Q

The pro and cons of replicating

What are the pros and cons of replicating (for DR) a database with a database replication tool like Quest Shareplex, vs. host-based or array-based replication software?

I believe that tools like Quest Shareplex are excellent for creating an alternate copy of a database for report generation, DSS, data mining and other external applications. However, I don't believe that they are adequate for DR. Database (-only) replication tools only replicate the data in the database and nothing more. They do not replicate the executable or configuration files associated with the database. They do not replicate any other files on the primary system, apart from the contents of the database.

Host- or array-based replication tools such as VERITAS Volume Replicator and Hitachi TrueCopy can replicate all of the data on the system. They do a much more thorough job, and are sure that a modified configuration file, or a patched executable are not overlooked in the replication process.

Most database replication tools do not copy the data, they send the transactions to the remote site, and re-execute them there. As a result, the database on the remote side is not exactly the same as it was on the primary side. Transaction numbers and locations of information will likely be different, confusing any tools that rely on that information.

While tools like Shareplex definitely have their place in the market, I believe that they are not adequate for DR, and tools that do full data replication should be used instead.

Thank you for question. If you have any other questions, please feel free to follow up.

Evan L. Marcus

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This was first published in July 2002

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