Re: [Ext2-devel] [rfc] Near-constant time directory index for Ext2

[Andreas Dilger]

Ted writes:
> Note that in the long run, the fully comatible version should probably
> have a COMPAT feature flag set so that you're forced to use a new enough
> version of e2fsck.  Otherwise an old e2fsck may end up not noticing
> corruptions in an index block which might cause a new kernel to have
> serious heartburn.

Actually, having a COMPAT flag also helps in other ways:

1) Turning indexing on and off is not a mount option as it currently is
   (or automatically done) so it will quell Linus' fears about priniciple
   of least surprise (i.e. not converting a filesystem without user action).
   A superblock COMPAT flag is more in keeping with other ext2 features.

2) Running a new e2fsck on a COMPAT_INDEX filesystem could create the
   index for existing "large" directories that don't have the BTREE/INDEX
   flag set, so the kernel only ever has to deal with incremental indexing
   after the first block.  The kernel would just do linear access on
   existing multi-block directories until e2fsck is run.

3) Clearing the COMPAT flag would make e2fsck remove the indexes, if the
   user so desires.  I think this would be the behaviour of existing
   e2fsck anyways.

Cheers, Andreas
-- 
Andreas Dilger  \ "If a man ate a pound of pasta and a pound of antipasto,
                 \  would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/               -- Dogbert