Data protection storage, essentially disk arrays purpose-built for the data protection process, have made their mark typically by offering unique software capabilities like deduplication, compression and replication. These capabilities are the very ones that the data protection software vendors are working on replacing. Reality is that this software intelligence is still going to be needed on the data protection appliance even after most backup applications have added deduplication and replication.
As an example, one of the problems with software-based deduplication is that all the data has to run through that software in order for maximum deduplication efficiency. In almost every data center, there are some, if not many, data protection tasks that are external to the primary data protection application, managed by a separate piece of software. Additionally, many of the backup software applications, even with their deduplication capabilities, are in their relative infancy when it comes to replicating their backup jobs. Many backup software vendors advise that you use the storage system's replication capability along with their deduplication to create the remote vault. Factor in that data protection storage vendors are not standing still; they are adding enhancements that make their deduplication and optimization capabilities more efficient than the software. There is also more intelligence than just deduplication; they add encryption, WORM and other features.
The other component is the hardware itself. Cost is not the only concern with data protection storage; it needs to perform well, scale and maintain data availability. Most of the hardware capabilities that you need in data protection storage are also the result of more software intelligence, but this time provided by the hardware vendor and tuned to make the storage system perform, scale or protect your data better.
Performance is a big concern and is typically the number one motivation for choosing to send backups to disk. Having a disk target that can not keep up with the backup server sending data makes it hard to achieve this goal. Scaling should be a bigger concern. As retention requirements increase, the need to be able to scale storage becomes more critical. Finally, your hardware has to keep your data safe. Data protection storage systems should take steps to verify data not only as it is being written but also for the life of that data by re-scanning periodically it to make sure it is still valid, so it is there when you need it.