Disk-Based Mirroring Is A Lot More Than Just Safe - Technology Information

Computer Technology Review, April, 2000 by James W. Baker

Disk-based mirroring is often the first line of defense for speeding the return to operational normalcy following data center outages. Yet today, this powerful, flexible technology can also be used proactively, to perform tasks that can dramatically increase the business value of the enterprise. Before examining just a few of the many innovative applications to which users are leveraging their disk-based mirroring investments in new and creative areas, let's take a look at the technology.

True disk-based mirroring uses the intelligence of the disk subsystem to make mirror images of the data residing on its surfaces. Modern disk subsystems have built-in microprocessors, cache, sophisticated microcode (also known as firmware), and software that can be employed to complete, indeed enhance, the task of writing and reading data to and from the disk or disk array.

Today, more and more functions are being moved from the domain of the host server or mainframe to the intelligent storage subsystem. Sophisticated storage-oriented software handles mapping functions and catalog issues so that the host need not encounter duplicate addresses, to take one example. Users can creatively employ the technology because the second copy of data can be made uniquely addressable to the host. Mirrors can be local or remotely located, that is, inside or outside the same disk subsystems. When outside the same disk subsystem, storage-oriented software is used to move the data from the source to the target location.

Disasfer Recovery

Disaster Recovery (DR) operations can be facilitated with disk-based mirrors. In a traditional DR scenario, backup tapes are shipped offsite via couriers to stand-by locations that hopefully are far enough away not to be affected by the original disaster. Unfortunately, these tapes are already out of date by hours or days because they were run previously as full backups or as incremental backups some time ago. Tapes are subject to human errors such as running them in the wrong sequence or even losing them during transit. Problems might also arise if tape degradation prevents a restore operation after a period of time. In contrast, disk-based mirrors can be established remotely and kept current with up-to-the-last-transaction data. In the event of a disaster, the remote mirror assumes the primary disk role since its data is current. Fast post-disaster restart is the result.

Data Vaulting

A data vault is an offsite repository of information located across campus or across hundreds or thousands of miles. Its purpose is to store and protect data for possible future use. Innovative companies are vaulting their information on RAID protected disks rather than old fashioned tape that is not protected and worse, is susceptible to considerable amounts of human and media error (Fig 1). Depending on the application, the mirror can be kept as current as the last transaction.

Data vaulting differs from Disaster Recovery in that a DR site usually has a standby computer ready to assume a processing role, usually during the restore activity. A data vault may not have such a host and may even be run in an entirely unattended highly automated site.

Point In Time Backups

By far, the most common use of disk-based mirroring is online simultaneous point in time backups (Fig 2). Users make mirrors of their production data at a particular point in time. The process of making the mirror is very fast. Once the mirror is established, the backup routine is run against the mirror image rather than the production image. Better, this backup process occurs in parallel with production processing.

Consider that backup routines occur in virtually every computing operation every day and that most of them require that the production system be taken offline until completion of the backup. By any measure, putting systems back into production immediately is a productivity enhancement of huge significance, not just for the firm, but for the Gross Domestic Product (GDP) of the nation and, all the while, organizational information is being protected by a strictly followed backup discipline.

Data Warehouse Loading

Data warehouses often contain information about customers: their buying patterns, credit histories, demographics, product preferences, and so on. In the new electronic world of commerce, this information is vital to companies seeking to attain competitive advantage by knowing their customers better. The fresher this information is the more reliable it is and, therefore, decisions based on that data are likely to be more accurate.

The process of updating a data warehouse is called "loading" or "refreshing." Unfortunately, all too often, data warehouse loading is a batch activity that is enormously time consuming. The process is conducted offline--taking down the production system until the batch update is completed--because it takes so long.

It is not surprising that the load is often deferred until a more convenient processing time. On the other hand, if users took advantage of quickly established disk-based mirrors, those mirrors could be used as the source from which to load the data warehouse. Data warehouses would be loaded from the mirror rather than the production volume. Simultaneously, production operations could continue operating in parallel. Updates could be conducted nightly instead of every six weeks, resulting in fresher data and better decisions.