VMware vSphere 7.x Study Guide for VMware Certified Professional – Data Center Virtualization certification. This article covers Section 1: Architectures and Technologies. Objective 1.5 – Describe instant clone architecture and use cases.
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 Instant Clone Architecture And Use Cases
In objective 1.5, you need to describe instant clone architecture and use cases. First, let’s study virtual machine cloning types and techniques, and also compare the critical capabilities of each one. Then we can move to the instant clone topic, study architecture, operation, and use cases.
I don't see why it is necessary to understand only about Instant Clones (at the moment of) studying this material. This objective (as other similars in the exam's blueprint) should be called "Identify and differentiate cloning types in vSphere" or "Describe cloning operations for vSphere." And in there later incorporate Instant Clones. Also, unless VMware has some particular plans for this technology in the future, I don't see actual use exclusively for this subject in this exam. Is the ordinary clone (full clone) too basic for this exam? But, what the heck, it is just my opinion… probably, I am awfully wrong.
So, in short, even though it is not part of the objective, I consider it essential to differentiate between additional cloning techniques before studying just instant clones. Prior opinion, especially if you are moderately new to virtualization and VMware.
I have spent quite some time cloning VMs while designing and testing VDI, but my focus was always on full clones and linked clones. Instant Clones is kind of new to me (not for VMware), and while studying this topic, I found pretty neat information from this white paper: Understanding Clones in VMware vSphere 7. Almost all the information in this article is taken from here.
1. VMware Clone Types
When you install a guest operating system and applications can be time-consuming. With clones, you can conveniently make copies of a virtual machine.
Virtual machine cloning is one of vCenter's most heavily used provisioning operations. vSphere
offers three different types of clones: full clones, linked clones, and instant clones. Each type of clone differs in memory and storage efficiencies and performance. The choice of a clone type depends on business goals, workloads, provisioning times, and performance.
1.1 Full Clone
A full clone is an independent child VM that shares nothing with the parent VM after the cloning operation. The ongoing operation of a full clone is entirely separate from the parent VM.
1.2 Linked Clone
A linked clone is a child VM that shares virtual disks with the parent VM in an ongoing manner. A linked clone is made from a snapshot of the parent and uses snapshot-based delta disks. The child disks employ a copy-on-write (COW) mechanism, in which the virtual disk contains no data in places until copied there by a write. This optimization conserves storage space.
1.3 Instant Clone
An instant clone is a child VM that shares virtual disks and memory with the parent VM in an ongoing manner. Like the linked clone, an instant clone also leverages delta disks to conserve storage space. An instant clone also shares the memory state of the parent VM to deliver efficient memory use.
Note: Because a full clone does not share virtual disks or memory with the parent virtual machine, they usually perform better than linked clones or instant clones. However, full clones take longer to provision than the other clone types.
2. Instant Clone Architecture
The instant clones feature leverages vSphere vmFork technology (available with vSphere 6.0 U1 and later) to quiesce a running base image, or parent virtual machine, and rapidly create and customize a pool of virtual desktops.
2.1 Instant Clone Overview
- Instant Clones are always created from a running VM instead of a powered-off VM, and function much like a container.
- Instant Clones, unlike full clones or linked clones, are always created in a powered-on state with the VM ready for users to connect with the guest applications running inside the parent VM prior to provisioning.
- The technology uses rapid in-memory cloning of a running parent virtual machine and uses copy-on-write to rapidly deploy the virtual machines.
- Instant Cloned virtual machines are fully independent vCenter Server inventory objects. You can manage Instant Cloned virtual machines like regular virtual machines without any restrictions.
- The result of an Instant Clone operation is a virtual machine that is called a destination virtual machine.
- The processor state, virtual device state, memory state, and disk state of the destination virtual machine are identical to those of the source virtual machine.
- You must create instant clones (and linked clones) through the ESXi host’s command-line interface.
- Instant clones reduce the required storage capacity by 50 to 90 percent. The overall memory requirement is also reduced at clone creation time.
2.2 Instant Clone Operation
- During an Instant Clone operation, the source virtual machine is stunned for a short period of time, less than 1 second.
- While the source virtual machine is stunned, a new writable delta disk is generated for each virtual disk and a checkpoint is taken and transferred to the destination virtual machine.
- A memory checkpoint is also generated (taken) from the source virtual machine.
- The destination virtual machine then powers on by using the source's checkpoint.
- After the destination virtual machine is fully powered on, the source virtual machine also resumes running.
3. Instant Clone Use Cases
With Instant Clone you can create new virtual machines from a controlled point in time. Instant cloning is very convenient for large-scale application deployments because it ensures memory efficiency and allows for creating numerous virtual machines on a single host.
The major use case of Instant Clones is found under VMware Horizon for VDI deployments, where hundred of clones must be created in a short time. As these VMs can be quickly created and deleted, using them for testing or DevOps environments is also a great advantage.
In general, uses cases for the Instant Clones are:
- Virtual Desktop Infrastructure (VDI)
- DevTest and DevOps
- Big data with the Hadoop framework
3.1 Instant Clone Benefits In Horizon 7 (VDI)
- Administrators can quickly provision from a parent virtual machine whenever new desktops are needed, just in time for a user to log in. With this type of speed, you can reduce the number of spare machines as required.
- Instant clones do not need to be refreshed, recomposed, or rebalanced. After a user logs out of a desktop, the desktop is always deleted and recreated as a fresh image from the latest patch.
- Desktop-pool image changes can also be scheduled during the day with no downtime for the users or for the availability of the desktop pool, so the View administrator has full control over the changes when the users receive the latest image.
4. Which VMware Clone Type Use
The white paper referenced in this article discussed both the performance aspects and provisioning rates of these different clones. Below are the remarks for the test.
Based on VMware’s test:
- Full clones generally perform better than linked clones and instant clones because full clones do not share any disk or memory state with a parent VM.
- Instant clones and linked clones are not good candidates for disk I/O–intensive workloads when using a VMFS datastore because they rely on snapshot-based disks and redo logs that result in additional disk I/O latency.
- vSAN and vVOL datastores are a better alternative to a VMFS datastore when using clones or VM snapshots. Instant clones are not supported on vVOL.
- With near-identical performance as full clones, instant clones and linked clones are great candidates for Horizon View desktop solutions.
- Beyond Horizon View, instant clones and linked clones perform very well with CPU- and memory-intensive workloads without a significant I/O component.
- Linked clones generally perform slightly better than instant clones because they do not share a memory state with the parent VM.
- The provisioning time of instant clones and linked clones is much shorter than that of full clones.
- The provisioning time of full clones is bound by two factors: total VM storage size and disk I/O throughput of source and destination datastores. The unused storage space of a VM’s thin disks has no impact on the provisioning time.
- Provisioning a cold clone (clone from a powered-off parent VM) compared to a hot clone (clone from a running parent VM) increases the vCenter concurrency during bulk clone creation and thereby provides a significant reduction in total provisioning time.
- The provisioning time of linked clones does not vary based on the VM size or disk I/O throughput.
- The provisioning time of instant clones does not depend on disk I/O throughput, although the provisioning time has some dependency on the VM’s memory size.
- Provisioning instant clones from a frozen parent VM is helpful both from a deployment standpoint and provisioning time.
- The provisioning time of remote full clones is much slower than that of local full clones.
Note: The provisioning time of an instant clone is higher than that of a linked clone because instant clones are always created in a powered-on state, ready for users to connect to with the guest applications running inside the parent VM prior to provisioning, which is a key requirement for some applications. Provisioning an instant clone is a tradeoff of time vs functionality.
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 1.5 – Describe instant clone architecture and use cases.