background writer, WAL and snapshot backups

I really like the idea of taking a snapshot backup with postgres, using
either volume manager or hardware splits in a disk array to get a
physical backup.

In other, lesser database systems :) the system is structured to prevent
problems arising from "split block" writes, meaning that though the
database is writing an 8k block at the instant of the split, the
intervening drivers may have written a partial block.   two ways to
avoid this problem, as I understand, are:

1.) freeze all block writes briefly (excluding WAL perhaps).
Q:) I saw a patch going out to do writes in a background process. is
this only to help write blocks before absolutely necessary, or is it to
do *all* writes? if so, this would make adding freeze logic simple

2.) enable logging of entire blocks to the WAL, so recovery can
overwrite any partial blocks caught writing at the instant of split.
Q.) Does the WAL write tuple-level changes or block-level changes.

I've been gradually browsing the code.(WAL at least)  I'm a professional
DBA but amateur C programmer, I'm in over my head trying to sort out
myself.