Hi Ryusuke, On Sat, 2014-01-18 at 07:55 +0900, Ryusuke Konishi wrote: > > Yes, I think too that such suggestion is valuable for NILFS2. But I > > suppose that the problem is more complex. I mean a situation with > > write-able snapshots. If we will have write-able snapshots then it means > > necessity to have independent version of some superblock's fields > > (s_last_cno, s_last_pseg, s_last_seq, s_mtime, s_wtime, s_mnt_count, > > s_state, s_c_interval, s_feature_compat_ro, s_feature_incompat). For > > example, snapshot can be made before xafile creation on a volume and > > write-able snapshot should continue to live without possibility to xattr > > creation, and so on. > > OK, please tell me what do you suppose about the writable snapshot. > > Do you think we should keep multiple branches or concurrently > mountable namespaces on one device ? > When I think about notion of snapshot then I have such understanding. Read-only snapshot is a "frozen" file system state. Writable snapshot is an isolated file system state. So, I started from such understanding in my considerations. And I suppose that when we keep in one super root info about several snapshots then we have "multiple branches" approach. But now we use "concurrently mountable namespaces" approach for read-only snapshots, as far as I can see. So, I think that users like and use "concurrently mountable namespaces" and we should keep and evolve this approach. I didn't think deeply about writable snapshots yet but maybe it will need to modify VFS for multiple writable snapshots support (but I haven't any concrete vision of it). Thereby, maybe, "multiple branches" and "concurrently mountable namespaces" approaches are not contradictory but complimentary. > I prefer to maintain only one super root block per partition even if > we support writable snapshots. Otherwise, I think we should use > multiple partitions to simplify the design. > > I mean keeping multiple branches in one super root block with a DAT > file and a sufile in such a case. Maintaining multiple DAT files and > sufiles on one device seems too complex to me. > I suppose that your vision is right. But, anyway, it needs to elaborate more concrete suggestions for discussion. I simply mentioned about it but I didn't suggest something concrete. And I am not ready right now to suggest several ideas. :) Firstly, I want to finish my consideration about changing of superblocks' in-place update policy. Anyway, I am going to return to discussion about writable snapshots. With the best regards, Vyacheslav Dubeyko. -- To unsubscribe from this list: send the line "unsubscribe linux-nilfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
