There are a few limitations with the hardware or VM virtualization, which leads to containerization. Let's look at a few of them.
VMs run a fully-fledged OS. Every time a machine needs to be started, restarted, or shut down it involves running the full OS life cycle and booting procedure. A few enterprises employ rigid policies for procuring new IT resources. All of this increases the time required by the team to deliver a VM or to upgrade an existing one because each new request should be fulfilled by a whole set of steps. For example, a machine provisioning involves gathering the requirements, provisioning a new VM, procuring a license and installing OS, allocating storage, network configuration, and setting up redundancy and security policies.
Every time you wish to deploy your application you also have to ensure application specific software requirements such as web servers, database servers, runtimes, and any support software such as plugin drivers are installed on the machine. With teams obliged to deliver at light speed, the current VM virtualization will create more friction and latency.
The preceding problem can be partially solved by using the cloud platforms, which offer on-demand resource provisioning, but again public cloud vendors come up with a predefined set of VM configuration and not every application utilizes all allocated compute and memory.
In a common enterprise scenario every small application is deployed in a separate VM for isolation and security benefits. Further for ensuring scalability and availability identical VMs are created and traffic is balanced among them. If the application utilizes only 5-10% of the CPU's capacity, the IT infrastructure is heavily underutilized. Power and cooling needs for such systems are also high, which adds up to the costs. Few applications are used seasonally or by limited set of users, but still the servers have to be up and running. Another important drawback of VMs is that inside a VM OS and supporting services occupy more size than the application itself.
Every IT organization needs an operations team to manage the infrastructure's regular maintenance activities. The team's responsibility is to ensure that activities such as procuring machines, maintaining SOX Compliance, executing regular updates, and security patches are done in a timely manner. The following are a few drawbacks that add up to operational costs due to VM virtualization:
- The size of the operations team is proportional to the size of the IT. Large infrastructures require larger teams, therefore more costs to maintain.
- Every enterprise is obliged to provide continuous business to its customers for which it has to employ redundant and recovery systems. Recovery systems often take the same amount of resources and configuration as original ones, which means twice the original costs.
- Enterprises also have to pay for licenses for each guest OS no matter how little the usage may be.
VMs are not easily shippable. Every application has to be tested on developer machines, proper instruction sets have to be documented for operations or deployment teams to prepare the machine and deploy the application. No matter how well you document and take precautions in many instances the deployments fail because at the end of the day the application runs on a completely different environment than it is tested on which makes it riskier.
Let us imagine you have successfully installed the application on VM, but still VMs are not easily sharable as application packages due to their extremely large sizes, which makes them misfit for DevOps type work cultures. Imagine your applications need to go through rigorous testing cycles to ensure high quality. Every time you want to deploy and test a developed feature a new environment needs to be created and configured. The application should be deployed on the machine and then the test cases should be executed. In agile teams, release happens quite often, so the turnaround time for the testing phase to begin and results to be out will be quite high because of the machine provisioning and preparation work.
Choosing between VM virtualization or containerization is purely a matter of scope and need. It might not always be feasible to use containers. One advantage, for example, is in VM virtualization the guest OS of the VM and the host OS need not be the same. A Linux VM and a Windows VM can run in parallel on Hyper-V. This is possible because in VM virtualization only the hardware layer is virtualized. Since containers share the kernel OS of the host, a Linux container cannot be shipped to a Windows machine. Having said that, the future holds good things for both containers and VMs in both private and public clouds. There might be cases where an enterprise opts to use a hybrid model depending on scope and need.