Neil Brown wrote:

quoted

while I consider zfs to be ~80% hype, one advantage it could have (but I 
don't know if it has) is that since the filesystem an raid are integrated 
into one layer they can optimize the case where files are being written 
onto unallocated space and instead of reading blocks from disk to 
calculate the parity they could just put zeros in the unallocated space, 
potentially speeding up the system by reducing the amount of disk I/O.
    

Certainly.  But the raid doesn't need to be tightly integrated
into the filesystem to achieve this.  The filesystem need only know
the geometry of the RAID and when it comes to write, it tries to write
full stripes at a time.  If that means writing some extra blocks full
of zeros, it can try to do that.  This would require a little bit
better communication between filesystem and raid, but not much.  If
anyone has a filesystem that they want to be able to talk to raid
better, they need only ask...
  

Some things are not achievable with block-level raid.  For example, with
redundancy integrated into the filesystem, you can have three copies for
metadata, two copies for small files, and parity blocks for large files,
effectively using different raid levels for different types of data on
the same filesystem.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.