There is a decent workaround until more diskpool space arrives. Set the storage 
pool parameter MAXSIZE to a setting that will ensure that these "open files" 
(which must be large and voluminous, based on the fact that they would take up 
almost half of a 42GB diskpool) go directly to tape rather than disk. Also, you 
could consider changing copy serialization so that the TSM server doesn't try 
to resend those "open files" so many times.

To tell the truth, if you have 20GB+ of "open files", consider moving your TSM 
server to version 5.2, which has better handling of "open files". What kind of 
files are they, anyway?

Fundamental flaw? I don't think so. Your situation contains an unfortunate 
combination of circumstances that don't normally in most IT environments.

--

Mark Stapleton (stapleton AT berbee DOT com)