Database Shadowing and Mirroring: How It Works
This paper provides a comprehensive overview of database shadowing, also known as mirroring, as a strategy for protecting data against hardware failure and corruption. It explains how shadow copies of a database are maintained in parallel with a master copy, ensuring continuous availability and rapid recovery. The paper covers the step-by-step check-in and check-out process of a database shadowing system, examines key vendors including Mimer SQL and Imaging Solutions, and evaluates the major advantages and disadvantages of the approach. Topics such as disk space requirements, data clustering issues, RAID alternatives, and real-time data recovery are also discussed.
- What Is Database Shadowing?: Definition and core concept of database shadowing
- How Database Shadowing Works: Step-by-step operational mechanics of shadow systems
- Key Vendors: Mimer SQL and Others: Overview of Mimer SQL and Imaging Solutions
- Advantages of Database Shadowing: Key benefits including rapid recovery and availability
- Disadvantages and Limitations: Data corruption risks and clustering issues
- Conclusion: Shadowing within broader disaster recovery strategies
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What makes this paper effective
- Provides a clear, structured definition of database shadowing before moving into technical detail, making the paper accessible to readers with varying levels of prior knowledge.
- Balances discussion of advantages with an honest treatment of limitations, giving the paper analytical credibility rather than reading as promotional material.
- Grounds abstract concepts in a real-world application (Mimer SQL) to illustrate how shadowing is implemented in practice.
Key academic technique demonstrated
The paper demonstrates the technique of definition-first exposition: it introduces and defines the central concept thoroughly before analyzing its components, applications, and trade-offs. This organizational strategy ensures that readers share a common understanding of the subject before encountering more complex technical claims.
Structure breakdown
The paper opens with a conceptual definition of database shadowing, then moves into the operational mechanics of how shadowing systems process data. It profiles key vendors, notably Mimer SQL, before transitioning into an extended evaluation of advantages and disadvantages. The conclusion situates shadowing within broader concerns about business continuity and disaster recovery. Each section builds logically on the previous one, moving from definition to mechanism to application to evaluation.
What Is Database Shadowing?
Database shadowing means that a database is able to work with two or more copies of itself at the same time. Among these copies, the original — referred to as the "master" — is the normal file from which data is retrieved. The other copies are called "shadows" or, at times, "mirrors." Whenever an alteration or change of any kind must be made to the master, the same changes must also be made to the shadows, so that each shadow is always up-to-date with the master copy. All changes must be made separately to each shadow so that there is no corruption of the master, and so that any physical corruption of the master is not inadvertently transferred to the shadows. (Resilience)
In the application Mimer SQL, shadowing is generally controlled at the databank level. In any databank where there is a minimum of transaction control, the database can be shadowed to as many copies as required at any given time. This means that important data can be well protected against corruption or disk crashes without having to rely on the traditional methods of maintaining backups. If a disk crash or failure occurs in the drive where the master data is stored, operations automatically continue using the shadows, without any interruption.
The key benefit of maintaining shadows is that work can continue uninterrupted. Without shadows, a disk failure would require a time-consuming process of restoring the master database from backup files stored on tapes or other media. With shadowing, the application is kept up and running at all times. The database manager can choose a convenient time, transform the shadows into master databanks, and create new shadows based on the updated data. Importantly, maintaining shadows has no impact on running time or response times. Background threads carry out the various changes made to the master databank and to the shadows without affecting the users of the application. (Resilience)
References
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Eng, Audra. (July, 2004) "Sygate Secure Enterprise, 4.0." Retrieved at http://www.sygate.com/solutions/datasheets/wp/WP-Sygate-Secure-Enterprise.pdf. Accessed 4 October, 2005.
Flesher, Tom. (19 August, 2004) "Critical Data Availability in Real Time." Retrieved at Accessed 4 October, 2005.
"Mimer 8, Product Overview." Retrieved at Accessed 4 October, 2005.
Norvag, Kjetil; Bratbergsengen, Kjell. "Write Optimized Object Oriented Database Systems." Retrieved at Accessed 5 October, 2005.
"Resilience." Retrieved at Accessed 4 October, 2005.
"Shadow Files." Retrieved at http://www.cvalde.net/features/f4/ShadowFiles.htm. Accessed 4 October, 2005.
"Welcome, a Cautionary Tale." (July, 2004) Vol. 7. Retrieved at Accessed 4 October, 2005.
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