zfs code - Aug 2007 - Need info about DMU Transactions & space modification

ZFS Experts,

I am looking for some answers:

1. How does dmu_tx_hold_*() routines calculate amount of space
required to modify? As actual object modification are done after calls
to dmu_tx_hold_*() routines, how does one know the amount of space
required?
2. Are dmu_tx_hold_*() routines act independent of type of object?
3. Is it possible to calculate transaction size or even start a
transaction without knowing the objects to modify?
4. Is it possible to add operations to transaction after calling
dmu_tx_assign()?

Thanks in advance.

Cheers,
-Atul

-- 
Atul Vidwansa
Cluster File Systems Inc.
http://www.clusterfs.com