39 lines
1.7 KiB
XML
39 lines
1.7 KiB
XML
<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"
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version="5.0"
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xml:id="section_configure-backing-storage">
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<title>Configure Compute backing storage</title>
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<para>
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Backing Storage is the storage used to provide the expanded
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operating system image, and any ephemeral storage. Inside the
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virtual machine, this is normally presented as two virtual hard
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disks (for example, <filename>/dev/vda</filename> and
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<filename>/dev/vdb</filename> respectively). However, inside
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OpenStack, this can be derived from one of three methods: LVM,
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QCOW or RAW, chosen using the <literal>images_type</literal>
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option in <filename>nova.conf</filename> on the compute node.
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</para>
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<para>
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QCOW is the default backing store. It uses a copy-on-write
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philosophy to delay allocation of storage until it is actually
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needed. This means that the space required for the backing of an
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image can be significantly less on the real disk than what seems
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available in the virtual machine operating system.
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</para>
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<para>
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RAW creates files without any sort of file formatting, effectively
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creating files with the plain binary one would normally see on a
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real disk. This can increase performance, but means that the
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entire size of the virtual disk is reserved on the physical disk.
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</para>
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<para>
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Local <link
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xlink:href="http://en.wikipedia.org/wiki/Logical_Volume_Manager_(Linux)"
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>LVM volumes</link> can also be used. Set
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<literal>images_volume_group = nova_local</literal> where
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<literal>nova_local</literal> is the name of the LVM group you
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have created.
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</para>
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</section>
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