Getting disk into the backup process; adding benefits of disk while supporting existing processes - Storage Networking

Computer Technology Review, Jan, 2004 by Scott Hamilton

The negative impact on the system is dramatically reduced--data is only moved once by the applications servers over the SAN. The actual creation of media is moved completely outside the backup window and off the public network. Installation and on-going support is simplified since the user only has to manage one system, the basic procedures do not change from a tape-only architecture, and the solution has a single point of vendor responsibility. And finally, the actual performance during tape creation can be significantly increased because the system is carefully tuned specifically for backup and it is not competing for resources with other kinds of services.

Tape Format and Media Management

The answer to the second question--the kind of media format and tape management system--is closely related to the issue of integrating the disk-based system into an existing backup process. The approach that we recommend is one that creates tapes that keep the same format that the backup application believed it wrote in the first place, and that allows the backup application's media management system to be retained without change.

This is the approach that we have used in the Pathlight VX disk-to-tape solution. The tapes that the new system generates are identical to tapes created by the backup application writing to a system that uses conventional library architecture. A central advantage to this approach is that the tapes that are created can be loaded and read in any compatible drive anywhere--they do not require a disk-to-tape system for restores. And because the system creates a one-to-one link between the virtual media created at the time of backup and the real tapes that are created for export and long term retention, it also allows the established disaster recovery and data retention policies to be continued without change. The end result is a system that uses existing software for the backup and restore process, that exports and manages real media using existing policies and procedures, but that inserts a layer of disk in the process to provide both fault tolerance and higher performance.

Optimizing Performance

Keeping performance optimized is the third major issue, which users who integrate disk components into backup face. Although it doesn't seem intuitive at first, careful system management is required to get consistent performance benefits from the addition of disk to backup. Tape is optimized for handling large data streams while disk is optimized for high I/O performance. Any time there is a need to stop and start the data flow or to move between random blocks, disk-based storage will excel. But if users just point a high band-width backup stream at a RAID array, they are likely to see reduced performance from the disk system when they compare it to tape.

The solution to getting increased performance across all backup operations is to manage the disk resource so that it can support streaming performance. The technique involves providing a combination of RAID volume management, controller balancing, contiguous space use, and the matching of block size with stripe groups. This configuration borrows technology from applications that have to get streaming performance from disk--large parallel compute operations and delivery of rich media content and digital entertainment.