Re: WAPBL and backwards compatibility

[Chris Gilbert <chris%dokein.co.uk@localhost>]

Simon Burge wrote:

> There's a couple of approaches for dealing with this:
> 
>  - Loudly warn "DON'T DO THIS".  This seems like a bit of a cop-out.
> 
>  - 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.
> 
>  - Bump the FFS magic number.  This seems like a bit of a sledge hammer
>    approach, but may indeed be the most practical solution.  Someone
>    asked privately about inter-operating system compatibility with
>    FreeBSD and OpenBSD - the magic number change does solve this too.
>    It was also suggested that we could have a tool (tunefs?) that could
>    "downgrade" the magic number if there was a clean (or no?) journal.
> 
>  - Add the ext2's "Feature Compatibility" functionality.  Unfortunately
>    that doesn't buy us backwards compatability with any previous NetBSD
>    version.
> 
> I'm currently of the opinion that the second option above is the best
> fix, but I can certainly see the benefits of a magic number change.

Would changing the magic number break compatibility with 3rd party
bootloaders?  I'm thinking about things like grub, and also hardware
bootloaders, eg shark & cats (and other firmware bootloaders)

Thanks,
Chris