This deals with the 'original' Hammer, this is way above my head, and I don't know how much applies to either OS or the HAMMER2 so this is just me posting another link but:
"There are several major differences:
* All VOP operations related to the namecache... those using named
path elements like lookup, rename, etc... those are completely
different.
The biggest one is that our VOP_NRESOLVE() API is completely different
from the old VOP_LOOKUP() API. The old VOP_LOOKUP() API had to handle
numerous side effects and I don't recall the FreeBSD namecache
subsystem doing any namespace locking (beyond locking vnodes, that
is).
Also our VOP_NREMOVE() is fairly radically different... it's an all
in one call whereas the older VOP_REMOVE was paired with a VOP_LOOKUP
that stored side-effects (not sure if FreeBSD has cleaned that up yet).
i.e. old was LOOKUP(save side effects) + REMOVE(use side effects).
And DragonFly's is just NREMOVE().
* HAMMER uses a far more sophisticated buffer cache call-back interface
which allows HAMMER to veto write requests from the buf_daemon's (to
enforce certain write ordering requirements).
* DragonFly uses a locking model that I don't think FreeBSD has an API
for (our token locking model which releases held locks within the
model when a thread switches out and reacquires them when the thread
switches back in). If FreeBSD also has an API for this model this
would be easy to port (if not for all of the above issues).
* DragonFly uses a slightly different stacked BUF/BIO mechanic for I/O.
This would be easy to port (if not for all of the above issues).
* DragonFly's BUF/BIO allows for different buffer sizes within the
file buffers (not just the device buffers). Generally speaking
there can be significant mixing of buffer sizes and the cluster code
has to be able to deal with it given appropriate argumetns. I don't
think FreeBSD's cluster code could handle it.
Personally I think it might be too much for a GSOC project.
-Matt"
"One does not simply own/maintain a new filesystem".
Also, I wonder how much of HAMMER2 is tailored towards the DF-BSD kernel?