Safeguarding data has never been more critical. Information technology managers must handle more data than ever before and must protect this data from more threats than ever before. They must not only contend with perennial hazards such as fires and natural disasters but also grapple with newer concerns such as post-Sept. 11 terrorism. In response, government agencies are reexamining their data protection systems and exploring the possibility of foregoing tape-based backups for a speedier alternative: disk mirroring. Many commercial entities such as those in financial services have already adopted this system.

Disk mirroring allows continuous replication of selected data from one disk array to another. This can be done locally, within a data center, or remotely, to a far-off center. The swiftness of disk mirroring sets it apart from tape-based backup, the time-tested method currently employed by many organizations to safeguard data. Restoring data from tape can take days. Tapes have to be retrieved from the backup facility and the data then placed on disk arrays at the production site. In contrast, data recovery through disk mirroring can be completed in a few minutes, particularly if the system has been programmed to automatically connect to the backup system.

While the main benefit of disk mirroring is speed, another major advantage is timeliness. The copy of data it produces is up-to-date, even "up to the minute," according to Robert Manchise, chief technology officer at Virginia-based Anteon Corp. Meanwhile, the timeliness of data stored on tape is subject to the frequency of backup sessions, which are usually scheduled every 12 or 24 hours.

Disk mirroring also has disadvantages, foremost of which is its cost. The price per megabyte of storage is higher than tape, although the disparity is decreasing. In addition, managers will have to spend more if they choose to mirror to the same brand and model of storage subsystem used in the production site. The expense, however, can be lessened through heterogeneous mirroring, utilizing more economical disk arrays at the backup site. Another consideration is network investment. Disk mirroring in general needs a separate network infrastructure, apart from IP traffic, because it may require high-bandwidth links between backup and production facilities. To combine data mirroring and voice traffic over IP, organizations could utilize Dense Wavelength Division Multiplexing, a technology that uses a single optical fiber to transmit light signals simultaneously. However, this is expensive. Fortunately, low-cost alternatives are in development. IP-based storage as well as solutions for running storage traffic on IP networks are emerging. In the meantime, technology managers are choosing to limit their use of disk mirroring to the most critical data.

There are two mirroring approaches: synchronous and asynchronous. In synchronous mirroring, the production facility does not store data until it receives confirmation from the target storage subsystem that it has successfully mirrored that data. This approach ensures data consistency, up to the latest transaction. Its primary disadvantage, however, is latency; data has to make its way from the production site to target storage and back again. This delay can lead to applications timing out. Latency does not pose a problem if the backup site is no more than 62 miles from the production facility. However, organizations may choose distant backup facilities to avoid site failure. The second approach, asynchronous mirroring, excels at mirroring data over great distances. Latency is not a stumbling block in this approach because the local storage device stores data without waiting for confirmation of data receipt from the remote system. The trade-off is data inconsistency, which occurs if an input or output fails on the backup site.

Asynchronous mirroring is for organizations who want "continental-level mirroring" and don't mind losing a few input/outputs, says John Selep, product marketing manager for storage solutions at Hewlett-Packard Co. Federal customers and integrators, on the other hand, prefer synchronous mirroring for their most crucial information. Even as many organizations are already switching to disk mirroring, scientists are still searching for ways to improve it. Pacific Northwest National Laboratory is looking into storage-area network (SAN) software technology that has the ability to take snapshots of a database, which will allow for the easy correction of any database corruption that has been mirrored.

Source: Looking into Disk Mirroring
John Moore
Federal Computer Week, May 20, 2002
http://www.fcw.com/fcw/articles/2002/0520/tec-disk-05-20-02.asp