547a3fa4fa
Just move implemented specs to the right place. Change-Id: Ia09ed6aceb9e9874e07c740d2fe94ce1b88af9f7
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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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Quiescing filesystems with QEMU guest agent during image snapshotting
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=====================================================================
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https://blueprints.launchpad.net/nova/+spec/quiesced-image-snapshots-with-qemu-guest-agent
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When QEMU Guest Agent is installed in a kvm instance, we can request the
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instance to freeze filesystems via libvirt during snapshotting to make the
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snapshot consistent.
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Problem description
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===================
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Currently we need to quiesce filesystems (fsfreeze) manually before
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snapshotting an image of active instances to create consistent backups.
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This should be automated when QEMU Guest Agent is enabled.
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(Quiescing on cinder's create-snapshot API is covered by another proposal [1]_)
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Use Cases
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---------
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With this feature, users can create a snapshot image with consistent
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file systems state while the instances are running (fsck will not run when
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the snapshot image is booted).
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It will be nice when:
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* taking a quick backup before installing or upgrading softwares.
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* automatically taking backup images every night.
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Project Priority
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----------------
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None
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Proposed change
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===============
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When QEMU Guest Agent is enabled in an instance, Nova-compute libvirt driver
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will request the agent to freeze the filesystems (and applications if
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fsfreeze-hook is installed) before taking snapshot of the image.
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For boot-from-volume instances, Nova will call Cinder's snapshot-create API
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for every volume attached after quiescing an instance. To avoid double
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quiescing, Nova should tell Cinder not to quiesce the instance on snapshot.
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For this purpose, 'quiesce=True|False' parameter will be added to
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Cinder's snapshot-create API.
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After taking snapshots, the driver will request the agent to thaw the
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filesystems.
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The prerequisites of this feature are:
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1. the hypervisor is 'qemu' or 'kvm'
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2. libvirt >= 1.2.5 (which has fsFreeze/fsThaw API) is installed in the
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hypervisor
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3. 'hw_qemu_guest_agent=yes' property and 'hw_require_fsfreeze=yes' property
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is set on the image metadata,
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and QEMU Guest Agent is installed and enabled in the instance
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When quiesce is failed even though these conditions are satisfied
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(e.g. the agent is not responding), snapshotting may fail by exception
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not to get inconsistent snapshots.
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Alternatives
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------------
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Rewrite nova's snapshotting with libvirt's domain.createSnapshot API with
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VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE flag, although it will change the current
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naming scheme of disk images. In addition, it cannot be leveraged to implement
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live snapshot of cinder volumes.
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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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None
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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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None
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Performance Impact
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------------------
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While taking snapshots, disk writes from the instance are blocked.
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Other deployer impact
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---------------------
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None
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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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tsekiyama
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Work Items
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----------
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1. Implement the automatic quiesce during snapshotting when it is available.
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2. Add a quiesced snapshotting scenario test with libvirt >= 1.2.5
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(Fedora experimental queue will be a good place to start testing.)
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Dependencies
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============
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None
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Testing
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=======
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Live snapshotting with an image with qemu-guest-agent should be added to
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tempest.
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Note that it requires environment with libvirt >= 1.2.5, so it would be
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Fedora experimental queue job with virt-preview repository enabled.
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Documentation Impact
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====================
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Need to document how to use this feature in the operation guide (which
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currently recommends you use the fsfreeze tool manually).
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References
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==========
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.. [1] Quiesce admin action for consistent snapshot:
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https://review.openstack.org/#/c/128112/
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