Our tape library is showing its years, and it's time to start moving the mountain required to get it replaced with something. So this afternoon I spent some quality time with google, a spread-sheet, and some oldish quotes from HP. The question I was trying to answer is what's the optimal mix of backup to tape and backup to disk using HP Data Protector. The results were astounding.
Data Protector licenses backup-to-disk capacity by the amount of space consumed in the B2D directories. You have 15TB parked in your backup-to-disk archives, you pay for 15TB of space.
Data Protector has a few licenses for tape libraries. They have costs for each tape drive over 2, another license for libraries with between 61-250 slots, and another license for unlimited slots. There is no license for fibre-attached libraries like BackupExec and others do.
Data Protector does not license per backed up host, which is theoretically a cost savings.
When all is said and done, DP costs about $1.50 per GB in your backup to disk directories. In our case the price is a bit different since we've sunk some of those costs already, but they're pretty close to a buck fiddy per GB for Data Protector licensing alone. I haven't even gotten to physical storage costs yet, this is just licensing.
Going with an HP tape library (easy for me to spec, which is why I put it into the estimates), we can get an LTO4-based tape library that should meet our storage growth needs for the next 5 years. After adding in the needed DP licenses, the total cost per GB (uncompressed, mind) is on the order of $0.10 per GB. Holy buckets!
Calming down some, taking our current backup volume and apportioning the price of largest tape library I estimated over that backup volume and the price rises to $1.01/GB. Which means that as we grow our storage, the price-per-GB drops as less of the infrastructure is being apportioned to each GB. That's a rather shocking difference in price.
Clearly, HP really really wants you to use their de-duplication features for backup-to-disk. Unfortunately for HP, their de-duplication technology has some serious deficiencies when presented with our environment so we can't use it for our largest backup targets.
But to answer the question I started out with, what kind of mix should we have, the answer is pretty clear. As little backup-to-disk space as we can get away with. The stuff has some real benefits, as it allows us to stage backups to disk and then copy to tape during the day. But for long term storage, tape is by far the more cost-effective storage medium. By far.