Open Stack

Folks,

Currently in Havana development, RBD as ephemeral storage has serious
stability
and performance issues that makes the Ceph cluster a bottleneck for using an
image as a source.

Nova has to currently communicate with the external service Glance, which
has
to talk to the separate Ceph storage backend to fetch path information. The
entire image is then downloaded to local disk, and then imported from local
disk to RBD. This leaves a stability concern, especially with large images
for
the instance to be successfully created, due to unnecessary data pulling and
pushing for solutions like RBD.

Due to the fact we have to do a import from local disk to RBD, this can make
performance even slower than a normal backend filesystem since the import is
single threaded.

This can be eliminated by instead having Nova's RBD image backend utility
communicate directly with the Ceph backend to do a copy-on-write of the
image.
Not only does this greatly improve stability, but performance is drastically
improved by not having to do a full copy of the image. A lot of the code to
make this happen came from the RBD Cinder driver which has been stable and
merged for quite a while.

Bug: https://code.launchpad.net/bugs/1226351
Patch: https://review.openstack.org/#/c/46879/1

Thanks,
Mike Perez
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