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backup requirements such as remote, unattended data centers; a five-minute RTO; a 15-minute RPO or a non-existent backup window.


All recovery point objective (RPO), recovery time objective (RTO) and synchronicity requirements must be business-centric. Before deciding what these requirements are, you should first analyze and prioritize the business functions, and assign each computer system the recovery priority of the business function it serves. Next, decide on an RTO and RPO for each system and type of disaster -- from the loss of a disk to the loss of a metropolitan area. Some systems will have the same requirements for all types of disasters; others may have tougher requirements for specific types of disasters.

Once you've determined an RTO and RPO for each system and disaster type, the final step is to determine how long it will take to back up the system and how much the backup will impact the production system. If your requirements are impossible to meet with a traditional backup system, the following technologies are worth considering.

Snapshots. The most common type of snapshot is a virtual copy of an original volume or file system. The reliance on the original volume is why snapshots must be backed up to provide recovery from physical failures. Snapshot functionality resides in a number of places, including advanced file systems, volume managers, enterprise arrays, NAS filers and backup software.

Snapshots can help you to meet aggressive backup requirements. For example, some snapshots can meet an RTO of a few seconds by simply changing a pointer. An aggressive RPO can be achieved by creating several snapshots per day and, because snapshots can be created in seconds, you can also meet stringent backup window requirements.

Replication. Replication is the practice of continually copying from a source system to a target system all files or blocks that have changed on the source system. Replication used to be what companies implemented after everything was completely backed up and redundant, which meant that few used replication. However, many people are now using replication as their first line of defense for providing backup and disaster recovery.

When used with snapshots, replication allows for tiny backup windows. The snapshot takes just seconds to create, and replication is the quickest way to back up that snapshot to another device. But replication software doesn't usually provide recovery features. The RTO, RPO and synchronicity requirements that you'll be able to meet will be based on how you're performing snapshots or backups, and how quickly they'll be able to recover.

DRB systems. Data reduction backup (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.

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.

For more information:

Any point in time backups


About the author: W. Curtis Preston is the vice president of GlassHouse Technologies, Framingham, MA. He is also the author of Using SANs and NAS, Unix Backup & Recovery and the Storage Security Handbook.

This was first published in July 2005

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