However, other variables play a role in this argument, including reliability, scalability, longevity, compliance, security, and usability. Scalability is not a problem for tape. Compliance and security do not seem to be barriers to its adoption and neither is longevity. Modern tape cartridges are expected to have a shelf life of between 15 and 30 years. This is longer than the time required for the vast majority of migrations--the time when data has to be migrated from one piece of media to newer technologies. With tape, this time is between 7 and 10 years, based on the fact that LTO (the most popular form of open system tape) averages a new generation between every two and three years, and the latest LTO generation can read tape two generations back.
In a typical scenario, the oldest tape would have to be migrated before yet another new generation of tape drive was put in place. Contrast this with disk, where the migration would most likely occur on a 3 to 4 year cycle. It is not that disks do not technically have a longer life than 4 years, but economically it typically makes sense to buy new disks (more capacity for the same dollar and less maintenance costs) every 3 to 4 years. Note that you may very well see a 10 year old tape library running with 7 year old tape drives, but you are unlikely to see a disk array that is 7 -- let alone 10 -- years old. (By the way, operating system and application obsolescence that would affect migration times are ignored as they affect both disk and tape equally.)
One key issue has always been tape reliability. Fred Moore, Horison Information Strategies, discussed this subject in his address entitled "Future Predictions on the Role of Tape and Disk Media in the Data Center." Now Fred might well be nicknamed Mr. Tape, but even more importantly he might well be nicknamed Mr. Storage for his logical, solid analysis. For those of you who want more detail, please read "Tape Technology Leaps Forward to the 3rd Era" .
And Fred did not get into the additional approaches that several vendors described to show how they are improving the integrity of tape. Note that the comparison has to be with the tape of today and the disk technology of today, not the tape technology of the previous generation or generations.
The other key issue to consider is usability and the Long Term File System (LTFS) that is available with LTO-5. LTO-5 drives have two partitions: Partition 0 can hold directory structure information and Partition 1 holds content information. LTFS can take advantage of the directory structure information to more effectively manage tape. That is a huge help for managing tape in an active archive environment. LTFS also provides other advantages, such as self-describing tape. The goal is to be able to read files, if necessary, in the future, using XML even if the native application that created the file is no longer available to read it.
