Back To The Real World We have seen how data can be made immortal by maintaining multiple copies and increasing the number of copies over time. However, in the real world, users do not keep and compare multiple symmetric copies. Instead they make asymmetric copies called backups. There are two reasons for creating asymmetric copies. First, in the real world, copies made for the purpose of protecting data are usually made on non random-access media such as tape.
Copies made on such media can be a lot cheaper and more stable than a rotating hard disk.
However, it's also a lot more inconvenient to read the data. The second reason for using asymmetric copies is that usually some of the data is changing, and it is imperative to designate one copy as the working copy. So, instead of having N symmetric copies that refresh each other, we typically have one working copy and N-1 backup copies that represent snapshots of the working copy at various times in the past. This is a suitable model for the backing up of a changing file system, but is worlds away from the multiple symmetric copy system required for data immortality. The asymmetry of the working copy means that the working copy is effectively acting in a staged broadcasting capacity.