Hi - disk-disk is 'better & faster' but it is also much more $, depending on defintion of backed up.
Things to consider:
1) with 120gb of source & 181 gb of backup, at a 2:1 compression, you only get about 3 backups in 181 gb (181 / (120 / 2) )That is only 3 day, 3 wk, 3 mth, depending on how often you backup. Want longer backup, another 181gb of disk, new server, expansion cabinet, etc.
2) disk-disk assumes that your backup disk will NEVER (and I mean NEVER) crash, or at least not when you need it to do a restore. With today's disks, maybe, I wouldn't be willing to take that chance
3) ask yourself, is there any situation where, 4 day, 4 wk, 4 mth, 4 yr, where for some valid business reason, a file 4 day, 4 wk, etc. old needsrestored. If answer is no, fine with disk-disk that holds 3 day, wk, etc. If not, when that occurs someone is going to ask you "Who's stupid idea was it to do disk-disk for only N periods?". If you are able to say, "My Boss" and still be working,they will then ask, "Did you discuss the ramifications of such a backup strategy?"
The real benefit of tape is large capacity backup for 'reasonable' $ & time. A really good DLT that holds 70+gb is about $2,500 drive and $75 for 1 tape. Calculate the disk-disk for this backup (we do similar at work wiht 2 35gb (os, applications, user files, etc.)and 1 20gb (database):
Daily: 45 days, 3 tapes each, 135
Weekly (done instead of daily): 18 weeks, 3 tapes each, 54
Monthly (done instead of daily): 6 months, 3 tapes each, 18.
Cost: 135 + 54 + 18 = 207 at $75 = $ 15,525, 2 tape drives at $2,500 each, $ 5,000, Total: 20,525 for backup that will give some level of recovery for 1.5 year. There are other costs in this, storage, aintenanceand replacement on drives, scraping tapes, etc. I don't know of any means that gives a good a backup for a reasonable price.
