[nova][cinder] Volume-backed instance disks and some operations that do not support those yet

Peter Penchev openstack-dev at storpool.com
Mon Nov 11 16:06:21 UTC 2019


First of all, thanks to everyone involved for all the work on Nova, Cinder,
os-brick, and actually all the rest of OpenStack, too! So, yeah, I guess it
is kind of weird that I'm asking this on the list just a couple of days
after the PTG where I could have asked in person, but here goes :)

There seem to still be some quirks with Nova and volume-backed instance
disks; some actions on instances are not allowed, others produce somewhat
weird results. From a quick look at the code it seems to me that currently
these are:
- taking a snapshot of an instance (produces a zero-sized file, no real
data backed up)
- backing an instance up (refuses outright)
- rescuing an instance (refuses outright)
...and maybe there are some that I've missed.

So, possibly stupid question here, but what are the project's plans about
these - is there an intention to implement them at some point, or are there
some very, very hard theoreitcal or practical problems (so something like
"guess not for the present"), or is somebody working on something?

The main reason that I am asking is that we, StorPool, have a
shared-storage Cinder driver, and every now and then a customer comes up
and asks about one or more of these actions. Every now and then we come
back to the idea of writing a vendor-specific Nova image backend, but,
first off, we are not really sure whether we want to do this, and second,
we are not really sure whether it will be accepted upstream. A couple of
years ago people told us "don't do that" and there was some talk about
having an image backend for storage drivers supported by libvirt, but that
effort seems to have stalled.

Of course, we know that in all software projects, including, but certainly
not limited to, the more-or-less volunteer free/libre/open-source projects,
there are many tasks and many demands on the developers so that it is only
natural that not everything is implemented or adapted at once; things
happen, priorities shift, people get redirected, nobody else steps up to
continue - it happens. With this in mind, where do things stand right now,
should we consider writing an image backend, are there other options or

So thanks for reading through my ramblings, I guess, and keep up the great

Best regards,
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