For years, the term backup has been synonymous (at least implicitly) with the phrase tape backup. On most networks and in most data centers, the job of storing copies of data for backup/restore and for disaster recovery has been handled nearly exclusively by tape technologies - drives, cartridges, autoloaders, and libraries. By contrast, hard drives and disk arrays have been reserved for online primary storage, not for backup. But, with disk prices dropping and data growth rates rising, users should consider the performance gains they can realize by making disk technology part of their backup strategies. D2D (disk-to-disk)-based backup processes can help companies reduce downtime while handling the complexity of networked storage environments. Most importantly, the RAID (redundant array of independent disks)-based design of D2D solutions helps ensure that restore and recovery operations aren't compromised by lost data.
D2D Smashes Backup Windows
As part of an overall backup strategy, D2D commonly refers to disk arrays used as intermediate holding tanks (the second "D") for data as it moves from primary online disk storage (the first "D") to offline archival storage on tape. According to tape and D2D vendor Quantum Corp.'s (Milpitas, CA) VP of Worldwide Marketing, Storage Solutions Group, Rob Pickell, the addition of disk to the backup mix addresses throughput concerns often exacerbated by the migration to pooled storage resources. "The medicine for storage management headaches - server consolidation and storage networking - can cause as many problems as the original problem itself," Pickell says. "As the size and complexity of centralized data centers and storage networks has gone up, confidence that backup will be complete has trended downward."
A consequence of growth, both in infrastructure complexity and in stored data, is the increased threat of downtime stemming from long backup windows. In a typical SAN (storage area network), for example, multiple servers share the same tape backup device. This arrangement can increase users' anxieties about backup duration and completion. "Building SANs can have companies worrying about how and when they are going to back up all of these disks that are now connected," says Tim Klein, president and CEO of D2D vendor ATTO Technology, Inc. (Amherst, NY). According to Klein, copying data from disk to disk, at least as an interim step, allows users to more efficiently carve out windows for backing up particular applications and servers. "Companies can back up 100% of their data to disk overnight and later move it to tape," Klein says. "Using disk arrays, a backup job that used to require a 14-hour window can be cut down to as few as 3 hours."
Live Up To Your SLAs
Along with the speed of disk processing, which enables shorter backup windows, backup reliability is another key factor driving D2D adoption. As Pickell puts it, "There is a much greater awareness of the need for data protection policies. The recovery piece is where a company's IT shop will be tested in terms of whether or not it is meeting its SLA [service level agreement] obligations to the rest of the organization."
For Diamond Lauffin, senior executive VP for disk array vendor Nexsan Technologies (Woodland Hills, CA), reliability is even more important than speed. That's because, he acknowledges, some stored data simply requires more secure, timely access than does other data. As Lauffin observes, "Most people are not restoring data from a month ago or a year ago. If the server goes down today, they're restoring yesterday's data. So, by front-ending an existing tape solution with a disk solution, companies can give their most commonly accessed data the five 9s [99.999%] uptime and reliability that RAID-protected disks provide." Lauffin also points out another potential danger of relying solely on tape for backup. "Tape vendors will soon be offering cartridges that can hold 2 TB of data," Lauffin says. "Users need to think about whether or not they want 2 TB of critical data sitting on what is potentially a single point of failure."
Back Up To Disk, Archive To Tape
Of course, the increasingly competitive pricing and performance benefits of disk-based backup solutions beg the question, "Why use tape at all?" Well, most companies can't afford to just toss out all of their tape resources and replace them with disks. And, even if they could, some data probably doesn't warrant being kept and managed in on-site, online disk arrays. Most organizations have a lot of data that is rarely - if ever - accessed yet must be retained for legal or regulatory purposes. "Even companies with hybrid environments, where online or nearline disk arrays play key backup roles, will continue to use tape for what it's really good for - cheap archival storage," says Pickell. "And, despite trends in remote networking for disaster recovery, the cost of moving data over large distances is still expensive. Even for large businesses, it's still substantially cheaper to ship a tape cartridge to another location than it is to send it over telecom lines."