nilfs2 has a 'continuous snapshotting' architecture that the ceph-osd can take advantage of for making fully-consistent checkpoints of state for recovering from a crash. If these checkpoints are efficient enough, in fact, you may even get away with not having a journal file/device at all. Provided ceph can trigger snapshot creation when it wants to, can roll back to a previous snapshot on startup, and gets the basic features it needs out of the fs (mainly xattrs), this might be something worth looking at, if someone is looking for a project!
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