file |
| Move storage location | Power down, unplug cables, physically move drive to new system, recable | Copy virtual disk file to new location |
This table shows how virtual disks offer more straightforward processes for reconfiguring and moving storage.
Disaster Recovery and Availability Benefits
Virtual disks also provide some advantages when it comes to disaster recovery and high availability.
Because virtual disks are just files, they can be backed up like any other data. Make a backup copy of the virtual disk file, and you have a full backup of that virtual drive that can be restored if needed.
Physical drives have to be backed up using specialized disk imaging solutions. So virtual disks integrate more seamlessly into normal backup processes.
In addition, replicating and distributing virtual disk files across multiple storage systems is straightforward. This can aid in disaster recovery, as the disks can be easily failed over to alternate storage in case of a localized outage. It also helps with high availability, since disk images can be rapidly deployed across many systems.
Overall, the file-based nature of virtual disks simplifies protecting those disks and providing redundancy. This can reduce downtime from outages and help maintain performance.
Virtual Disk Backup/DR Examples
| Goal | Physical Disk Process | Virtual Disk Process |
|---|---|---|
| Backup | Specialized disk imaging solution | Standard file backup process |
| Replication | Purpose-built SAN synchronization | File copy/sync |
As shown above, the processes for backup and replication are easier with virtual disk files.
Testing and Development Environment Flexibility
In addition to production benefits, virtual disks are also useful for creating flexible testing and development environments.
Development teams can use virtual disks to simulate different storage configurations without physical hardware changes. This allows efficiently testing things like:
- Application performance with different disk types/sizes
- RAID configurations
- Effect of disk failures
- Storage migration processes
By using virtual disks to simulate these conditions, development can be accelerated without physical hardware changes.
Virtual disks are also useful for quickly spinning up test and QA environments. Rather than procuring physical hardware for temporary test labs, teams can use virtual disks to quickly deploy and reconfigure virtual test environments. These can be sized appropriately for each specific test effort.
Virtual Disk Testing Examples
| Testing Goal | Physical Disk Process | Virtual Disk Process |
|---|---|---|
| Simulate slower disk | Replace with actual slower physical disk | Configure virtual disk with slower access rates |
| Test application on larger disk | Obtain larger physical disk | Expand size of virtual disk |
| Test RAID recovery | Build physical RAID and fail drives | Simulate RAID across multiple virtual disks |
This illustrates how virtual disks can simplify and accelerate test processes without physical hardware changes.
Potential Drawbacks of Virtual Disks
Despite their advantages, virtual disks also come with some potential downsides to consider:
- Slower performance – Virtual disk access can be slower than directly-attached physical disks, especially for write operations.
- Added storage layer – The hypervisor/host managing the virtual disks adds a layer of abstraction between the guest OS and physical storage.
- Limited functionality – Virtual disks may not support all advanced storage hardware features like caching.
- Host dependence – Virtual disks are tied to their host environment and can’t be directly accessed without it.
The slower speed and added abstraction layer in particular can impact performance for I/O intensive applications. So virtual disks may not be ideal for high performance production databases or other extremely disk-demanding uses.
However, for many general computing uses and the benefits described in this article, virtual disks provide a compelling set of advantages.
Typical Use Cases for Virtual Disks
Based on their pros and cons, some typical use cases where virtual disks offer strong benefits include:
- Test and development environments – Rapid provisioning and reconfiguration aids development.
- General computing uses – Light/moderate I/O requirements like web servers, file servers, application servers, etc.
- Server consolidation – Allows efficiently running multiple workloads on shared hardware.
- Production applications with bursting capacity – Handling fluctuating workloads by dynamically resized disks.
Virtual disks are less advantageous for high I/O databases, transaction processing systems, and other extremely disk performance dependent applications.
But for many use cases, especially development/test and general computing, virtual disks provide compelling flexibility, efficiency, and management benefits.
Hypervisor Platform Options for Virtual Disks
Key platforms that support creation and management of virtual disks include:
- VMware ESXi – Enterprise hypervisor allows virtual disks for guest VMs.
- Microsoft Hyper-V – Native to Windows Server, manages virtual disks.
- Linux KVM – Open source hypervisor with CLI and GUI for disk creation.
- Citrix XenServer – Enterprise platform with tools for efficient virtual disk provisioning.
- Oracle VM VirtualBox – Free desktop solution supporting advanced disk images.
Many other hypervisors and virtualization platforms also use virtual disks under the hood as well.
So while the specifics vary, the ability to create and manage virtual disks is common across enterprise and open source virtualization solutions.
Feature Comparison of Major Hypervisor Virtual Disk Options
| Hypervisor | Max Disk Size | Disk Types | Disk Allocation | Management Tools |
|---|---|---|---|---|
| VMware ESXi | 62TB | Thin/Thick | Lazy Zeroed, Eager Zeroed | vSphere Client |
| Hyper-V | 256TB | VHDX | DynamicallyExpanding, Fixed | Hyper-V Manager |
| KVM | 2TB | qcow2 | Sparse, Full | virsh, Virtual Machine Manager |
| XenServer | 2TB | VHD | Dynamic, Fixed | XenCenter |
This table provides an overview of key capabilities for the most common enterprise hypervisor platforms. As you can see, advanced virtual disk creation and management is a core component across these solutions.
Getting Started With Virtual Disks
If you are looking to start taking advantage of virtual disks, here are some tips:
- Choose your hypervisor platform – Whether VMware, Hyper-V, KVM, XenServer, etc.
- Allocate storage capacity – Ensure you have sufficient physical storage available on hosts/SAN.
- Create virtual disks – Use hypervisor tools to create disks for VMs as needed.
- Experiment with configurations – Test out different disk sizes, types, locations, etc.
- Monitor performance – Use platform tools to view virtual disk IOPS, latency, throughput.
- Consider backups/replication – Factor into disaster recovery strategy.
Starting small with a test/dev environment is recommended to gain experience before broader production rollout.
Overall virtual disks provide a range of strong benefits, but also introduce a layer of abstraction that needs to be accounted for especially from a performance perspective.
Conclusion
In summary, the compelling advantages of using virtual hard disks include:
- Much more efficient physical storage usage – Only occupy space they need
- Flexibility to dynamically resize and easily move disks
- Integrate with standard backup processes
- Accelerate test/dev by rapidly provisioning disks
Potential downsides like added abstraction and possible slower performance should be factored in for high I/O applications.
But for many use cases including test/dev, general computing, and server consolidation, virtual disks provide very significant benefits. By moving from physical to virtual disks, organizations can drive cost savings, flexibility, and agility through more efficient and dynamic storage provisioning.
