Re: WAPBL and backwards compatibility

[Alan Barrett <apb%cequrux.com@localhost>]

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}."

--apb (Alan Barrett)