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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