Experimenting with backupsets.

I understand that TSM treats the backupset as one file, and discovered that
it mounts all the volumes before sending data.

What I don't know is, what is TSM doing when it's mounting the volumes?  Is
it verifying the integrity, or availability of all the volumes?  Is it
cacheing the data?

I actually cancelled my session before it finished reading all the volumes.
So I didn't get any data.  Does it remount the volumes a second time to
send the data?

Thanks

John Johnson