Continuous data protection

Once an acronym becomes popular, altering it — even to better reflect the underlying technology — is difficult business. CDP, the natural evolution of conventional backups, would probably benefit from a name change to something along the lines of "data recovery preparedness". After all, the objective of adopting a CDP solution is to ensure that your enterprise — or selected business processes within — can survive a disruption without data loss.

As many enterprises have been made painfully aware, the traditional backup paradigm has never provided an impenetrable data protection shield. In essence, conventional backup applications take a picture of selected databases or files at recurring intervals, typically each day at business closing time.

The approach, however, has severe limitations, most notably its long intervals between protective copies, which in the event of a disruption, translates to lost data. In today's world of highly interactive applications, a prolonged risk of data loss is fast becoming increasingly unacceptable, which is probably what triggered GlassHouse Technologies CTO James Damoulakis to title a recent BusinessWeek white paper "Best Practices: Are Backups a Waste of Time?"

Backups may not always be a waste of time, but the fact that just about every backup-software vendor has added CDP to its portfolio is probably the most unbiased acknowledgement of the importance of CDP — and the limitations of traditional backup wares. CDP moves beyond backup's limitations by providing virtually infinite recovery points, an enormous improvement that leaves very little or no data at risk.

In the main, CDP solutions take one of two approaches: Either they use a host agent to intercept and replicate every write to disk, or they schedule frequent snapshots to create numerous volume images from which to restart in case of damage.

Less granular though easier to implement, the snapshot approach is worthwhile — and perhaps less burdensome — when full recoverability is not needed. For example, scheduling snapshots every 30 minutes can adequately protect an accounting system. In the event of a disruption, users can easily re-enter the last half hour of transactions after the proper files are restored from the latest volume image.

With more storage systems offering snapshot capabilities, this quasi- or near-CDP snapshot approach is certain to become more popular. Another notable advantage is that the snapshots can be the source of traditional backup operations so that tape copies for vaulting or data exchanges with other parties can be created offline.

However tempting near-CDP may be, it is no substitute for the no-bits-left-behind approach of a true, host-agent CDP solution. Recovery is not as easy with the host-agent approach because it requires applying data changes against a known good copy of the affected file or database. But true CDP makes it possible to bring data back at the very instant preceding the damage, which is quite a departure from the a priori, fingers-crossed decision one has to make when scheduling near-CDP and backups.

The kind of flexibility true CDP offers brings the VCR — or TiVo — rewind capability to mind, making it the ideal data protection safeguard for applications without a safety net, such as e-mail, database updates, word processors, and CAD/CAM. Not surprisingly, a number of lightweight CDP applications on the market protects user files, including what's stored on desktop and laptops. But the heavy lifting of data recovery for many companies revolves around e-mail and database-centred applications.

When considering a CDP solution, however, keep in mind that CDP alone does not provide application recovery. For that, you will need additional software. That said, CDP is an important first step toward ensuring that your data is safe — and that the lifeblood of your business can easily be restored should something damaging transpire.

Data deduplication

Few technologies can claim the quick road to success that data deduplication can. Just four years ago, the technology was proposed by but a few pioneers, largely ignored by major storage vendors. Today it is difficult to single out a vendor that doesn't have a data deduplication slide in its marketing materials.

In hindsight, the quick success of data deduplication is easy to explain: It's the most effective strategy for offsetting a significant portion of the data currently deluging companies. And with some enterprises doubling the amount of data they must manage every year, it's not surprising to see how data deduplication's promise to shrink data capacities by a factor of 20 to 1 would appeal to most.

To achieve that level of capacity reduction, data deduplication technologies use algorithms that essentially replace identical globs of data with pointers to a single instance. Implementations differ in how they apply those algorithms; for example, Sepaton pursues file-based byte-level comparisons, whereas Data Domain looks for equal fragments within files.

Moreover, the size of the fragments replaced can be either fixed or variable, another key differentiator among data deduplication solutions. Avamar, for example, uses a variable-size segment to identify duplicates. According to the vendor, which was recently acquired by EMC, the approach remains effective even when minor changes, such as inserting a single line in a document, could defeat comparisons between fixed-length segments. Despite such comparative claims, however, vendors' declared average deduplication ratios differ very little.

Other differences in how vendors implement the technology can have a more significant impact on the effectiveness of data deduplication in your daily operations. Adding traditional, hardware-implemented compression, for example, can further reduce data capacities by 50 percent — a nontrivial gain that essentially doubles your dedup ratio.

Putting the dedup magic wand to work in line with your backups may seem the smart thing to do, but only if the added overhead doesn't extend your backup windows into business hours. Because of this, some companies may benefit from a more prudent offline, off-band approach to deduplication.

Nevertheless, because of the unprecedented data reduction ratios offered by data deduplication, it has become an indispensable addition to VTLs (virtual tape libraries). In fact, by storing more data per gigabyte, data deduplication narrows the cost gap between tape reel and SATA storage, which makes it economically viable to keep all but the oldest data online.