Ceph

we need to consider to make a tradeoff between multi-client support and single-client support for librbd. In practice, most of the volumes/images are used by VM, there only exist one client will access/modify image. We can't only want to make shared image possible but make most of use cases bad. So we can add a new flag called "shared" when creating image. If "shared" is false, librbd will maintain a object map for each image.

We can easily find the advantage of this feature:
1. Avoid clone performance problem
2. Make snapshot statistic possible
3. Improve librbd operation performance including read, copy-on-write operation.

See more(https://wiki.ceph.com/Planning/Blueprints/Submissions/librbd%3A_shared_flag%2C_object_map)