84 lines
3.1 KiB
XML
84 lines
3.1 KiB
XML
<?xml version="1.0" encoding="utf-8"?>
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<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0"
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xml:id="storage-concepts">
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<title>Storage concepts</title>
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<para>The OpenStack stack uses the following storage types:</para>
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<table rules="all">
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<caption>Storage types</caption>
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<col width="33%"/>
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<col width="33%"/>
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<col width="33%"/>
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<thead>
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<tr>
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<td>On-instance / ephemeral</td>
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<td>Volumes block storage (Cinder)</td>
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<td>Object Storage (Swift)</td>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>Runs operating systems and provides scratch space</td>
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<td>Used for adding additional persistent storage to a virtual
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machine (VM)</td>
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<td>Used for storing virtual machine images and data</td>
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</tr>
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<tr>
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<td>Persists until VM is terminated</td>
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<td>Persists until deleted</td>
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<td>Persists until deleted</td>
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</tr>
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<tr>
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<td>Access associated with a VM</td>
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<td>Access associated with a VM</td>
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<td>Available from anywhere</td>
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</tr>
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<tr>
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<td>Implemented as a filesystem underlying OpenStack
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Compute</td>
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<td>Mounted via OpenStack Block-Storage controlled protocol
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(for example, iSCSI)</td>
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<td>REST API</td>
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</tr>
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<tr>
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<td>Administrator configures size setting, based on
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flavors</td>
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<td>Sizings based on need</td>
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<td>Easily scalable for future growth</td>
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</tr>
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<tr>
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<td>Example: 10GB first disk, 30GB/core second disk</td>
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<td>Example: 1TB "extra hard drive"</td>
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<td>Example: 10s of TBs of data set storage</td>
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</tr>
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</tbody>
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</table>
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<?hard-pagebreak?>
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<para>Other points of note include: <itemizedlist>
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<listitem>
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<para><emphasis>OpenStack Object Storage is not used like a
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traditional hard drive.</emphasis> Object storage is all
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about relaxing some of the constraints of a POSIX-style file
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system. The access to it is API-based (and the API uses
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http). This is a good idea as if you don't have to provide
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atomic operations (that is, you can rely on eventual
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consistency), you can much more easily scale a storage
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system and avoid a central point of failure.</para>
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</listitem>
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<listitem>
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<para><emphasis>The OpenStack Image Service is used to manage
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the virtual machine images in an OpenStack cluster, not
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store them. </emphasis> Instead, it provides an
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abstraction to different methods for storage - a bridge to
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the storage, not the storage itself.</para>
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</listitem>
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<listitem>
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<para><emphasis>OpenStack Object Storage can function on its
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own.</emphasis> The Object Storage (swift) product can be
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used independently of the Compute (nova) product.</para>
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</listitem>
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</itemizedlist>
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</para>
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</section>
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