are redo records always flushed in order?

Dear Gurus,

Could you please help me with my questions below?
if TX1 starts and commits (no matter what mode: immediate, wait, batch, nowait). after that TX2 starts and commits (whatever mode). are TX1's redo records (including the commit record) always flushed no later than TX2's? is there ANY chance that TX2's records (including the commit record) would be flushed first? i can't think of any but i could be missing something.
a related question: (maybe i'm worrying too much) is redo write always atomic? when single redo write size is bigger than max OS I/O write size, 1 redo write may take several physical writes to finish. if there's instance crash in the middle of the several physical writes, is the db able to discard the already written to disk commit records and roll back ALL transactions associated with this redo write? if yes, how does the db achieve that? using on-disk RBA?
would the zero-copy redo and private redo strands features introduce new issue on this matter?
the DB is 10gR2 (since i mentioned the different commit methods), OS are mainly hp-ux 11, solaris 10, windows xp. 

thanks a lot!

Jessica Mao
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