[ceph-users] Question about per MDS journals

Hi. I'm a newbie in CephFS and I have some questions about how per-MDS journals work. In Sage's paper (osdi '06), I read that each MDSs has its own journal and it lazily flushes metadata modifications on OSD cluster. What I'm wondering is that some directory operations like rename work with multiple metadata and It may work on two or more MDSs and their journals, so I think it needs some mechanisms to construct a transaction that works on multiple journals like some distributed transaction mechanisms. Could anybody explains how per-MDS journals work in such directory operations? or recommends some references about it? Thanks. kyujin.