A system image uploaded into an UEC cluster.
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A UEC cluster doesn't do anything by itself. It's just a container, inside which you place any number of virtual machine images.
Virtual machine images for UEC have a specific format, too. You can't simply copy a disk image or .ISO and boot that into a UEC cluster, as you might be able to with a virtualization product like VMware or VirtualBox. Instead, you have to "package" the kernel and a few other components in a certain way, and then feed that package to the cluster.
Canonical has several of these images already available. They're representative of the most common Linux distributions out there, so most everyone should be able to find something that matches what they need or already use.
The process for uploading a kernel bundle is a multi-step method that you should take as slowly and gradually as possible. In fact, you may be best off taking the instructions described on the page, copying them out, and turning them into a shell script. That way, you won't be at the mercy of typing the wrong filename or passing the wrong flags -- and not just once, but potentially over and over. The same goes for the scripts used to build custom images (read on for more on that).
Finally, once an image has been uploaded and prepared, the administrator can start the instance through one of Eucalyptus's command-line scripts. Note that an image might take some time to boot depending on the hardware configuration (and whatever else might also be running on the cluster), so you might see the image show up as "pending" when you use the euca-describe-instances command to list running instances.
Walrus And Amazon S3
Those who've used Amazon's EC2 will be familiar with Amazon S3. That's the storage service provider for EC2, which lets you preserve data in a persistent fashion for use in the cloud. Eucalyptus has a similar technology, Walrus, which is interface-compatible with S3.
If you're familiar with S3 and have already written software that makes use of it, retooling said software for Walrus shouldn't be too difficult. They use many of the same command-line functions -- e.g., S3's implementation of Curl -- but you can also use Amazon's own EC2 API/AMI toolset to talk to Walrus as if it were Amazon's own repositories.
Note that Walrus and S3 have a few functional differences. You can't, for instance, yet perform virtual hosting of buckets, a feature typically used for serving multiple sites from a single server. This is something that's probably more useful on Amazon's services than in your own cloud, so it's not terribly surprising that Walrus doesn't support it (yet).