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Wednesday, August 12, 2009

AWS EBS Volumes and Snapshots

AWS Elastic Block Store Volumes are invaluable for setting up EC2 nodes.

You can clone and/or backup Volumes into static Snapshots, also extremely useful. But be aware that the process of creating a Snapshot can be extremely slow and this can be a major problem if you are not expecting the delay.

For example, creating a snapshot from a 50GB volume took almost 2 hours for me today. Now, times will vary for all sorts of reasons. Most importantly, the first time you create a snapshot off a given volume will take the longest. Subsequent snapshots are just recording the changes since the previous one.

Being prevented from using your Volume for an extended period of time can be a huge problem, so plan ahead and make your snapshots overnight, for example.

Having said that, if your Volume contains Static or Read-Only data, then you have more flexibility:
- You can create a Snapshot from a mounted Volume, without having unmount or detach
- You can mount an unmounted Volume that is in the process of Snapshot creation.

This is convenient and does not appear to be explicit in the AWS docs, as far as I can see.
But this is limited to Volumes that will not change during snapshot creation.

WARNING: DO NOT try this with a Volume that is being written to, such as one holding a database.


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