I looked at the info on Apple's support page searching on "journal log". It tells how to enable/disable, plus some useful stuff.
Also, although the info alludes to jounaling a disk, thinking of it as journaling all the filesystems on a certain disk. Filesystems have 2 tipes of information:
(1) Data - your applications stuff
(2) Metadata - time, ownership, permissions, pointers to all disk block containing the data.
During filesystem writes, jounaling places the Metadata in a log file until the data is written to disk.
On my imac running 10.2, #df-k shows my whole disk as 1 filesystem taking-up the entire disk.
As to whether or not to journal:
The whole disk is 1 filesystem. A power disruption while writing to disk could corrupt your filesystem. You cannot mount a corrupted filesystem.
You then run #fsck to repair the filesystem. With jounaling, the journal log gets replayed 7 almost always fixes the problem(s).
Without journaling #fsck must read the disk, and sometimes fixes the problem.
So, from a reliability standpoint we have "almost always" versus "sometimes".
Hey, don't worry, if your filesystem won't #fsck clean, there is a workaround:
use #mkfs/#newfs to rebuild the filesystem, then do a full restore from backup. Couldn't get worse, right?
Remember, on my system, the whole disk is 1 big filesystem.
How can I run #mkfs/#newfs you say? Ah, from your O/S CD!!
As to the performance hit, if Apple's jfs is like Veritas & AIX, only writes get journaled. As a single user host, you will probably not be write-intensive unless you create big streaming files. It isn't so much the performance hit of an additional write, but the real hit is when the journal log fills & the system is waiting for some journal log space to do more writes.
