On Fri, Oct 20, 2006 at 01:37:05PM -0700, Jason Thorpe wrote:
> - Contiguously allocated from within the file system's free space.   
> Pros: simple, fast.  Cons: hard to enable journaling on a pre-existing  
> file system if the free space is heavily fragmented.
> 
> - Allocated from file system free space as contiguously as possible,  
> but no different than any other file.  Pros: easy to enable journaling  
> on pre-existing file systems because it will work regardless of how  
> the free space is laid out.  Cons: have to use BMAP to translate  
> journal offset -> location on disk.
> 

Somebody will know better than me but, if the log area isn't contiguous
I would have thought that it would impact transaction I/O so much that the
benefit of metadata-consistency wouldn't be worth it.

> Also, IMO, the journal should be hooked up to a reserved inode.

Wouldn't this rule out the possibility of adding journaling to a pre-existing
filesystem?