Let me just issue a word of caution... about "on the fly". Also, I understand that some applications don't allow for downtime and you must do some things in full production. So this argument may not work in all situations.
Sure you can do many things "on the fly".
For HPUX, JFSOnline has many functions that can be performed while the box is in full production. However, no SA worth his/her salt, me included, will perform these functions while the box is in full production mode. I will find a time that is low utilization or arrange downtime, which I know is not always possible in some environments.
Any time you intend to reduce a filesytem, you are in danger of destroying data. Granted that Veritas does a great job of handling the reduce functionality. My vendor HPUX has warned that reduction with JFSOnline is risky at best, and granted that their tool in less functional than Veritas Volume Manager /Volume manager. They recommend to backup, blowaway the Volume and then rebuild it (without Vertias).
Please backup all data before you attempt this!!
Even with Veritas products, I am sure that if you read the fine print for your version of Veritas Volume manger, there will be a caution for reducing... and a possible recommendation to backup, and recreate the volume group.
Don't misunderstand, I LOVE VERITAS products!!!!!! They are great, BUT just don't take a vendor's word for the functionality of a product because they won't be responsible for restoring the data that may be lost by the product not performing up to it's billing.
Please Just BE CAREFUL!! Okay!!
In the final analysis, Veritas is by far a better tool than JFS (which is nothing more than a cheap imitation of Veritas)