AIX does this. There is one journal LV (logical volume) for each VG
(volume group). This (rather than one journal per whole system) is
because the VG is intended to be the unit of portability, i.e. you can
eject a ZIP disk with a VG on it and the VG on it will work elsewhere.
AIX's JFS is restricted to the LV/VG universe (it starts calling you
nasty names if you try to create one on just any old block device), so
this scheme works well.
I don't think AIX makes any effort to load-balance the log between
physical disks. It can't -- the size of the journal LV is the minimum.
I agree that for load-balancing reasons it would be nice if ext3 moved
in the direction of supporting journal-outside-fs and even a shared
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