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This Question has 2 parts:
1) What snapshots can be safely removed, do I need to retain the ones from which a running instance was launched?
2) How are snapshots appearing from a shut down instance?

Forgive my ignorance, the manual(?) http://aws.amazon.com/ebs/faqs/ says "customers can create volumes based on any of their existing snapshots". I take it that means I only need 1 snapshot of a volume at any given time, and I do not need to keep snapshots from which running instances were launched? I know that's what it says but just want to be really careful here.

About a month ago, some logging filled the root partition of an EC2 instance. Afterwards it wasn't behaving and rather than fussing with it, I created a new instance from the most recent snapshot and shut down the original instance. Now I see weekly auto-snapshots appearing from both the new and old instance. It runs from the default user's crontab, but since the old instance is shut off, what's generating the snapshots? I checked the new instance and it has one line to run its own snapshot weekly.

I'm ready to delete the old instance and snapshots but having not done this before, I'm trying to read as much as I can before trashing something important...

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An EBS Snapshot is a point-in-time backup of an EBS Volume. A new volume can be created from a snapshot, and the new volume will then contain exactly the same content as was on the original volume when the snapshot was taken. (By the way, as opposed to a VMWare snapshot that allows roll-back, an EBS Snapshot is more like a backup.)

Creating a snapshot causes the used blocks on the source volume to be copied to the Amazon S3 infrastructure. This means it is replicated in three facilities and is available to all Availability Zones. (The snapshots won't appear in an S3 bucket -- they are accessed via EC2.)

Snapshots are incremental, so only modified blocks are copied to S3.

You can safely delete any snapshot. EBS/S3 will keep any blocks are that are also used in another snapshot so that you can always create a new volume from any snapshot, and it will contain everything that was on the volume when the snapshot was taken. Deleting snapshots will not affect any existing volumes or instances.

There is no facility to automatically create snapshots, so your "auto-snapshots" are being created by something running in your own environment. If you wish to track-down the cause, turn on AWS CloudTrail and view the log files to see the log of API calls and identify the source of the snapshot request.

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