Nothing happens to existing volumes: they remain as readable and writable as 
they always have been.  TSM *should* have been writing the volumes to the 
maximal capabilities of the tape drive technology; and the tape drive senses 
the format used for partially written volumes so as to continue using them that 
way.

Estimated Capacity has no functional impact on tape usage: it is used to 
compute a Pct Util for reports of volumes that are Filling.  It is common for 
volumes to be written to hold more than the Estimated Capacity number, as the 
are filled until End Of Volume is reached.  (See Technote 1104698.)

    Richard Sims   http://people.bu.edu/rbs

On Jul 7, 2010, at 8:20 AM, Mehdi Salehi wrote:

> Thanks Richard,
> What happens to existing volumes if I change the devclass format to
> compressed? I mean, should TSM update the "estimated capacity" of volumes?
> 
> Regards