openstack-manuals/doc/config-reference/block-storage/section_block-storage-overview.xml

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