This is actually a VxVM question. I have a volume/filesystem spread over 4*146G disks. Now I want to shrink the filesystem - which I can do using vxresize. However, I want to shrink so that two of the four disks that the filesystem occupies are removed from the volume. Can I do that?

Yes, you can.

Assuming disks called disk1, disk2, disk3, disk4, volume "vol" and diskgroup "mydg" , and that you want to free disks 3 and 4.

So I should run vxresize to resize the filesystem to, say, 200G and then run vxassist? I just want to know exactly what I am going to have to do, because there is a netbackup master server running on that filesystem and obviously we do not want any data loss...

Yes, that's exactly it.

Do the resize to make sure you are under the capacity of two of the four disks, then the vxassist. I have had to do this recently when someone decided to grow a volume on a systems where the second plex in a mirror was detached and disassociated and a someone else decided to grow the volume and allocated all of the disks is the diskgroup as available. As a result an additional subdisks were allocated on on of the disks that should only have been assigned to the mirror plex.

I'll see if I have a machine handy and I'll a screen dump of this scenario, and the exact sequence.