nova/releasenotes/notes/avoid_muli_ceph_download-40...

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Nova now has a config option called
``[workarounds]/never_download_image_if_on_rbd`` which helps to
avoid pathological storage behavior with multiple ceph clusters.
Currently, Nova does *not* support multiple ceph clusters
properly, but Glance can be configured with them. If an instance
is booted from an image residing in a ceph cluster other than the
one Nova knows about, it will silently download it from Glance and
re-upload the image to the local ceph privately for that
instance. Unlike the behavior you expect when configuring Nova and
Glance for ceph, Nova will continue to do this over and over for
the same image when subsequent instances are booted, consuming a
large amount of storage unexpectedly. The new workaround option
will cause Nova to refuse to do this download/upload behavior and
instead fail the instance boot. It is simply a stop-gap effort to
allow unsupported deployments with multiple ceph clusters from
silently consuming large amounts of disk space.