Scenario 8: Continuous Reboots After OS Patching

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Problem:

• After applying an OS patch, a production EC2 instance keeps rebooting.
  
  

• The instance does not reach a stable running state.
  
  

• Application availability is affected due to the continuous reboot loop.
  
  

Step-by-Step Troubleshooting:

1. Examine System Logs and Console Output:
   
   

   • Use the EC2 console’s “Get System Log” feature to retrieve boot logs.
     
     

   • Look for kernel panics, missing files, or driver errors during boot.
     
     

   • Identify if the reboot is caused by specific services or system processes failing.
     
     

2. Assess Recent Changes:
   
   

   • Review which OS patches or updates were applied just before the issue started.
     
     

   • Check any user data scripts for errors or recent changes that might run on each boot.
     
     

   • Look for configuration changes in system files that could cause boot failure.
     
     

3. Boot in an Alternate Mode:
   
   

   • Stop the instance and detach its root EBS volume.
     
     

   • Attach the volume to another healthy instance as a secondary volume.
     
     

   • Mount the volume and check system files for errors, especially in /etc/fstab, init scripts, and kernel modules.
     
     

   • Look for corrupted files, misconfigured drivers, or inappropriate startup commands.
     
     

4. Rollback or Reapply Patches:
   
   

   • If a specific patch caused the reboot loop, remove or roll back the patch from the attached volume.
     
     

   • Test the problematic patch in a cloned instance or staging environment before retrying it in production.
     
     

   • Ensure all dependencies and configurations are compatible with the patch.
     
     

5. Validate Instance Type Compatibility:
   
   

   • Confirm that the instance type supports the patched OS, especially in terms of kernel version and drivers.
     
     

   • Some instance types require specific kernel settings or drivers to boot properly.
     
     

   • Consider switching to a compatible instance type if necessary.
     
     

Key AWS Terms:

• System Log: Boot-time logs available from the EC2 console.
  
  

• Root EBS Volume: The primary disk for an EC2 instance that can be detached and attached to another instance.
  
  

• Kernel Panic: A critical error from the OS kernel that stops the system from running.
  
  

• User Data Script: A script executed at instance launch that may affect startup.
  
  

• Instance Type Compatibility: Ensuring hardware and OS configurations align.
  
  

Interview Insight:

• When an EC2 instance enters a reboot loop after patching, I check system logs to find kernel errors or service failures.
  
  

• I isolate the root volume, investigate configuration or patch issues, and roll back problematic updates.
  
  

• I always validate kernel and driver compatibility with the instance type to prevent hardware-OS conflicts.