VMware vSphere 7.x Study Guide for VMware Certified Professional – Data Center Virtualization certification. This article covers Section 1: Architectures and Technologies. Objective 1.6 – Describe ESXi cluster concepts
This article is part of the VMware vSphere 7.x - VCP-DCV Study Guide. Check out this page first for an introduction, disclaimer, and updates on the guide. The page also includes a collection of articles matching each objective of the official VCP-DCV.
Describe ESXi cluster concepts
Objective 1.6 is a parent of multiple key topics discussing vSphere clustering. Therefore, this article is a quick introduction and an overview of all the essential concepts to understanding ESXi clusters.
In VMware vSphere, there is no logic to talk about clusters without mentioning Distributed Resource Scheduler (DRS) and High Availability (HA); these are the two essential services that support the management of VMware clusters. However, we will discuss these concepts in the following topics.
1. VMware Clustering
When I began studying VMware a couple of years ago, I had the same question that many in the virtualization realms had when the technology was just rising. What if a single server fails? Why put all your eggs (VMs) in one basket (Server)? It wouldn't be worth the benefits of saving computing resources by placing a couple of virtual servers into a physical one, as if this fails, you could lose all services.
Due to this conception, for many, it was hard to flip the mental switch about the advantages of virtualization. People were still uncertain about virtualization, even though VMware included similar and even better clustering technologies than physical machines. These new virtual services delivered capabilities that previously required complex or expensive solutions to implement.
These services also provide significantly higher hardware utilization and better alignment of IT resources with business goals and priorities. In the past, companies have had to assemble a patchwork of the operating system or software application-specific solutions to obtain the same benefits.
While using virtualization, even a powerful server can be overloaded with multiple computing tasks. To ensure better performance and reliability, we can use clustering technologies.
2. VMware Hosts and Clusters
Let's go back to the basics.
2.1 ESXi Host
A Host represents the aggregate computing and memory resources of a physical server. For example, suppose the physical server has two 12-core CPUs running at 3.0 GHz each and 256 GB of system memory. In that case, the Host will have 72 GHz of computing power and 256 GBs of memory available for running virtual machines assigned to it.
2.2 ESXi Cluster
A Cluster represents the aggregate computing and memory resources of a group of physical servers sharing the same network and storage arrays. For example, if the group contains four servers, each has two 12-core CPUs running at 3.0 GHz and 256 GB of memory. The Cluster will then have 288 GHz of computing power and 1,024 GB of memory available for the running virtual machines assigned to it.
Admins (the virtual resource owners) do not need to be concerned with the physical composition (number servers, quantity, and type of CPUs) of the underlying cluster to provision resources. Admins set up the resource provisioning policies based on the aggregate available resource. vSphere will automatically assign the appropriate resources dynamically to the virtual machines within the boundaries of those policies.
2.3 Resource Pools
Resource pools simplify control over the resources. They provide a flexible and dynamic way to divide and organize computing and memory resources from a Host or a Cluster. vSphere virtualizes and aggregates industry-standard servers (processors, memory, their attached network, and storage capacity) into logical resource pools (from a single ESX Server host or a VMware cluster) that can be allocated to virtual machines on demand. Any Resource Pools can be partitioned into smaller Resource Pools at a fine-grain level to divide further and assign resources to different groups or for various purposes.
3. VMware DRS and HA
In general, the two essential services that support the management of VMware clusters are VMware DRS and HA.
VMware DRS provides automatic initial virtual machine placement and makes automatic resource relocation and optimization decisions as hosts are added or removed from the cluster or the load on individual virtual machines changes.
Now, suppose an ESXi host goes offline. It will take down all virtual machines running on that single host. VMs won't come back online until you restore the server or restore them to another server using a backup and restore process. These processes can be very time‑consuming. VMware HA allows these virtual machines running on the host to be switched over to use other host resources in the cluster in the case of host machine failures.
More about these topics and concepts in following vSphere 7 objectives.
The topic reviewed in this article is part of the VMware vSphere 7.x Exam (2V0-21.20), which leads to the VMware Certified Professional – Data Center Virtualization 2021 certification.
Section 1 - Architectures and Technologies.
Objective 1.6 – Describe ESXi cluster concepts