Hot adding memory in Linux

Products

• VMware Desktop Hypervisor
• VMware vSphere ESXi

Issue/Introduction

Memory you have hot added in Linux is not registered in the guest using the free or top commands.

Environment

VMware vSphere Esxi 6.x 

VMware vSphere Esxi 7.x 

VMware vSphere Esxi 8.x 

Cause

This issue occurs when Linux requires manual intervention to expose the newly added memory in the guest. The process is called setting the memory to online.

Resolution

To resolve this issue, set the memory online in RHEL, CentOS, and SLES OS.

• To set the memory online in RHEL and CentOS:

1. Look for memory that appears offline.
   
   Run this command to check the state of the memory:
   
   grep line /sys/devices/system/memory/*/state
    
2. When memory appears offline, run this command to set it to online:
   
   echo online >/sys/devices/system/memory/memory[number]/state
    

• To set the memory online in SLES OS:

1. 1. Run this command to load the acpiphp module for SLES to detect hot add devices:
      
      modprobe acpiphp
       
   2. Run this command to enable acpi_memhotplug within the SLES virtual machine:
      
      modprobe acpi_memhotplug
       
   3. Using vSphere Client edit the virtual machine settings to increase the memory assigned.
      
      
   4. Run this command to bring the memory online in /sys/devices/system/:
      
      echo online >/sys/devices/system/memory/memory[number]/state
       
   5. Run this command to check the state of the memory.
      
      Look for memory that appears offline:
      
      grep line /sys/devices/system/memory/*/state
       
   6. When memory appears offline, run this command to set it to online:
      
      echo online >/sys/devices/system/memory/memory[number]/state
       
   7. Verify you can see the extra memory by running this command:
      
      free -m