113 lines
6.7 KiB
XML
113 lines
6.7 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"
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version="5.0"
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xml:id="section_block-storage-overview">
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<title>Introduction to the Block Storage service</title>
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<para>The OpenStack Block Storage service provides persistent block storage resources that
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OpenStack Compute instances can consume. This includes secondary attached storage similar to
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the Amazon Elastic Block Storage (EBS) offering. In addition, you can write images to a
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Block Storage device for Compute to use as a bootable persistent instance.</para>
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<para>The Block Storage service differs slightly from the Amazon EBS offering. The Block Storage
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service does not provide a shared storage solution like NFS. With the Block Storage service,
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you can attach a device to only one instance.</para>
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<para>The Block Storage service provides:</para>
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<itemizedlist>
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<listitem>
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<para><systemitem class="service">cinder-api</systemitem>. A WSGI app that authenticates
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and routes requests throughout the Block Storage service. It supports the OpenStack
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APIs only, although there is a translation that can be done through Compute's EC2
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interface, which calls in to the Block Storage client.</para>
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</listitem>
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<listitem>
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<para><systemitem class="service">cinder-scheduler</systemitem>. Schedules and routes
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requests to the appropriate volume service. Depending upon your configuration, this
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may be simple round-robin scheduling to the running volume services, or it can be
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more sophisticated through the use of the Filter Scheduler. The Filter Scheduler is
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the default and enables filters on things like Capacity, Availability Zone, Volume
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Types, and Capabilities as well as custom filters.</para>
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</listitem>
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<listitem>
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<para><systemitem class="service">cinder-volume</systemitem>.
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Manages Block Storage devices, specifically the
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back-end devices themselves.</para>
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</listitem>
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<listitem>
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<para><systemitem class="service">cinder-backup</systemitem>.
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Provides a means to back up a Block Storage volume
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to OpenStack Object Storage (swift).</para>
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</listitem>
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</itemizedlist>
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<para>The Block Storage service contains the following components:</para>
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<itemizedlist>
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<listitem>
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<para><emphasis role="bold">Back-end Storage Devices</emphasis>. The Block Storage
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service requires some form of back-end storage that the service is built on. The
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default implementation is to use LVM on a local volume group named "cinder-volumes."
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In addition to the base driver implementation, the Block Storage service also
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provides the means to add support for other storage devices to be utilized such as
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external Raid Arrays or other storage appliances. These back-end storage devices may
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have custom block sizes when using KVM or QEMU as the hypervisor.</para>
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</listitem>
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<listitem>
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<para><emphasis role="bold">Users and Tenants (Projects)</emphasis>. The Block Storage
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service can be used by many different cloud computing consumers or customers
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(tenants on a shared system), using role-based access assignments. Roles control the
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actions that a user is allowed to perform. In the default configuration, most
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actions do not require a particular role, but this can be configured by the system
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administrator in the appropriate <filename>policy.json</filename> file that
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maintains the rules. A user's access to particular volumes is limited by tenant, but
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the user name and password are assigned per user. Key pairs granting access to a
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volume are enabled per user, but quotas to control resource consumption across
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available hardware resources are per tenant.</para>
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<para>For tenants, quota controls are available to
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limit:</para>
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<itemizedlist>
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<listitem>
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<para>The number of volumes that can be
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created.</para>
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</listitem>
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<listitem>
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<para>The number of snapshots that can be
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created.</para>
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</listitem>
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<listitem>
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<para>The total number of GBs allowed per tenant
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(shared between snapshots and volumes).</para>
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</listitem>
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</itemizedlist>
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<para>You can revise the default quota values with the Block Storage CLI, so the limits
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placed by quotas are editable by admin users.</para>
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</listitem>
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<listitem>
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<para><emphasis role="bold">Volumes, Snapshots, and Backups</emphasis>. The basic
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resources offered by the Block Storage service are volumes and snapshots which are
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derived from volumes and volume backups:</para>
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<itemizedlist>
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<listitem>
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<para><emphasis role="bold">Volumes</emphasis>.
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Allocated block storage resources that can be
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attached to instances as secondary storage or
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they can be used as the root store to boot
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instances. Volumes are persistent R/W block
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storage devices most commonly attached to the
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compute node through iSCSI.</para>
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</listitem>
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<listitem>
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<para><emphasis role="bold">Snapshots</emphasis>. A read-only point in time copy
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of a volume. The snapshot can be created from a volume that is currently in
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use (through the use of <parameter>--force True</parameter>) or in an available
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state. The snapshot can then be used to create a new volume through create
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from snapshot.</para>
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</listitem>
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<listitem>
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<para><emphasis role="bold">Backups</emphasis>. An
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archived copy of a volume currently stored in
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OpenStack Object Storage (swift).</para>
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</listitem>
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</itemizedlist>
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</listitem>
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</itemizedlist>
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</section>
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