Re: [CFD] disk format fixing

[Ryusuke Konishi <ryusuke-sG5X7nlA6pw@public.gmane.org>]

On Mon, 10 May 2010 01:45:35 +0900, Jiro SEKIBA wrote:
> > As an alternative, I'm thinking to add a new state flag which
> > indicates segments are allocated physically continuously from
> > the super root to which super blocks point.
> >
> >
> > The aim of this flag is allowing nilfs to find out the latest segment
> > with bisection search.
> > 
> > Envisioned changes are as follows:
> > 
> > * add the new flag (for example, NILFS_INORDER_FS) for sbp->s_state.
> > 
> > * set the flag if a new mount options is specified (for example
> >   "-o bisect-root").
> > 
> > * Do not update super block when the filesystem is unmounted, and keep
> >   the state: s_state.NILFS_INORDER_FS = 1, s_state.NILFS_VALID_FS = 0.
> > 
> > * Stop periodic update of super blocks if the flag is set.
> > 
> > * If s_state.NILFS_VALID_FS = 0 && s_state.NILFS_INORDER_FS = 1 when
> >   the filesystem is mounted, then do bisect search to find out the
> >   latest segment.
> 
> Here are the question, to do bisect search, you need to know the end of
> the segments to divide physically continuous blocks.  Which is the
> end of the block for bisect?  Is this going to be the physical partition end?

The end position is the segment immediately in front of the current
segment since disk space is typically used like a circular buffer.

> > * If a new segment is allocated discontinuously and a new super root
> >   is created, then write out super blocks to catch up the position.
> > 
> > * If GC breaks the series of physically continous segments, then
> >   update super blocks to catch up the latest super root position.
> > 
> > * Add a new option to cleanerd to pass the "-o bisect-root" mode
> >   and let it select the rotational GC algorithm (current default).
> > 
> > * If the "-o bisect-root" option is not specified, then use a
> >   conventional algorithm and drop the NILFS_INORDER_FS flag.
> 
> I prefer option name like "-o async_sb" or "-o no_sync_sb" or so.
> Because users would rather curious how to udpate super block than
> how to find the latest log.

Thanks for the comment.  In a narrow sense, "no_sync_sb" is difficult
in this approach, so "few_sync_sb" or "async_sb" sounds better to me.
Anyway, I agree with your point.
 
> > This doesn't break forward compatibility, because the "next segment
> > chain" is still maintained and older implementations will ignore the
> > NILFS_INORDER_FS flag.  Older implementations and the current grub2
> > module can find the latest super root though they incur penalty for
> > mount time.
> > 
> > Another drawback of this approach is that it depends on garbage
> > collection algorithm, but at least at present, this seems not to
> > matter.
> > 
> > How does that sound?
> 
> It would be much better than scanning whole segments from
> boot loader point of view.

Yes, I believe it doesn't complicate boot loader so much.

Thanks,
Ryusuke Konishi
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