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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:
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.
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.
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.
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.
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.