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spaces on the disk, so if that log is inaccessible (or locked) for some reason, then 
attempting to allocate new space will have exactly the impact you're seeing.

If the reboot doesn't fix it (and possibly even if it does) it may make sense to stop 
everything and force a full fsck (even if the OS thinks the partition is clean). Unmount it, 
and then "fsck.jfs -v -f <device> | tee /tmp/fsck_log".  That'll log to a file as well as 
chucking it to the screen, which may be of use to someone that knows JFS well.

One last check (that will probably take quite a while) would be to attempt to read the block 
device using dd - that'll tell you if the problem is with jfs or at a lower level.  Just "dd 
if=<block device> of=/dev/null bs=16M" and leave it running overnight ;)

Graham