Diagnosing Common Issues When Using Packer’s Amazon‑EBS Builder with Incremental Snapshots

Problem Statement

When building Amazon Machine Images (AMIs) with Packer’s amazon-ebs builder, you may notice long build times, frequent failures, or AMI naming collisions. These symptoms often stem from misconfigured incremental snapshot settings, IAM permissions, or naming conventions.

Key Takeaway

Use incremental snapshots and a deterministic ami_name template, while ensuring proper IAM policies and network access. Follow the diagnostic steps below to pinpoint and fix the root cause.

Recognizable Conditions

Condition	Typical Error
Build stalls or takes >30 min	Snapshot creation slow or duplicate
Build fails with AccessDenied	Missing IAM permissions
AMI name already exists	Template does not generate unique names
Post‑processor errors after instance shutdown	Instance unreachable for SSH

Diagnostic Table

Check	What to Verify	Command / Action
IAM Permissions	ec2:CreateSnapshot, ec2:CreateImage, ec2:RegisterImage, ec2:DescribeImages	aws iam list-policies --query "Policies[?PolicyName=='AmazonEC2FullAccess'].Arn"
Network Connectivity	Instance can reach api.ec2.amazonaws.com and outbound internet for the snapshot upload	aws ec2 describe-instances --instance-ids i-xxxx | grep PublicIpAddress
Snapshot Size	Snapshot matches root volume size	aws ec2 describe-snapshots --owner-ids self | grep SnapshotId
AMI Naming Uniqueness	Template uses time‑based placeholders	Check ami_name field in template
Post‑processor Reachability	Instance remains SSH‑able until post‑processor runs	ssh -i key.pem ec2-user@ -o ConnectTimeout=10

Ordered Checks and Fixes

1. Verify IAM Permissions
   • Ensure the role used by Packer (or the credentials you pass) has the required EC2 actions.
   • If missing, attach the managed policy AmazonEC2FullAccess or create a custom policy:

     {
       "Version": "2012-10-17",
       "Statement": [
         {"Effect":"Allow","Action":["ec2:CreateSnapshot","ec2:CreateImage","ec2:RegisterImage","ec2:DescribeImages"],"Resource":"*"}
       ]
     }

2. Check Network Access
   • From the build host, confirm you can reach the EC2 API endpoint:

     curl -s https://ec2.amazonaws.com | head -n 1

   • If behind a proxy, set HTTP_PROXY and HTTPS_PROXY environment variables before running packer build.
3. Ensure Incremental Snapshots Are Enabled
   • In the template, set snapshot_create_volume_size to match the root volume and enable snapshot_create_volume_size.
   • Verify that the snapshot ID returned in the build log ends with -snap-* and that subsequent builds reuse the same snapshot ID if no changes occurred.
4. Validate ami_name Template
   • Use time‑based placeholders to avoid collisions:

     "ami_name": "{{user `ami_base`}}-{{timestamp "20060102-150405"}}"
     

   • Run packer build -var 'ami_base=test' template.json and check the EC2 console: the AMI should have a unique name ending with a timestamp.
5. Confirm Snapshot Size
   • After build, run:

     aws ec2 describe-snapshots --owner-ids self --query "Snapshots[*].{Id:SnapshotId,Size:VolumeSize}"

   • Size should match the instance’s root volume (e.g., 30 GB).
6. Post‑processor Reachability
   • Ensure the instance remains running until the post‑processor (e.g., communicator or file transfer) completes. Add a pause_before or pause_after setting if needed.
   • Example: "pause_before": "30s" to give the instance time to finish provisioning.

Escalation Criteria

• If IAM permissions and network checks pass but the build still fails, examine the AWS CloudTrail logs for denied API calls.
• Persistent snapshot failures may indicate limits on ec2:CreateSnapshot per hour; contact AWS support for quota increase.
• Repeated ami_name collisions after timestamping suggest clock skew on the build host; synchronize NTP.

Practical Verification

After applying the fixes, run a test build:

packer build -var 'ami_base=demo' template.json

Then list the AMIs:

aws ec2 describe-images --owners self --query "Images[*].{Name:Name,Id:ImageId}"

Verify the snapshot size and that the AMI is available for launch.

Limitations

• Incremental snapshots are only supported for EBS‑backed instances; instance store volumes are ignored.
• Snapshot reuse requires the builder to run in the same AWS account and region.
• Time‑based naming assumes the build host’s clock is accurate; consider using {{timestamp "20060102-150405"}} with UTC.