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Re: WAPBL and backwards compatibility

On Fri, Aug 01, 2008 at 11:03:39AM +0200, Alan Barrett wrote:
> On Fri, 01 Aug 2008, Simon Burge wrote:
> > There's a couple of approaches for dealing with this:
> > 
> > [1] Loudly warn "DON'T DO THIS".  This seems like a bit of a cop-out.
> > 
> > [2] Add a check in the current FFS WAPBL code (and fsck_ffs) to
> >    explicitly not replay the log if the filesystem is marked as clean.
> >    This would be on the assumption that something else has fixed it for
> >    you since you last mounted it with a WAPBL-aware kernel.
> > 
> > [3] Bump the FFS magic number.  [...]
> > 
> > [4] Add the ext2's "Feature Compatibility" functionality.  [...]
> [I added numbers above].
> I think that we should do option 2 in the short term, option 4 if
> anybody has a good design with code, and a bit of option 1 for good
> measure.
> I am not familiar with ext2fs, but I assume that each "feature" under
> option 4 would have flags saying things like "If you don't understand
> this feature then you should {not even try to read, mount the file
> system read only, go ahead and write to the file system provided you set
> the "is dirty" flag, whatever}."

Features are added in one of three sets. The sets are "read/write if you 
don't understand," "read/only if you don't understand," and "don't touch 
if you don't understand."

I like Thor's suggestion of bumping the "level" of the file system. We 
should add a plist containing three arrays of strings. Each string is a 
feature name, and there is an array for each of the buclets above.

Take care,


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