Add API to os-brick to allow extending an attached volume

Add API to os-brick to allow extending an attached volume

https://blueprints.launchpad.net/cinder/+spec/brick-extend-attached-volume

Add extend_volume API to os-brick connector objects to allow updating the host kernel size information for an attached volume.

Problem description

Currently, Cinder has the ability to extend volumes that aren’t attached. Unfortunately, this doens’t work for attached volumes, due to the host kernel that the volume is attached to, won’t see the new size unless actions are taken on the host. The linux kernel doesn’t automatically see the size changing on the remote storage server. This spec outlines the idea of adding those required actions on the host server when an attached volume is extended in size.

Use Cases

An OpenStack user has a Cinder volume attached to a host/VM and they want to extend/grow the volume while it’s attached.

Proposed change

Add a new os-brick Connector api called “extend_volume” that does the work on the host to get the volume size updated. This will also have to take into account the raw device and the multipath device, if available. It will return the size of the volume after the resize is complete or None if it failed.

This spec doesn’t cover the work needed by Cinder and Nova to make the solution complete. This spec simply covers the work needed by os-brick, to ensure the volume that’s attached to the nova compute host can update it’s host kernel to see the volume’s new size. Cinder will have to change, to allow attached volumes to be extended, and when complete call a new Nova API to initiate the work on the compute host. Then Nova will have to issue a blockresize in virt to get the VM’s virtual device resized. For the volume types such as RBD, where Nova doesn’t use os-brick as the library for managing the volume, Nova will have to do the work of issuing the commands to extend the volume itself.

Alternatives

Unfortunately, if a volume is attached to a VM, there is nothing an end user can do, because the Nova compute host kernel has to see the changes before the VM can see the new size.

Data model impact

None

REST API impact

None

Security impact

None

Notifications impact

None

Other end user impact

The end user, that owns the VM will may have to issue a filesystem resize to get the native filesystem on the volume to recognize the new volume size. For ext* style filesystems, users can use resize2fs.

Performance Impact

None

Other deployer impact

None

Developer impact

None

Implementation

Assignee(s)

Primary assignee:
walter-boring

Work Items

Add the new API to the base Connector class and then add the common required code to the linuxscsi.py to do the real work.

Dependencies

  • In order for this to be used by the OpenStack end user, Cinder will have to optionally allow for attached volumes to be extended.
  • There will also need to be some Nova work done to initiate the call into os-brick’s new API to do the work, and then notify the VM after it’s successful.

Testing

Until the end to end Cinder -> Nova capability is in place, there isn’t a way to automate the testing of this. For now, it’s a manual process.

Documentation Impact

What is the impact on the docs team of this change? Some changes might require donating resources to the docs team to have the documentation updated. Don’t repeat details discussed above, but please reference them here.

References

A related cinder-spec that has been up for a while here:

Related information on how to manually do the work of notifying the host kernel:

Creative Commons Attribution 3.0 License

Except where otherwise noted, this document is licensed under Creative Commons Attribution 3.0 License. See all OpenStack Legal Documents.

cinder-specs 0.0.1.dev432