[openstack-dev] [nova] Device tagging: rebuild config drive upon instance reboot to refresh metadata on it

Daniel P. Berrange berrange at redhat.com
Mon Feb 20 15:46:43 UTC 2017

On Mon, Feb 20, 2017 at 02:24:12PM +0000, Jeremy Stanley wrote:
> On 2017-02-20 13:38:31 +0000 (+0000), Daniel P. Berrange wrote:
> [...]
> >    Rather than mounting as a filesystem, you can also use NVDIMM directly
> >    as a raw memory block, in which case it can contain whatever data format
> >    you want - not merely a filesystem. With the right design, you could come
> >    up with a format that let you store the role device metadata in a NVDIMM
> >    and be able to update its contents on the fly for the guest during hotplug.
> [...]
> Maybe it's just me, but this begs for a (likely fairly trivial?)
> kernel module exposing that data under /sys or /proc (at least for
> *nix guests).

The data is exposed either as a block device or as a character device
in Linux - which one depends on how the NVDIMM is configured. Once
opening the right device you can simply mmap() the FD and read the
data. So exposing it as a file under sysfs doesn't really buy you
anything better.

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