Re: [writable snapshots discussion] Does nilfs2 do any in-place writes?

[Vyacheslav Dubeyko <slava-yeENwD64cLxBDgjK7y7TUQ@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.


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