
I get paid to sell enterprise backup software.
The first thing you should consider is not the Data itself but the application which uses the data. How important are these applications to your business? What compliance issues are there? From these you can work out the recovery time objective (how long does it take me to get the application data available) and the recovery point objective (how old is the application data that I can now access). Now these can be anywhere from seconds (ok milliseconds but that's not going to be solved by data protection software, you'll need storage replication and clustering) to a few days for RTO and years for RPO (litgation issues). Consider also that the older the saved data is, the longer the RTO becomes.
So you can define RPO and RTO for all your application data, now you can set about working out which technologies you can use to try to meet these objectives.
Yes, short term RTO and RPO can be best met with various disk based technologies, Continuous Data Protection, Snapshots, backup to disk, VTL, but as many have pointed out Disk arrays are cheap to buy, but they are expensive to run (power and cooling), they break (cheap disks break more often) and do i really want to keep multiple copies what is essentially the same data on another disk array?
Storage Technology companies love to sell you more disks, because they know that you'll have to renew them every 3-5 years and then they'll come up with all sorts of software options for which they'll charge you (probably) per Terabyte, and then there is the maintenance on top.. So they come up with VTL (let's treat a random access device as a serial device and pretend we're a tape library) De-Duplication software ( they nearly always replicate, aha! A second array). Don't get me wrong, there is a place for these (and other) disk based data protection technologies, it's as a first backup destination for nearly all data protection. I'd go as far as to say that de-duplicated disk is a pretty good place for most medium term (around 1 year) fileserver or NAS content. (bear in mind that compressed formats don't de-duplicate well), But I would never recommend that this is the only copy to be kept. Array based Snapshots and Snapshot replication are great for keeping multple backups of databases, but they soon mount up (2% of the size of the database 10 times a day is 100% of the database in 5 days and you've replicated the array anyway so double that again ) if you sell disk storage a great idea, otherwise medium-long term storage on Tape please, so I can move it physically between sites and it costs me very little to store it.
Then there is compliance. Most of these rules are pretty vague but what they boil down to is pretty much always the same. You must be able to provide a particular piece of data from a date sometime in the past and prove that this data has not been altered and possibly who had has access to this data from the time it was written. Some of these compliance issues can be met by backup software, many can only be met by an Archiving Solution specifcally designed for this purpose.
Usually these involve WORM technology and are often Tape based.
Tape in all it's different forms is going to be around for a long, long time. There is simply no alternative method of economically meeting those medium and long term RPOs and RTOs .