Merge "libvirt: Volume snapshots for network disks"
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specs/juno/libvirt-volume-snap-network-disk.rst
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specs/juno/libvirt-volume-snap-network-disk.rst
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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==========================================
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Volume Snapshots for Network-Backed Disks
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==========================================
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https://blueprints.launchpad.net/nova/+spec/libvirt-volume-snap-network-disk
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Nova currently supports creating and deleting snapshots of file-backed
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Cinder volumes via libvirt's snapshot mechanism. This work extends
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that capability to create and delete snapshots for network-backed disks
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in a similar fashion.
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This enables more complete Cinder volume functionality for deployments using
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qemu network-backed volumes through a mechanism like libgfapi.
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Problem description
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===================
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Nova does not support creating a snapshot via libvirt for a network-backed
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Cinder volume that is attached to an instance. Currently, attempting to
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snapshot a Cinder volume configured this way will result in a failed snapshot
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operation.
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This is important for deployers who use qemu network-backed storage for Cinder
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volumes. (Typically for performance reasons.)
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Proposed change
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===============
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Nova needs to be able to construct a <domainsnapshot> XML entity with
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the required fields to snapshot a network-backed disk via libvirt.
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Nova similarly needs to be able to pass in arguments for libvirt's
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blockCommit and blockRebase operations to delete snapshots for network-backed
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disks. libvirt is adding support for a different style of parameters to the
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blockjob APIs to support this, which allows referencing an existing item in
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the disk snapshot change by index rather than by path name.
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Alternatives
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------------
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There is no alternative for deployers wishing to use Nova-assisted snapshots
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of Cinder-backed storage. Nova must be able to interact with libvirt to
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enable this functionality.
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Data model impact
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-----------------
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None
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REST API impact
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---------------
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This work is used by the os-assisted-volume-snapshots extension APIs with no
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API-level changes.
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Security impact
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---------------
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None
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Notifications impact
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--------------------
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None
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Other end user impact
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---------------------
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End-user impact is that Cinder volume snapshots now work when Nova
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is configured to use libgfapi for the GlusterFS Cinder driver.
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(qemu_allowed_storage_drivers=['gluster'])
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Performance Impact
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------------------
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Deleting (merging) a GlusterFS volume snapshot may be more efficient,
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particularly for simultaneous snapshot deletes for different volumes, as
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this work uses qemu direct storage access (via libgfapi) rather than a
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FUSE-mounted file system.
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No direct performance impact within Nova itself.
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Other deployer impact
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---------------------
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This change is relevant when using the Cinder GlusterFS driver and Nova
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is configured with qemu_allowed_storage_drivers=['gluster'].
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Developer impact
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----------------
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None
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Implementation
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==============
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Assignee(s)
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-----------
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Primary assignee:
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eharney
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Work Items
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----------
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* Support for creating a volume snapshot of a network-backed disk
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* Support for deleting a volume snapshot of a network-backed disk
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- Parse backing chain information from libvirt's domain XML
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- Pass new-style arguments to blockCommit and blockRebase
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Dependencies
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============
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* This functionality depends on libvirt changes which are currently targeted
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for libvirt 1.2.6.
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- The libvirt capability is detected without using the libvirt version.
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* The libvirt changes also require fixes within qemu (targeting 2.1).
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* Currently only relevant for GlusterFS Cinder deployments.
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Testing
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=======
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This should be tested via Tempest volume snapshot test cases. Since it is
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dependent on having a GlusterFS deployment this is not currently tested in
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the gate.
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When third-party CI is enabled for the GlusterFS driver within Cinder, it
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should cover this.
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Documentation Impact
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====================
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None
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References
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==========
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* Required libvirt changes:
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- https://www.redhat.com/archives/libvir-list/2014-June/msg00492.html
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* Required QEMU changes:
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- https://lists.gnu.org/archive/html/qemu-devel/2014-06/msg04058.html
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* Based on work done in
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- https://blueprints.launchpad.net/nova/+spec/qemu-assisted-snapshots
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* Patch series: https://review.openstack.org/#/c/78748/
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