Feature

How disk has changed backup

Ezine

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CDP. A CDP system is basically an asynchronous, replication-based backup system. The software runs continuously on the client to be backed up, and each time a file changes, the new bytes are sent to the backup server within seconds or minutes. But unlike replication, a CDP system can roll back to any changes at any time.

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Pros and cons of alternative backup methods
CDP products transfer data to the backup server in different ways. Some transfer changed blocks immediately, while others collect changed blocks and send them every few minutes. They also differ in how they do recoveries. Some products are able to restore only the blocks that have changed from a particular point in time, while other programs operate in a more traditional manner by recovering the entire file or filesystem. The first method accommodates more aggressive RTOs and RPOs than the second method. Also, CDP products can meet any type of synchronicity requirement because they can recover one, 10 or 100 systems to any synchronized point in time.

Another difference in CDP products is that some are database-centric and work only with a particular database, such as Microsoft Exchange or SQL Server. Most file-based CDP products aren't going to provide interfaces for your database apps. These CDP products copy blocks to the backup destination in the same order they're changed on the client. Restarting your database causes it to go into the same mode that it would go into if the server were to crash (i.e., crash recovery mode). It examines the data files, figures out what's inconsistent, rolls backward or forward any necessary transactions or blocks, and then the database is up. If the CDP product puts the blocks back in the exact order in which they were changed, then the database should be able to recover from any point in time. Some products can even present a logical unit number or volume to your database that it can mount and test before you do the recovery.

Some CDP vendors, like Kashya and Mendocino, integrate with database vendors. In addition to continuously copying blocks from source to destination, they integrate with your apps to create consistent recovery points that can be used to recover your database without it having to go into crash recovery mode. Keeping the app out of recovery mode can save a lot of time during a restore.

Your database vendor may have a different opinion about CDP: If you're not using their supported backup method, they may not be helpful if something goes wrong. Discuss the support issue with your database vendor and include your DBA in the discussion.

This was first published in November 2005

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