Snapshots are an important tool for backing up instances before doing work on them, or to create new instances based on another existing instance.
Creating a Snapshot
To snapshot an existing instance, first ensure that you have enough volume storage for the snapshot. From the dashboard, under the "Compute" section of the menu bar, select "Overview" to see how much volume storage you have remaining. You will require enough volume storage both for the snapshot itself, and for a new instance built from the snapshot (if that is what you wish to do with the snapshot). When creating a snapshot of an instance, a copy is made of every volume currently attached to that instance. So (for example) if your instance has a 20GB root disk volume plus an additional 100GB storage volume attached, you would need at least 120GB for the snapshot to copy both the root disk and storage volumes; if you intend to use the snapshot to create another instance, you will need another 120GB on top of that again.
Secondly, you want to make sure that the size of your root disk volume matches the size of the disk specified by the instance flavour you have chosen. For example, if you are going to snapshot an instance that uses the m2.medium flavour, then the root disk for your instance should be 20GB. There is currently a bug in Nova (https://bugs.launchpad.net/nova/+bug/1646740) that means if you create a snapshot where the root disk of the original instance is smaller than the disk size for the flavour, then when you go to create a new instance from that snapshot, you will get an error message like the following:
Volume is smaller than the minimum size specified in image metadata. Volume size is xxx bytes, minimum size is yyy bytes. (HTTP 400)
When you are ready to create a snapshot, simply go to the "Instances" section of the dashboard (under "Compute" from the left-hand menu bar), then from the "actions" pull-down menu to the right of the instance you wish to snapshot, select "Create Snapshot". Enter a name for the snapshot, then click on "Create Snapshot". The snapshot will be listed in the "Images" section of the dashboard, while the volumes associated with that snapshot will appear under the "Volume Snapshots" tab of the "Volumes" section of the dashboard. The snapshot itself will be listed as being 0 bytes in size; it is actually the associated volumes that take up the storage space.