What is the functionality of the option "reserve all guest memory (all locked) within the virtual machine ?

Products

• VMware vCenter Server
• VMware vSphere ESXi

Issue/Introduction

The option called "Reserve all guest memory" allows you to allocate and reserve all the memory assigned to a virtual machine (VM). When this option is enabled, VMware guarantees that the full amount of memory allocated to the VM will be reserved and available exclusively for that VM. This ensures that the guest operating system running inside the VM always has the specified amount of memory available.Please note that reserving all guest memory means that the allocated memory will not be shared or used by other virtual machines or the host system. This can result in memory overcommitment issues, where the total memory assigned to all VMs exceeds the available physical memory. Careful consideration should be given to the memory requirements of other VMs and the host system before enabling this option.

 

Environment

VMware vCenter Server Appliance 6.7.x
VMware vSphere ESXi 8.0.0
VMware vCenter Server 7.0.0
VMware vSphere 7.0.x
VMware vCenter Server 8.0.0

Resolution

What is the difference between enabling it or not, please find the details below if you enable it:

• Memory reserved to a VM is guaranteed never to swap or balloon.
• If an ESXi host does not have enough unreserved RAM to support a VM with a reservation, the VM does not power on.
• Adding a vSphere DirectPath I/O device to a VM sets memory reservation to the memory size of the VM. Typically this is required by pass-thru-related features such as DirectPath I/O and SR-IOV that allow the VM to access PCI devices directly bypassing the virtualization layer.

Selecting this check box ensures that all of the VM's memory gets reserved even if you change the total amount of memory for the VM. The memory reservation is immediately readjusted when the VM's memory configuration changes.

• VMs never consume more physical RAM than is specified by the memory allocation limit.
• VMs might use the VM swap mechanism (.vswp) if the guest OS attempts to consume more RAM than is specified by the limit.

For more information please refer to this 3rd-party article.

Additional Information

https://frankdenneman.nl/2013/02/21/reserve-all-guest-memory-all-locked/1