Exploring backup alternatives in a SAN environment (page 2 of 2)

Many of the technologies required to implement second-host-backup and third party copy in a SAN environment are also necessary (or at least useful) in non-SAN environments. In this section we look at the prerequisite technologies supporting these advanced backup techniques.
One such technique is Object Mapping.

Object Mapping is the process of mapping logical file information into absolute physical addresses. The mapping process must take into account various levels in the data management stack, including applications, database management systems, filesystems, and volume managers.

This technique is required to ensure that a backup product can understand and respond to changes in how the data is layed out and addressed on disk. Some maintenance and redundancy techniques, if they occur at a specific point in time, may invalidate what the backup product thinks it is backing up. The net result would be a corrupted (potentially undetected) backup.

Snapshot technology provides reliable SAN backup, the basic premise is to create a consistent point-in-time image of a given file system or data volume, and use this image as the reference point for the backup operation. Once a snapshot is in place, the primary data can continue being modified without affecting the backup operation.

The concept of Application Quiesce is closely related to the snapshot technology. In order to obtain a consistent and useful version of the data on backup media, it is vital to coordinate the snapshot (and the resulting backup operation) with the application that manages the data. Application Quiesce is the process of temporarily pausing application updates and flushing application buffers at a point in time where application consistency exists.

The high-profile benefit of such SAN backup techniques is the ability to export the actual backup copy operation from the primary application host to an off-host entity. The off-host copy service can reside in either a second host on the storage network or any other SAN element with visibility of both the backup source and destination.

Conclusion
After examining the various backup architectures and techniques, both SAN and LAN, and exploring the data environments, several observations and conclusions can be reached:

1. There is no "one size fits all" backup architecture. Combining multiple backup methodologies can often provide the best solution.

2. SAN backups are not trivial operations. There are several key technologies required for successful and reliable SAN backup deployments. Although the potential benefits are high, the current reality is that due to the complex prerequisites, only a limited number of environments will be able to take advantage of SAN backups.

3. Snapshot technologies can drastically shrink or eliminate the backup window, but they require additional online disk storage, up to three times the storage if the mirror technique is used.

4. Compared to conventional backup solutions that are host-resident, SAN backup solutions may not have built in features like load balancing and clustering/failover. In this regard, solutions such as second-host backup need to be evaluated versus third-party copy data movers.

5. While SAN backup solutions with exported data copy services do drastically reduce (or eliminate) the I/O overhead on the application host, the potential for I/O contention between the application updates and the backup data copy still exists.

In summary, one must closely examine the variety of backup architectures and techniques being offered against the environments (physical and logical data) being protected. The potential benefits of SAN backup solutions are huge, but it is important to fully understand the caveats and limitations before deployment.