Tiered storage: new strategies match new demands and opportunities

Computer Technology Review, March, 2004 by Kevin Honeycutt

Integrated Disk-Based Backup Solutions

Another very practical use of multi-resource storage environments is the creation of specialized solutions that present themselves to applications as a single resource, but that have a range of characteristics that no single storage resource can provide. A perfect example is the idea of including disk as part of the backup process.

With the advent of lower-cost ATA disk systems, the idea of including disk within the backup process has become more appealing. Disk offers random access to data blocks, allowing it to handle some kinds of write and read operations much more efficiently than tape. And it can be organized easily into highly resilient redundant arrays so that data can remain readily available even through a fault condition that affects a single spindle.

But, for a variety of reasons, backup still needs tape for the vast majority of end-user applications and environments. Tape drives handle large data streams very effectively, and backup applications have mature tape library interfaces. Tape is a static medium that is unaffected by viruses, and it provides the lowest cost bulk storage available--much less expensive than even the lowest cost disk. And tape is a wonderful medium for long-term storage: durable, stable, easy to transport and store.

Various systems have been offered to give backup the characteristics of both media types, but most of them have created new problems and increased the integration and management burden for IT departments. What they share in common is that they write data to a single disk resource first, and then operate a separate process to move the data a second time to a tape resource. This two-stage operation has many problems. It doubles the amount of data movement that the backup servers perform, so it degrades their other operations, clogs networks, and increases the management burden on IT departments and integrators. It also may increase licensing costs for backup software, since most of the backup software that first writes to disk and later to tape requires separate licensing for the disk and tape resources. And of course it makes management of the task much more complex, leading to confusion when it comes to providing integration and support services.

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Using a unified storage environment with managed data migration to combine disk and tape offers a much more elegant way to create a better backup solution. ADIC's Pathlight VX is a good example. The Pathlight VX looks like a single resource--a tape library--to backup software but, in reality, it creates an integrated disk-tape storage environment with the characteristics of both media types. When backup jobs are first written, the data is written on a RAID-5 ATA array that presents itself to the backup application as if it were a tape library with multiple drives and media. Then, in the background and off-line, the Pathlight VX system writes the data to real media in a real tape library and exports the tapes for long-term storage triggered by commands issued by the backup software. All data movement is carried out by data movers within the Pathlight VX system, so there is no negative impact on production servers, and the data movement occurs completely off-line so there is no increase in network traffic. Most importantly, the system integrates the functions so there is one management interface for all operations--the conventional backup application software that users currently use.