Open Stack

All,

Nikola has been working on some very helpful improvements to the block
device mapping code.  These are being tracked here:

https://blueprints.launchpad.net/nova/+spec/improve-block-device-handling

We have been using these new features during development of our native
image building tool for Nova.  We're slightly uncertain about which
elements of this feature will be landing in Havana and wanted to discuss
on-list.

Our understanding is that the following types of mappings are possible
with code changes targeted for Havana:

1) When using purely cinder volumes, essentially any combination of the
mapping types listed in the feature specification is now possible.

2) Glance images may also be specified in the device mapping structure,
but they will be converted to volumes prior to instance launch.

3) A single glance image may be passed in at launch time, in concert
with any of the mappings above, and will be mapped and otherwise behave
in the same way as it would in the absence of any mappings.


One feature that we do not expect, but would like to see in future
revisions:

4) RFE: Multiple glance images mapped in as arbitrary block devices but not
converted to volumes.  These would instead be temporary/ephemeral copies
of the glance content, ideally with the possibility of caching.  This
would allow us to, among other things, launch an instance with a "blank"
glance root disk image as well as a glance backed version of the
installation ISO image for a given OS.  It would also allow for ISO
backed installations even in environments where cinder is not present. 


Thus far we have encountered the following issues:

1) When mapping a volume as a cdrom device, it is still presented as a
disk device.  This is reported here:

https://bugs.launchpad.net/nova/+bug/1223890

2) When passing in a glance image combined with the v2 block device
mappings, the glance image seems to be ignored - Only the mapped block
devices are present.  Having this work in the way described above would
be very helpful as it would allow us to continue to save the results of
our native installs via glance snapshots.  Without this change, any
installs that require a mapped block device (which means any install
that uses an ISO image) must be done to a volume backed block device.
This, in turn, makes delivering the result as a glance image
significantly more complicated.

Thoughts?

-Ian