Snapshots on Amazon EC2

Question

I used CreateImageRequest to take a snapshot of a running EC2 machine. When I log into the EC2 console I see the following:

1. AMI - An image that I can launch
2. Volume - I believe that this is the disk image?
3. Snapshot - Another entry related to the snapshot?

Can anyone explain the difference in usage of each of these? For example, is there any way to create a 'snapshot' without also having an associated 'AMI', and in that case how do I launch an EBS-backed copy of this snapshot?

Finally, is there a simple API to delete an AMI and all associated data (snapshot, volume and AMI). It turns out that our scripts only store the AMI identifier, and not the rest of the data, and so it seems that that's only enough information to just Deregister an image.

Answer 1

The AMI represents the launchable machine configuration - it does NOT actually contain any of the machine's data, just references to it. An AMI can get its disk image either from S3 or (in your case) an EBS snapshot.

The EBS Volume is associated with a running instance. It's basically a read-write disk image. When you terminate the instance, the volume will automatically be destroyed (this may take a few minutes, note).

The snapshot is a frozen image of the EBS volume at the point in time when you created the AMI. Snapshots can be associated with AMIs, but not all snapshots are part of an AMI - you can create them manually too.

More information on EBS-backed AMIs can be found in the user's guide. It is important to have a good grasp on these concepts, so I would recommend giving the entire users guide a good read-over before going any further.

If you want to delete all data associated with an AMI, you will have to use the DescribeImageAttribute API call on the AMI's blockDeviceMapping attribute to find the snapshot ID; then delete the AMI and snapshot, in that order.

Answer 2

This small PS script takes the AMI parameter (stored in a variable), grab the snapshots of the given AMI ID by storing them into an array, and finally perform the required clean up (unregister & remove the snapshots).

# Unregister and clean AMI snapshots
$amiName = 'ami-XXXX' # replace this with the AMI ID you need to clean-up
$myImage = Get-EC2Image $amiName
$count = $myImage[0].BlockDeviceMapping.Count

# Loop and store snapshotID(s) to an array
$mySnaps = @()
for ($i=0; $i -lt $count; $i++)
{
 $snapId = $myImage[0].BlockDeviceMapping[$i].Ebs | foreach {$_.SnapshotId}
 $mySnaps += $snapId
}

# Perform the clean up
Write-Host "Unregistering" $amiName
Unregister-EC2Image $amiName
foreach ($item in $mySnaps)
{
  Write-Host 'Removing' $item
  Remove-EC2Snapshot $item
}

Clear-Variable mySnaps