If I understand you correctly, the following are the Commvault components you need to deploy in the exact order outlined below:
Step 1: Setup a single Disk Library to store deduped data on disk:
This disk library will serve as the designated target disk library for the Global Dedupe Policy you will deploy in step 3.
Step 2: Setup a single Tape Library :
This tape library will serve as the designated Tape library for the Secondary Policy Copy you will create in step 5.
Step 3: Create a Global Deduplication Policy:
a) As you follow the wizard, select the Disk Library you created in Step 1 as the target library.
b) For SP18 deployments, once you have created the GDP, you will notice in the Commcell Console under 'Deduplication Engines' that 3 DDBs have been created. This "Horizontal DDB Scaling" feature is ON by default in this service pack.
Step 4: Create a new Storage Policy:
a) As you follow the wizard, enable deduplication.
b) Select the GDP you created in Step 3
c) After the Storage Policy creation is complete, you should be able to see a single Storage Policy Copy, "Primary Copy", under the new Storage Policy Copy you created. This is the default policy copy in which ALL associated subclients will write their backups to first.
Step 5: Create a new Policy Copy (to hold data to be written to tape) under the new Storage Policy you created:
a) Do not enable Deduplication for this copy, leave the box unticked
b) Select the tape library you created in Step 2 as the target library
c) Select the desired Drive pool and scratch pool for this Tape copy
Step 6: Associate your subclients to the Storage Policy you created. Do not start a backup just yet.
Step 7: If you do not wish for some of your client data to be written to tape, simply do the following:
a) Find the Storage Policy you created, right click on the Tape Policy Copy created, go to Properties
b) Under the 'Associations', deselect the subclients you do not wish to be copied to tape.
Step 8: Setup a backup schedule for your subclients.
As for the GDP you wish to delete, simply go to old Storage Policy(s) you created that are associated with the old GDP, right click the storage policy copy, view jobs, select all jobs that are there, right click and select 'delete jobs'.
Run a 'data aging' job, right click Commserve icon>>All tasks>Data aging>run immediate job
Under each DDB>>right click>>all tasks>>Validate and Prune Aged Data
In about 1hr or so, the old DDBs should just dissapear.