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Building CentOS AMIs

These steps detail how to build AMIs that can be started on AWS EC2 instances using cloud images provided by CentOS.

Prerequisites

  • An Amazon EC2 instance with sufficient disk space (~15GB) to run this script. I run this script on an EC2 instance running Amazon Linux 2 because the transfer speeds are much faster to S3.
  • AWS credentials with permissions to create snapshots, etc.
  • The qemu-img RPM installed.

S3 Bucket

You'll need an S3 bucket to hold raw disk images that are used by the snapshot creation process.

To create a bucket, use the following (this example assumes the us-west-2 region):

ACCOUNT=$(aws sts get-caller-identity --query Account --output text)
BUCKET_NAME="${ACCOUNT}-vm-imports"
BUCKET_REGION="us-west-2"

aws s3api create-bucket \
    --bucket ${BUCKET_NAME} \
    --create-bucket-configuration LocationConstraint=${BUCKET_REGION}

IAM Role

AWS requires a vmimport role to be defined for import/export operations. See this documentation for more information.

If you don't have this role, it can be created using the following commands:

aws iam create-role --role-name vmimport --assume-role-policy-document "file://trust-policy.json"

Add a policy to the newly created role:

ACCOUNT=$(aws sts get-caller-identity --query Account --output text)
BUCKET_NAME="${ACCOUNT}-vm-imports"
sed -e "s/___BUCKET_NAME___/${BUCKET_NAME}/g" role-policy.json.template > role-policy.json
aws iam put-role-policy --role-name vmimport --policy-name vmimport --policy-document "file://role-policy.json"

Upload raw disk image to S3

  • Environment variables

    The following environment variables should be set for the subsequent steps:

    ACCOUNT=$(aws sts get-caller-identity --query Account --output text)
BUCKET_NAME="${ACCOUNT}-vm-imports"

IMAGE_URL="http://cloud.centos.org/centos/8/x86_64/images/CentOS-8-ec2-8.1.1911-20200113.3.x86_64.qcow2"
IMAGE_FILE="CentOS-8-ec2-8.1.1911-20200113.3.x86_64.qcow2"
IMAGE_ID="CentOS-8-ec2-8.1.1911-20200113.3"
IMAGE_DESCRIPTION="CentOS 8 ec2 8.1.1911 20200113.3"

  • Download the image provided by CentOS:

    wget ${IMAGE_URL}

  • Convert the provided qcow2 file to raw format:

    qemu-img convert -S 0 -p ${IMAGE_FILE} -O raw ${IMAGE_ID}.raw

  • Run dracut on the raw image (required for CentOS 8, optional for CentOS 7):

    This step requires root level permissions.

    IMAGE_ID="CentOS-8-ec2-8.1.1911-20200113.3"

mount -o loop,offset=1048576 ${IMAGE_ID}.raw /mnt
mount -o bind /dev /mnt/dev
mount -o bind /proc /mnt/proc
mount -o bind /sys /mnt/sys
mount -t tmpfs tmpfs /mnt/run

chroot /mnt
dracut -f /boot/initramfs-4.18.0-147.3.1.el8_1.x86_64.img 4.18.0-147.3.1.el8_1.x86_64
# Run any other customizations here
exit

umount /mnt/run
umount /mnt/sys
umount /mnt/proc
umount /mnt/dev
umount /mnt

  • Upload the raw image to S3:

    aws s3 cp ${IMAGE_ID}.raw s3://${BUCKET_NAME}/${IMAGE_ID}.raw

Run import-snapshot operation

  • Execute:

    aws ec2 import-snapshot --disk-container \
    "Description=CentOS,Format=RAW,UserBucket={S3Bucket=${BUCKET_NAME},S3Key=${IMAGE_ID}.raw}"

    This will return an ImportTaskId. Set an environment variable to the ImportTaskId (substitute the example below with your task ID):

    TASK_ID=import-snap-087cd7311afba675f

  • Wait for snapshot completion:

    Creating the snapshot will take several minutes. You can check the status of the task by running:

    aws ec2 describe-import-snapshot-tasks --import-task-ids ${TASK_ID}

    While it is running, you will see output with a Status of active:

    {
    "ImportSnapshotTasks": [
        {
            "SnapshotTaskDetail": {
                "Status": "active", 
                "Format": "RAW", 
                "DiskImageSize": 0.0, 
                "UserBucket": {
                    "S3Bucket": "123456789012-vm-imports", 
                    "S3Key": "CentOS-8-ec2-8.1.1911-20200113.3.raw"
                }, 
                "Progress": "2", 
                "StatusMessage": "pending"
            }, 
            "ImportTaskId": "import-snap-087cd7311afba675f"
        }
    ]
}

    When it's complete, you'll see output with a Status of completed:

    {
    "ImportSnapshotTasks": [
        {
            "SnapshotTaskDetail": {
                "Status": "completed", 
                "SnapshotId": "snap-0fecb321718c3dcd9", 
                "DiskImageSize": 10737418240.0, 
                "UserBucket": {
                    "S3Bucket": "123456789012-vm-imports", 
                    "S3Key": "CentOS-8-ec2-8.1.1911-20200113.3.raw"
                }, 
                "Format": "RAW"
            }, 
            "ImportTaskId": "import-snap-087cd7311afba675f"
        }
    ]
}

Register an AMI

  • Collect information about the snapshot that was created:

    # Substitute the snapshot ID using the output from the previous step
SNAPSHOT_ID=snap-0fecb321718c3dcd9

SNAPSHOT_SIZE=$(aws ec2 describe-snapshots --snapshot-ids ${SNAPSHOT_ID} --query Snapshots[].VolumeSize --output text)

  • Register the AMI:

    aws ec2 register-image \
    --architecture x86_64 \
    --block-device-mappings "DeviceName=/dev/sda1,Ebs={DeleteOnTermination=true,SnapshotId=${SNAPSHOT_ID},VolumeSize=${SNAPSHOT_SIZE},VolumeType=gp2}" \
    --description "${IMAGE_DESCRIPTION}" \
    --ena-support \
    --name ${IMAGE_ID} \
    --root-device-name /dev/sda1 \
    --virtualization-type hvm

    The returned ImageId from the previous command can be used to launch EC2 instances running CentOS.

Cleanup

Clean up any unused resources (such as S3 buckets, EC2 instances, etc.) that are no longer required.

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