Hallo,

not even as  strange idea as you think.
In fact, this configuration is recommended for backing-up directories
since years,
so this is only an uncommon idea expansion.

I am just going to implement this for file server backup,
but not for database backups.

Given the high price tag for library slots and unfriendly price for TSM
library licenseces
(you do can start with library cheaper than corresponding tsm library
license!!)
as in contrast to falling prices for raid subsystems I find
this scenario to be only naturall.

I still will setup backup storage pool on tapes, but not in library,
only on 2 single tape drives. No collocation, so I only hope
for 1-2 tape mounts per week, plus daily tsm-database backups.
Tape library will be only used for databases and some special data,
like picture library from our PR department, so I can keep the library
small
an relatively cheap.

Do calculate with correct Raid costs, inkluding your laboring, expensive

housing and cabling, external support contracts.

Do think about practicability of large raid sets - how long will it take
to rebild a large raid, how long to format or fsck/chkdsk it.

No known limits about size of diskpools - you can use many diskpool
files to create one large diskpool.

In one point I am not with you : of raid is by far not as  simple as
expansion
of tape pool - in tape pool you simply define additional tapes.
Well, you can end with too many tape checkouts, but you will have the
space.

You forgot one advantage: 100% usage of storage space (correlates to "no
reclamation")

Add tape pool with manual tape library (library in TSM sense, no
necessary a real tape robotic system)
and you will have both overflow space if your disk pool should grow too
quickly
and system for tsm database backup and system for backup of your primary
backup.


best regards
juraj salak