AWS Difference between a snapshot and AMI

问题:

So I am having trouble working out what, specifically is the difference between these two.

As I understand it, a snapshot is simply a backup of the disk drive, whereas the AMI is a backup of the entire system (or instance I should say), but isn't the entire system technically wholly located on the disk drive? and if that's the case then there is no clear difference and I am missing something?

回答1:

There are two types of AMIs (and corresponding instances):

1. instance-store (sometimes called S3-based). These are less common and I don't recommend them for beginners. An instance-store AMI is a copy of the root instance-store volume plus some metadata, all saved in an S3 bucket in a special format

2. EBS boot. This is probably what you are using. An EBS boot AMI is an EBS snapshot of the EBS root volume plus some metadata like the architecture, kernel, AMI name, description, block device mappings, and more.

You can take a snapshot of an EBS boot volume and turn it into an EBS boot AMI by registering it with the appropriate metadata. The trickiest part of this is specifying the correct AKI id (kernel) so that it boots correctly.

回答2:

The major difference is between the type of service referred to. A snapshot is of an EBS volume where you are able to save state and reboot with the same data at a certain point in time.

An AMI is similar, but its for the EC2 instances themselves. You cannot take a snapshot of a non ebs backed instance, but you can create a AMI (system image) of one.

Generally I use EBS snapshots as backup solutions for a database volume and I use an AMI to save instance configuration

回答3:

An AMI can be created using a snapshot. For example, using a single "snapshot" you can create multiple AMIs, say one PV and one HVM AMI using the same snapshot.

So, snapshot has the system / OS data. AMI is (snapshot + machine/hardware metadata).

回答4:

I was confused by that as well. Here's simplest way of understanding it:

• EBS Snapshot very often represents a backup of specific EBS volume, it might be any volume (Root volume, data volume, etc.)

• AMI (Amazon Machine Image) is a backup of Entire EC2 instance. For example, with proper configuration it's possible to create AMI which includes multiple EBS volumes.

Now, it might sound confusing, but they are both stored as a "EBS Snapshots".

Just think of that way:

• EBS Snapshot is a just a data backup.
• AMI is a representation of system state at specific time. You can also boot from it.

回答5:

Here are few more differences between AMI and EBS Snapshots:

1) An AMI is launchable and contains links to the root device and may contain links to snapshots of the other data volumes.

2) The data image contained in an AMI does not represent a well-defined point-in-time, unless the instance is rebooted, something that is typically not acceptable for production environment. Snapshots can be taken in a consistent manner since their exact point-in-time can be controlled, so before the snapshot starts it can be made sure that everything is "ready for backup".

3) An AMI can be created from existing snapshots of the root device for Linux but not for Windows.

回答6:

Snapshots are less expensive to use as a backup strategy, because when you have multiple snapshots, you only pay for one full backup and ther rest are, in essence, just diffs, and usually much smaller.

回答7:

You can think of an AMI as a generic template for a machine with OS and installed components being preserved.

A Snapshot can include everything an AMI does, but also saves the disk data of an EBS volume.

Which one you decide to use will generally be dictated by whether your instances are EBS backed and if you want to exactly recreate a machine with all data intact or just want a generic machine template.

回答8:

From the definition provided by AWS it clarifies the difference - An Amazon Machine Image (AMI) is a template that contains a software configuration (for example, an operating system, an application server, and applications). From an AMI, you launch an instance, which is a copy of the AMI running as a virtual server in the cloud. Whereas for Snapshots You can back up the data on your EBS volumes to Amazon S3 by taking point-in-time snapshots. Snapshots are incremental backups, which means that only the blocks on the device that have changed after your most recent snapshot are saved. When you delete a snapshot, only the data exclusive to that snapshot is removed.

回答9:

As per the definition provided by AWS,

An AMI is a template from which you can start an EC2 instance. An EBS Snapshot is a block level copy of an EBS volume. The EBS volume might be a boot volume (i.e. containing an operating system), or a data-only volume (containing database files for example). You use RegisterImage to create an AMI (from a snapshot).

These are two different concepts, applied at different levels (EBS volumes vs EC2 templates) However, there are some dependencies between the two concepts.

For EBS backed EC2 instances (i.e. EC2 instances that boot from an EBS volume), the AMI is implemented as an EBS Snapshot of a boot volume + a couple of meta data (the architecture of the machine - 32 vs 64 bits -, the type of virtualization - HVM vs PV - etc ...)

So, for EBS backed EC2 instances, an AMI is an EBS snapshot + an XML file. You can even create your own AMI based on any snapshot of a boot volume you own.