Re: [CFD] disk format fixing

[Jiro SEKIBA <jir-hfpbi5WX9J54Eiagz67IpQ@public.gmane.org>]

At Mon, 10 May 2010 03:29:22 +0900 (JST),
Ryusuke Konishi wrote:
> 
> 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.

Ah, I got it.  The continuous log is the one to make bisect search possible,
yet roll forwarding can reach the latest log.

I think, roll forwarding may be enough for boot loader,
for it still reach correct log.
And once correctly mounted, super block will be corrected.

Thanks!

regards,

> > > * 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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> 
> 


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Jiro SEKIBA <jir-hfpbi5WX9J54Eiagz67IpQ@public.gmane.org>
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