From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vyacheslav Dubeyko Subject: Re: [writable snapshots discussion] Does nilfs2 do any in-place writes? Date: Mon, 20 Jan 2014 15:54:09 +0400 Message-ID: <1390218849.3034.121.camel@slavad-ubuntu> References: <1389787304.3913.28.camel@slavad-ubuntu> <20140116.002353.94325733.konishi.ryusuke@lab.ntt.co.jp> <1389866912.2565.44.camel@slavad-ubuntu> <20140118.075519.43661574.konishi.ryusuke@lab.ntt.co.jp> Reply-To: slava-yeENwD64cLxBDgjK7y7TUQ@public.gmane.org Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=dubeyko.com; s=default; h=Mime-Version:Content-Transfer-Encoding:Content-Type:References:In-Reply-To:Date:Cc:To:Reply-To:From:Subject:Message-ID; bh=hIsAFVp7ChkByVYodkwFNvWPq4IZshKOMW+j9DPWY7k=; b=LpL0yEsnznb0XNyOJsk1/ZIdxm2XROIY3iBNuBJJKY4Q1lliFDe7Y5Kb+1PCbHGOy8zYw3obq21+lxHjXUUeeoGGOdFMQySQ/YAdKgrW3q9vbHwMsG3YY4a7qfVZ9JAT; In-Reply-To: <20140118.075519.43661574.konishi.ryusuke-Zyj7fXuS5i5L9jVzuh4AOg@public.gmane.org> Sender: linux-nilfs-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Ryusuke Konishi Cc: Clemens Eisserer , linux-nilfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org 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-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html