[sheepdog] obbject placement

[Corin Langosch]

Hi Yua,

Am 11.02.2015 um 11:44 schrieb Liu Yuan:
> Yes, you are right if we don't consider the virtual nodes added to consistent hashing to mitigate the object migration
> problem. But with virtual nodes in the picture, we can add one more rule to make sure all the data and ec-chunks are
> on the different physical nodes. Thanks Yuan 
Thank you for your reply. I digged some more meanwhile and if I'm correct, you just walking the rb-tree and to avoid
using the same node again, you simply skip nodes already in use. See include/sheep.h#67 (oid_to_vnodes).

Thanks
Corin