I find to determine the amount of tape needed is not an exact science; however, there are somethings to be aware of to help you get a feel for how many tapes you need.
A few questions you need to do your best to find out:
How much data will be protected?
How many versions will be kept?
What percentage will reclamation be ran?
How long to keep the tapes empty before overwriting?
- aka This allows you to roll back TSM and still have the data on the tapes.
Any collocation going on?
A simple example:
(note LTO4 is 800GB or 1.5625 TB compressed @ 2:1 ratio)
I have a node that will have 1 TB of data to be backed up.
I want to keep at most 31 days or 10 versions.
I have LTO4 tapes, which is 800GB or 1.6TB compressed - I choose compression.
I want a tape 60% empty to reclaim.
1TB (worst case if it changes daily I will hit the 10 versions before 31 days) I will have 10TB including versions. I will need minimum of 7 tapes (1.6TB * 7 = 11.2TB) just for the backup. Now if I want a copy off site then, another 7 tapes. However, don't forget the data will expire... so as the data begins to expire, when do I want to reclaim the tape? 1.6TB that is only 40% full (or 60% reclaimable space) is really .64TB so up the tapes from 7 to 16 tapes for each tape pool.
Now I am at 16 tapes for each pool, 32 in total just for the data needs. Add about 10% to 20% (32* 20% = 6.4 or 7 tapes) for scratch volumes; up to 39 tapes. Now TSM 'reusedelay' as well collocation (I recommend googling those terms if you have questions) will increase the amount of tapes needed as well as other settings I failed to mention (in my defense this is off the top of my head). Math may not be entirely accurate but gives a good idea of what to look for.