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|A tale of two divisions|
DRB systems. DRB systems were designed to answer the following questions: If only a few bytes in a file change, why back up the entire file? If the same file resides in two places on the same system, why back it up twice? Why not store a reference to the second file? And why waste server and network resources by backing up the same file across multiple systems?
By backing up a file once, and then backing up only the changed bytes, backup windows are substantially reduced. Tape copies of disk-based backups can usually be created at any time, depending on your requirements. Some DRB products can meet aggressive RTO requirements by restoring only the blocks that have changed since the file was last backed up. The RPO and synchronicity abilities of DRB products are based on how often you back up, but it's common to back up hourly.
The biggest advantage to DRB products is that, from the user adoption perspective, they're the closest to what users know. Their interfaces are similar and they often have database agents like traditional backup software. They're also able to back up faster and more often, and use much less bandwidth. 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.
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 file system. Obviously, 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. Remember that, unlike traditional backup products, file-based CDP products aren't going to provide interfaces for your database applications. 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. 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.
Of course, 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 June 2005