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Disk libraries and tape libraries: aiming for peaceful coexistence.

Tape libraries have been the primary technology deployed in traditional backup and restore environments. Until recently, they have offered an overwhelming cost advantage over disk. Customers that require the highest recovery service levels use disk-to-disk data replication, but this requires changing their traditional backup processes to implement the technology. Today, a new disk library category is evolving from these two traditional solutions. While not as fast as disk-to-disk replication, disk libraries integrate with existing processes and provide better service levels than tape.

Disk-based backup and recovery is gaining attention because of changes in both the business and the IT environment. As the volume of data organizations must backup and recover continues to expand exponentially, the window in which applications can be taken off line to do the backup stays the same or shrinks. Business managers want their data restored and their applications recovered more quickly than ever. For these reasons, the old tape-based backup and restore operations no longer meet today's business requirements. This paves the way for a new, disk-based paradigm for backup and recovery.


Some customers have avoided disk-to-disk backup and restore not only because disk storage hardware costs more than tape hardware, but also because they would need to change their backup processes and infrastructure. Disk libraries now offer businesses the opportunity to experience the benefits of disk-based backup at lower cost and with less effort than ever before. These libraries can be "dropped into" existing environments without upgrading backup software, retraining staff, or changing existing backup and business processes. Disk libraries also allow operations managers to provide faster, more reliable backup and restore for critical business applications--without the costs of changing their environment or processes.

However, disk libraries will not necessarily mean the end of tape. Tape's portability assures it a place within the information lifecycle--as the preferred media for long-term, off-site storage of data. To understand how the two technologies will work together, consider their respective advantages and disadvantages.

Tape vs. Disk in Traditional Data Centers

Tape has had two traditional recovery roles in most enterprises. One is for long-term, off-site storage of data that is unlikely to be accessed often (or ever again). The second role is to backup and restore applications, so that applications can be recovered if a system failure or disaster occurs.

As an off-site storage medium, tape is still the media of choice because it is the least expensive to purchase on a permegabyte basis and is the easiest to transport off site. Most medium to large organizations already have tape libraries, which can store and transfer data to and from hundreds or thousands of tape cartridges.

The use of new, lower-cost ATA drives is driving down the cost of disk-based backup and restoration of critical data. ATA is the enabling technology making disk-based backup and restore more affordable, because ATA drives cost roughly 50% less than the Fibre Channel drives used in more demanding, online transaction processing applications. Disk libraries consist of arrays of inexpensive, ATA disk drives linked by high-performance, redundant storage controllers. Because these same disk libraries emulate the software used to manage existing tape libraries, the backup application thinks it is still writing to tape, while actually writing to disk. This makes the migration from tape to disk libraries seamless.

The advantages of disk libraries include:

Higher sequential performance: Tape drives based on the Linear Tape-Open (LTO) 2 format transfer sequential data at approximately 25MB/sec in real-world environments. By comparison, a high-performance disk library may move as much as 400MB/sec, allowing a single disk library to offer the sequential performance of nearly 16 high-performance tape drives. In normal backup environments, high-performance disk libraries should improve backup performance roughly 30-60% over existing tape solutions.

Faster random-access performance: Disk libraries also shine when it comes to restore performance, which involves accessing data stored in many scattered locations on a storage media. Disk libraries deliver incremental data restores in one-tenth the time of a tape library because there is no need to load tape cartridges and rewind tape, in order to find the required data. Disk libraries leverage the random-access nature of disk compared with the sequential access nature of tape. Tape libraries require mounting the physical tapes during restore operations. In a disk library, all the volumes are online and immediately accessible.

Higher availability: Because disk libraries are built around storage controller technology, they can be fully redundant and use RAID protection against media failure.

Higher utilization of media: Tape cartridges come in fixed sizes, which do not always correspond to the amount of data an organization needs to backup at a specific time. As a result, companies must purchase, handle, and store many tape cartridges that are only partly used, which increases backup and restore costs. "Stacking" volumes on the same tape can increase utilization, but requires so much effort and overhead that many organizations simply choose to live with low media utilization. However, intelligent disk libraries allocate capacity as needed, allowing near-100% utilization of backup media. In addition, many disk libraries support data compression, which further increases utilization.

Consolidation: Some disk libraries can emulate multiple tape libraries simultaneously. This allows users to consolidate and simplify their tape environment in the same way that Enterprise Storage consolidated disk systems in the late 1990s. Customers with several smaller backup applications and tape solutions can write their backup data to a single disk library that appears identical to each of their tape libraries dedicated to individual backup applications. This consolidation reduces the overhead associated with managing multiple environments.

However, disk libraries will generally continue to be more expensive to acquire on a per-megabyte level than tape libraries for the foreseeable future. Although costs vary, for most enterprise customers disk libraries will continue to be 40-55% more expensive than their tape equivalents. In exchange for that premium, users receive higher performance, better availability, and an environment that is easier to manage.

Tape and Disk Together

Clearly, tape will exist for the long term. Where will tape libraries fit in the future, as enterprises adopt disk libraries for traditional backup and restore operations? Disk library vendors know that customers will want to use both technologies side-by-side and are providing capabilities for both platforms to work together.

One example is automating the off-site storage process. As data ages and the need to access it becomes less likely, an operations manager might need to move it from a high-performance disk library to tape for transfer to off-site, long-term storage. Some disk library products ship with physical connections to tape solutions that can be accessed through the disk library's management software. Since that software emulates the tape management application the customer already uses, the disk library interprets the vault or eject commands of the backup application and writes out to a tape library in native tape format. With the tape library physically connected, the disk library offloads this "second write" to tape from the backup server and SAN. This allows the organization to continue to use tape where it fits best, for the low-cost storage of long-term, off-site data. Another area of coexistence is migration of data on existing tape media to a disk library. Some disk library products support bar codes in a manner similar to tape libraries. A request for a tape volume not mounted in the disk library generates a message indicating that the missing tape should be mounted in the attached tape library, allowing its contents to be read into the disk library for future use.

Customers do not need to go through an expensive and difficult data migration to introduce a disk library into their environment. Whatever data has already been stored on tape libraries can stay there, with newly generated, time-sensitive data stored on the disk library until it is ready to be moved to tape for longer-term, off-site storage. Finally, the customer gets the advantages of much faster backup and restore without the need to purchase additional software licenses or to retrain their staff.

What to Look for in a Disk Library

For all their benefits, disk libraries are more expensive than tape. Before purchasing a disk library, know the critical features that will deliver a long-term return on your investment.

First, look for a proven disk-based architecture that provides the greatest performance, scalability and availability. Consider only vendors with a proven track record of building and supporting storage systems that support mission-critical applications. High availability is a crucial feature that elevates a disk library over its tape equivalent. Therefore, customers should choose disk libraries that support dual, redundant controllers and RAID protection for data.

Another critical bench-mark is the speed with which the disk library can read and write sequential data. To recoup the added cost of a disk library, it should deliver high enough sequential access to replace ten or more tape drives.

For some customers, disk libraries will hold weeks of backup data. Given this requirement, any disk library should scale to at least 50 tera-bytes of usable capacity. Customers will benefit most from libraries that use high-capacity ATA drives and data compression.

Disk libraries also must precisely emulate existing tape libraries and tape drives, avoiding the need for customers to change existing backup applications. Consider disk library vendors with proven interoperability testing for backup application and tape emulation combinations. While these combinations should work, ask your vendor to see their qualification testing to prove that it does indeed work.

Disk libraries also should support the use of physical tape for creating off-site archival media, and on-demand migration from tape to the disk library. These capabilities should be easy to configure and manage through graphical user interfaces and wizards.

These pre-configured appliances should be capable of being installed, configured, and deployed within a couple of hours. Finally, because both tape and disk libraries are vital parts of an organization's traditional backup and restore operation, customers should choose vendors who can deliver the highest levels of service and support.


Operations managers are all too familiar with technology that is supposedly faster and better than its predecessors, but requires painful and expensive upgrades. Disk libraries are a welcome exception in that they can be "dropped in" to an existing storage environment without the need for data migration, new software licenses, or retraining of the operations staff. The right disk library gives the customer a new tier of storage for faster, more reliable backups and restores with minimum pain.

Chuck Hollis is vice president of Platform Marketing at EMC Corporation (Hopkinton, MA)
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Title Annotation:Regulatory Compliance
Author:Hollis, Chuck
Publication:Computer Technology Review
Geographic Code:1USA
Date:May 1, 2004
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