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Vinchin Blog What Is VMware HotAdd/HotPlug and When Should I Enable It?

What Is VMware HotAdd/HotPlug and When Should I Enable It?

2022-07-18 | Echo

Table of contents
  • What is VMware HotAdd?
  • Requirements for using HotAdd
  • Pros and cons of VMware HotAdd
  • When should I enable HotAdd?
  • Wrap up:

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Few types of hardware can be added when the VM is running such as network adapter, hard disk, and USB adapter, but when you want to add RAM, CPU, or other hardware, you have to power the VM off. However, things are different when it comes to the VMware vSphere environment in which a feature is developed to put memory devices “hot” while a virtual machine is on.

What is VMware HotAdd?

VMware HotAdd refers to a feature that enables virtual devices to be added "hot" to a virtual machine while it’s working. Virtual machines can increase their CPU and memory capacities in addition to SCSI disks.

You’ve probably seen the term HotPlug and wondered about its connection to HotAdd. In short, both features work under the same logic for adding vCPU and vRAM, so they can be seen as interchangeable.

Requirements for using HotAdd

Before you use the feature, there are some requirements you need to know.

VMs with minimum hardware version 7 (vmx-7), which was introduced in vSphere 5.

Fault Tolerance cannot be enabled on the virtual machine.

A VMware vSphere Advanced, Enterprise or Enterprise plus license is required.

VM OS must support HotAdd/HotPlug. 

The guest OS has VMware Tools.

To enable HotAdd/HotPlug, the VM needs to be powered off.

The limitations of the licensing of Guest OS need to be carefully considered.


Pros and cons of VMware HotAdd

Pros:

No VM downtime: By using HotAdd CPU or Hot Add RAM, you may instantly add more RAM or vCPUs to your VMware virtual machines without turning them off.

Simple configurations: While your VM is running, you can easily adjust the RAM or CPU configurations if you require extra resources.

Cons:

Overhead produced: The HotAdd brings some overhead when it works.

Cannot work with vNUMA (Virtual Non-uniform Memory Access): Once HotAdd is enabled, the VM will automatically disable vNUMA, which is turned on by default for VMs with 8 or higher vCPUs to optimize memory distribution in VMware vSphere and avoid memory-bandwidth chokepoints. This will result in VM performance degradation.

When should I enable HotAdd?

There’s a reason why VMware leaves HotAdd disabled by default, they want you to decide when and how to use it. The only suggestion is to use it only when you need it. You don’t want to do any harm to your VM performances, after all, why would you allow such an effect on your VM when you take every precaution to maximize the performance of your virtual infrastructure?

It’s also worth noting that the support for Hot “Add” doesn’t mean you can do it reversely for Hot “Remove”. This is because the VM is using these resources, so pulling them away directly is not allowed. So, when you want to remove any added vCPU/vRAM, please make sure to do it when the VM is at rest.


Wrap up:

VMware HotAdd/HotPlug is an advanced feature that helps VMware users add vCPU/vRAM on a working VM. Useful as it is, it’s still disabled by default due to some possible inconvenience it may cause on the guest OS’s computing performance, so careful considerations need to be taken before you decide whether to enable the feature.

HotAdd can also be used in data protection scenarios for higher backup efficiency: a VM’s virtual disk data can be transferred to a backup appliance (or backup proxy) and then sent to the storage media. You can simply enable and make the most of this advanced HotAdd transport mode for VMware VM backups with Vinchin Backup & Recovery. 60 days of the full-featured trial of the product is now available to download.  

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