Restoring a volume from a snapshot

Question

Let's say I have an AMI with an attached EBS Volume.

I also have a snapshot.

I want to "restore" the EBS Volume to the snapshot.

What's the best process to do this?

Answer 1

I don't know of a way that you can 'restore' an attached volume, but the way i would do it is to create a volume from the snapshot, then detach the original and attach the new one.

Answer 2

If you have a running EC2 instance, and you want to restore it to the state captured in an earlier snapshot, then you need to stop the instance, detach its current volume, create a new volume from the snapshot, attach the new volume to your instance, and restart your instance. Furthermore, there are a couple subtleties around specifying the availability zone of the new volume, and the device name when detaching/re-attaching the volume.

The logic might easier to see if you do it from the command line, instead of from the AWS web UI.

The following bash script is not fit for production use, since it lacks any error-checking and it just uses sleep instead of polling to ensure AWS commands have completed. But it does perform all these steps successfully:

#!/bin/bash

set -e

# IN PARAMS
INSTANCE_ID=<YOUR_INSTANCE_ID_HERE>
SNAPSHOT_ID=<YOUR_SNAPSHOT_ID_HERE>
# OUT PARAMS
VOLUME_ID=

# begin execution

echo "Gathering information about the instance"
DEVICE_NAME=`ec2-describe-instance-attribute ${INSTANCE_ID} --block-device-mapping | awk '{print $2}'`
OLD_VOLUME_ID=`ec2-describe-instance-attribute ${INSTANCE_ID} --block-device-mapping | awk '{print $3}'`
echo "Found instance ${INSTANCE_ID} has volume ${OLD_VOLUME_ID} on device ${DEVICE_NAME}"

echo "Creating new volume from snapshot"
AVAILABILITY_ZONE=`ec2-describe-availability-zones --filter state=available | head -n 1 | awk '{print $2}'`
VOLUME_ID=`ec2-create-volume --availability-zone ${AVAILABILITY_ZONE} --snapshot ${SNAPSHOT_ID} | awk '{print $2}'`
echo "Created new volume: ${VOLUME_ID}"

sleep 20

echo "Stopping the instance"
ec2-stop-instances $INSTANCE_ID

sleep 20

echo "Detaching current volume"
ec2-detach-volume $OLD_VOLUME_ID --instance $INSTANCE_ID --device $DEVICE_NAME

sleep 20

echo "Attaching new volume"
ec2-attach-volume $VOLUME_ID  --instance $INSTANCE_ID --device $DEVICE_NAME

sleep 20

echo "Starting the instance"
ec2-start-instances $INSTANCE_ID