Designing Storage Area Networks: A Practical Reference for Implementing Fibre Channel and IP SANs (2nd Edition)

Prepress refers to the creation and modification of graphical images for advertisements, catalogs, and posters. Graphics can be as simple as low-resolution black and white newspaper ads or as sophisticated as large, four-color, high-resolution images that are applied to billboards or city buses.

Unlike full-motion video applications, computerized prepress data traffic is bursty. A single graphical image is read from disk, rendered for several hours by a graphic artist at a workstation, and written back to disk. The file may pass through multiple revisions, and therefore multiple workstations, as other detail, titles, and legends are added. When graphical editing is complete, the file is then read by a preprint processor for conversion from digital format to hardcopy or print negative.

Because a graphical image must go through a series of editing steps, each by a different artist, as it passes through the production process, file ownership is critical for maintaining data integrity. If the same image is inadvertently opened and modified at the same time, hours of work can be lost. Software companies that specialize in prepress operations resolve this potential problem by providing file sharing middleware. This software resides on each workstation. By intercepting calls from the operating system to the file system, the application allows file ownership to be serially transferred from one user to another as the file is read from and written back to disk.

In addition to file ownership, the amount of time it takes to read a large graphical image from disk for editing, and to write back the modified version, is an important issue for prepress. Read/write time is, for the user, down time, and the accumulated down time between edits can impact the entire production process. For large prepress operations in particular, the bandwidth supplied by a Fast Ethernet LAN is not sufficient for concurrent file transfers of image files that are often in the range of hundreds of megabytes. To increase productivity, gigabit speeds (Gigabit Ethernet or Fibre Channel) are required.

As with video applications, prepress has a voracious appetite for storage. A catalog production, for example, may require terabytes of storage for high-resolution photographic images and formatting information. A major brands consumer catalog may have three or more editions per year, with revisions of some product images and introduction of new ones. All this data must be maintained and accessible for updates. File compression helps to reduce the overall storage requirement, but it is less effective for high-resolution images.

SANs were introduced into prepress operations primarily by vendors of file access software. As a total solution, the combination of file access middleware, higher bandwidth and shared storage via a storage network, and increased storage capacity provided by SAN-attached disk arrays addresses most of the data infrastructure issues faced by prepress operators.

As shown in Figure 12-2, graphics workstations can be attached to departmental SAN switches in each work area, with trunked links running between the departments and the processing/storage center. The distribution of users is scalable: Additional users can be accommodated with additional switches, and the population of each switch can be adjusted according to workload and bandwidth requirements. Shared storage in this configuration is also scalable, both by the addition of drives into the RAID enclosures and by attachment of new arrays over time. Specialized preprint processors can be brought into the SAN via SAN bridges. And finally, file access software on each graphics workstation ensures that a file can be modified by only one user at a time and that the identity of the current owner is known. This SAN solution also provides great efficiency by transporting graphics data using the block SCSI protocol, as opposed to file-based transports.