Disk-to-disk backup: Buyer beware

Posted on May 01, 2004

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By Michele Hope

Faced with shrinking backup windows, time-intensive, or outdated tape architectures, and increasingly stringent service level agreements (SLAs) that require rapid restores, many IT storage professionals are evaluating disk-based architectures to solve their backup problems.

However, analysts agree that with disk-to-disk (D2D) backup it pays to know what you're getting first and why you think you need it.

According to W. Curtis Preston, an author of books about data protection and president of The Storage Group consulting firm, most IT shops investigating D2D fall into one of two categories: those that are having challenges with their current backup system (e.g., failed backups, write errors, performance problems), and those that already require a "forklift" upgrade or replacement of their outdated backup system.

In the first case, it may be tougher to cost-justify a move to disk-based backup, and disk may not be the right solution to solve the problem. "If you already have a significant investment in tape, you may just want to get a more reliable tape system," says Preston.

Or, you could start smaller by taking D2D for a test drive. In this case, Preston often recommends investing in just a "little bit of disk and a lot of tape," and using disk first for incremental backups.

Dorian Cougias, founder of IT consulting firm Network Frontiers and an author of books about backup, says the decision to use disk as a primary backup medium "should be made with regard to price and frequency of restores...The question, beyond price, is based upon how often you have to restore files from tape," he says.

Although a strong proponent of disk-based backup, The Storage Group's Preston is quick to question the blanket statement that disk is faster than tape. It's not in all cases, he warns. While disk is often faster for random access, such as individual files or directories, performing volume backups and restores from disk versus tape may be a different story.

In any case, once you decide disk-based backup is for you, your choice now turns to which approach works best.

If you are evaluating larger disk-based backup solutions, Preston says you can choose between buying a disk array for use with your existing backup software and file system, or you can buy some type of virtual tape library with disk behind it that emulates tape. Neville Yates, CTO of Diligent Technologies (one of a handful of vendors that offer virtual tape solutions), divides virtual tape options into two types: those that use disk only and those that use disk combined with a physical tape library behind the disks.

Preston categorizes virtual tape offerings a little differently, however, into those that can integrate with a physical tape library and those that can't. He cites Alacritus, ADIC, FalconStor Software, and Copan Systems as examples of the first type; Diligent, Quantum, and Sepaton exemplify the second. In terms of lower price points or innovative designs, he also gives a nod to upcoming disk-based products from vendors such as Spectra Logic. (For more about Spectra Logic's product, see "Users rate D2D backup top priority," p. 1.)

Your ultimate choice may relate to the size and complexity of your backups. For smaller shops, using backup software with its own file system to write directly to a disk array may work fine, Yates says. But he says that this approach can quickly break down in an enterprise backup environment where you need to back up several terabytes of data.

Maintenance efforts involved with multiple file systems could quickly become overwhelming to backup administrators who have to continually allocate disk space to complete the backups. Issues such as disk fragmentation could also pose administration challenges. Many virtual tape products, Yates says, allow capacity planning to be simplified via a shared disk pool versus "islands" of storage. They can also automate other management tasks.

While Network Frontiers' Cougias notes that many companies use network-attached storage (NAS) devices for disk-based backup, he prefers virtual tape libraries due in part to their ease-of-use, built-in security features, and inherent tuning for disk-based backup.

Preston agrees that virtual tape libraries will likely offer faster performance and easier integration than a file system-based approach. But he says users should be prepared to pay more for these benefits.

According to Preston, another hidden cost with virtual tape may come in the form of robotic license fees charged by the backup software vendor. Even though the backup software addresses disk instead of tape, since virtual tape presents itself to the backup application as if it's physical tape, tape license fees may still apply.

Most backup software vendors charge license fees based on either the number of tape drives in use or the number of slots. In either case, Preston advises finding out how the addition of a virtual tape solution will impact software license fees.

According to Diligent's Yates, there are some other questions you should ask when evaluating solutions from either of the two virtual tape camps. One is whether the disk portion of the virtual tape system accommodates disk sharing. If it does, this might allow any unused portions of disk to be utilized elsewhere in a storage area network (SAN) or shared pool of storage. He also thinks users should ask about whether the tape library behind the server is a closed or open store, and how they can manage capacity planning elements associated with it.

Yates maintains that in the event of rare problems with physical tape media, an open store implementation may sometimes break down the catalog information developed by the backup application to track where the backup data resides. "The most fragile link is the link that exists between the database catalog, the database administrator, and the actual bar code of the tape," he says. "The challenge with decoupling data movement from disk to tape is non-trivial."

Yates recommends learning what other tracking mechanisms you'll need to deploy other than cataloging. You should also know the procedures you'll need to follow to restore specific files.

For final advice, The Storage Group's Preston recommends using an independent resource to help evaluate your current backup architecture and look for opportunities to improve on it before you invest your dollars in new acquisitions.

Michele Hope is president of Data Concepts, a technical communications firm specializing in storage and networking technologies. She can be reached at michelehope@earthlink.net.


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