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<?xml version="1.0" encoding="UTF-8"?>
<!--This work by Eucalyptus Systems is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. See the accompanying LICENSE file for more information.-->
<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept outputclass="docs one-sidebar sidebar-first" id="using_images">
<title>Using Images</title>
<p>An image is a snapshot of a system's root file system and it provides the
basis for instances. When you run a new virtual machine, you choose a machine image to use
as a template. The new virtual machine is then an instance of that machine image that contains its own copy of everything in the image. The instance keeps running until you stop or terminate it, or until it fails. If an instance fails, you can launch a new one from the same image. You can create multiple instances of a single machine image. Each instance will be independent of the others.</p>
<p>You can use a single image or multiple images, depending on your needs. From a single image,
you can launch different types of instances. An <term id="instancetype">instance type</term> defines what hardware the instance has, including the amount of memory, disk space, and CPU power. </p>
<p>A machine image contains all the information needed to boot instances of your software.
For example, a machine image might contain software to act as an application server or Hadoop
<!--instance-store vs. block-store-->
<note type="note">This section describes instances that are not backed by dynamic block volumes,
or Eucalyptus block storage (EBS) devices. For information about EBS, see <xref
<title>Available Images</title>
<p>Existing machine images available at the <xref
href="" scope="external" format="html">Eucalyptus Machine Images</xref> page.</p>
<p>You might find that an existing machine image meets your needs. However, you might want to
customize a machine image or create a machine image for your own use. For more information about
creating your own machine image, see <xref href="creating_images.dita"/>. </p>
<title>Image Types</title>
<p>Machine images are either backed by persistent storage or backed by instance
store. An image backed by persistent storage means that the root device is a snapshot and appears as a
persistent volume when an instance is launched from the machine image. A machine that is backed by instance store means
that the root device is stored in Walrus and
appears as instance store when an instance is launched from the machine image</p>
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