Impossible right now. While the on-disk format is the same, the
ext2 and ext3 drivers are totally different.
It's like asking if you can switch between IDE and SCSI drivers
automatically for a disk when you go on battery.
That said, it would be possible to update the ext3 code to allow it
to mount a filesystem without a journal by updating the journal
wrappers in include/linux/ext3_jbd.h.
Even so, the reasoning you give is a bad one - you are far safer
with ext3 on your laptop when it is running out of batteries,
because you are likely to run out of battery without unmounting
the filesystem cleanly, and that's just when you want ext3.
You should just change the flush parameters to be longer, or
figure out exactly why ext3 is writing stuff to disk when there
is no real I/O happening.
-- Andreas Dilger http://www-mddsp.enel.ucalgary.ca/People/adilger/ http://sourceforge.net/projects/ext2resize/
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