Short answer:  No worries.  

Is it possible to use a mixed (model 3 and model 9) environment for the
TSM DB ? (and reclog and storage pools)

>> Yes, that's fine.  TSM will put your DB anywhere you create the data
sets/volumes for it to use.

Do I have to create a new devclass e.g. DISKmod9 ? Or can I just use our
current DISK devclass (see below) ?

>> No.   (The only difference between a 3390-3 and a 3390-9 is the
number of cylinders on the volume; the track format is the same.)

Can I just continue using the same allocate and format job to take new
disk volumes in production for TSM ?

>> Yes.

Other points to consider ?

>> Traditionally, the main thing to consider when moving from 3390-3 to
3390-9 was performace.  You have 3 times as much data on a 3390-9, so
you run the risk of having contention for access to the volume.  BUT, in
an ESS or DS8100 environment, the volumes are all virtual anyway, and
the performance effect is almost entirely masked by the large cache
sizes IF your system is genned to use PAV's (Parallel Access Volumes).
If you don't have PAV's turned on, Z/OS will not start more than one I/O
operation to the same physical device address (UCB).  Perhaps you should
discuss that with your Z/OS systems programmer before loading all your
TSM data on 3390-9's.  But that's a performance issue, not really a TSM
issue.  

Other than that, TSM won't know or care that you have switched to
3390-9's.

Wanda Prather
"I/O, I/O, It's all about I/O"  -(me)