Merge "NVMe connector support MD replication spec."
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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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NVMe Connector Support MDRAID replication
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==========================================
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https://blueprints.launchpad.net/cinder/+spec/nvme-of-add-client-raid-1
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NVMe connector replicated volume support via MDRAID.
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Allow OpenStack to use replicated NVMe volumes that are distributed on
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scale-out storage.
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Problem description
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===================
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When consuming block storage, resilience of the block volumes is required.
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This can be achieved on the storage backend with NVMe and iSCSI though the
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target will remain a single point of failure. And with multipathing, the HA
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of paths to a target does not handle volume data replication.
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A storage solution exposing high performance NVMeoF storage would need a
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way to handle volume replication. We propose achieving this by using RAID1
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on the host, since the relative performance impact will be less significant
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in such an environment and will be greatly outweighed by the benefits.
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Use Cases
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=========
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When resiliency is needed for NVMeoF storage.
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Taking advantage of NVMe over a high performance fabric, gain benefits of
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replication on the host while maintaining good performance.
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In this case volume replica failures will have no impact on the consumer,
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and will allow for self healing to take place seamlessly.
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For developers of volume drivers, this will allow support for OpenStack
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to use replicated volumes that they would expose in their storage backend.
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End users operating in this mode will benefit from performance of NVMe with
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added resilience due to volume replication.
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Proposed change
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===============
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Expand the NVMeoF connector in os-brick to be able to take in connection
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information for replicated volumes.
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When `connect_volume` is called with replicated volume information, NVMe
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connect to all replica targets and create a RAID1 over the devices.
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.. image:: https://wiki.openstack.org/w/images/a/ab/Nvme-of-add-client-raid1-detail.png
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:width: 448
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:alt: NVMe client RAID diagram
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Alternatives
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------------
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Replication can also be actively maintained on the backend side, which is
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the way it is commonly done. However, with NVMe on a high performance fabric,
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the benefits of handling replication on the consumer host can outweigh the
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performance costs.
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Adding support for this also increases the type of backends we fully support,
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as not all vendors will chose supporting replication on the backend side.
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And we can support both without any kind of impact on each one.
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Data model impact
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-----------------
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NVMe Connector's `connect_volume` and `disconnect_volume` param
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`connection_properties` can now also hold a `volume_replicas` list, which will
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contain necessary info for connecting to and identifying the NVMe subsystems
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for then doing MDRAID replication over them.
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Each replica dict in `volume_replicas` list must include the normal flat
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connection properties.
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If `volume_replicas` has only one replica, treat it as non-replicated volume.
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If `volume_replicas` is ommitted, use the normal flat connection properties
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for NVMe as in the existing version of this NVMe connector.
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non-replicated volume connection properties::
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{'uuid': '96e25fb4-9f91-4c88-ab59-275fd354777e',
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'nqn': 'nqn.2014-08.org.nvmexpress:uuid:...'
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'portals': [{
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'address': '10.0.0.101',
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'port': '4420',
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'transport': 'tcp'
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}],
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'volume_replicas': None}
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replicated volume connection properties::
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{'volume_replicas': [
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{'uuid': '96e25fb4-9f91-4c88-ab59-275fd354777e',
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'nqn': 'nqn.2014-08.org.nvmexpress:uuid:...'
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'portals': [{
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'address': '10.0.0.101',
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'port': '4420',
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'transport': 'tcp'
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}]},
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{'uuid': '12345fb4-9f91-4c88-ab59-275fd3544321',
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'nqn': 'nqn.2014-08.org.nvmexpress:uuid:...'
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'portals': [{
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'address': '10.0.0.110',
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'port': '4420',
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'transport': 'tcp'
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}]},
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]}
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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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Requires elevated priviliges for managing MDRAID.
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(Current NVMe connector already does sudo executions of nvme cli, so this
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change will just add execution of `mdadm`)
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Active/Active HA 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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Working in this replicated mode will allow for special case scenarios where for
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example a MDRAID array with 4 replicas loses connection to two of the replicas,
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keeps writing data to two remaining ones. Then, after a re-attach from a reboot
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or a migration, for some reason now has access to only the two originally
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lost replicas, and not the two "good" ones, then the re-created MDRAID array
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will have old / bad data.
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The above can be remedied by storage backend awareness of devices going faulty
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in the array. This is enabled by the NVMe monitoring agent, which can recognize
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replicas going faulty in an array and notify the storage backend, which will
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mark these replicas as faulty for the replicated volume.
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Multi attach is not supported for NVMe MDRAID volumes.
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Performance Impact
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------------------
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Replicated volume attachments will be slower (need to build MDRAID array).
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It's a fair tradeoff, slower attachment for more resiliency.
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Other deployer impact
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---------------------
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NVMe and MDRAID and their CLI clients (`nvme` and `mdadm`) need to be
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available on the hosts for NVMe connections and RAID replication respectively.
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Developer impact
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----------------
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Gives option for storage vendors to support replicated NVMeoF volumes via
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their driver.
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To use this feature, volume drivers will need to expose NVMe storage that is
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replicated and provide necessary connection information for it when using this
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feature of the connector.
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This would not affect non-replicated volumes.
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Implementation
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==============
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Assignee(s)
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-----------
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Zohar Mamedov
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zoharm
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Work Items
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----------
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All done in NVMe connector:
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- In `connect_volume` parse connection information for replicated volumes.
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- Connect to NVMeoF targets and identify the devices.
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- Create MD RAID1 array over devices.
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- Return symlink to MDRAID device.
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- `disconnect_volume` destroy the MDRAID.
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- `extend_volume` grow the MDRAID.
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Dependencies
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============
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NVMe and MDRAID and their CLI clients (`nvme` and `mdadm`) need to be
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available on the hosts for NVMe connections and RAID replication respectively.
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Fail gracefully if they are not found.
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Testing
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=======
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In order to properly test this in tempest, programmatic access will be needed
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to the storage backend. For example, to fail one of the drives of a replicated
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volume.
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We could also slide by with just a check of connected NVMe subsystems
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(`nvme list`) and scan of MDRAID arrays (`mdadm -D scan`) to see that multiple
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NVMe devices were connected and a RAID was created.
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In either case tempest will need to be aware that the storage backend is
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configured to use replicated NVMe volumes and only then do these checks.
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Aside from that, running tempest with NVMe replicated volume backend
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will still fully test this functionality, it's just the specific assertions
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(nvme devices and RAID info) that would be different.
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Finally, we will start with unit tests, and have functional tests in os-brick
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as a stretch goal.
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Documentation Impact
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====================
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Document that NVMe connector will now support replicated volumes and that
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connection information of replicas is required from the volume driver to
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support it.
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References
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==========
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Architectural diagram
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https://wiki.openstack.org/wiki/File:Nvme-of-add-client-raid1-detail.png
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