Backups maintain a permanent or redundant record in case of lost data or disaster--you probably run daily, weekly and monthly backups of data to tape, in full or in increments. You may also maintain and archive your monthly backups both on- and off-site for years, depending on your organization's backup policy. Once near-line data is no longer accessed and becomes obsolete, it's typically sent to offline status and available only from an archived tape backup or in analog format.
In the short run, it makes economic sense to migrate data from online to near-line and then to offline storage--this frees hard-disk space for the data that demands regular and immediate access. In the long run, however, it can hide some of your true storage costs.
Today's magnetic disks have a life expectancy of about three years. That's a good benchmark for planning and budgeting data migration from older disks to newer ones. Tape formats have a life span of about five to seven years if they are stored properly. But if you recycle the old tapes, beware that these tapes may contain records that by law must be retained after this seven-year period if no permanent record exists in analog form.
In that case, you have to copy the data onto new tapes--a costly process. For example, 4-mm tapes that hold 12 GB to 40 GB of data cost $10 to $20 per cartridge. Newer tapes holding 100 GB to 200 GB cost more than $100 per cartridge. You can preserve the old storage hardware and software to access the media even if you upgrade your tape technology, but it's not easy. Once maintenance and support contracts for old equipment expire, you can't renew them. Besides, vendors would rather sell you new equipment than maintain their old stuff.
Another option is to migrate data from older tape formats to newer ones, or copy it to optical discs. Storage media vendors claim that 5.25-inch or 12-inch optical disc media will last 100 years if stored properly. But this media will evolve, too, requiring hardware and software upgrades someday.