CDP: Strengths and weaknesses

Downtime to recover terabytes of data can cost a company dearly in terms of income (and customer satisfaction). Continuous data protection (CDP) is emerging as an attractive supplement to traditional backup schemes when downtime isn't an option. It's clear that CDP isn't a perfect or universal backup solution. Some implementations involve agents, or can impact network performance. CDP users must also watch for potential configuration and interoperability issues. But CDP solutions are appearing and proving themselves in very demanding enterprise applications.

Continuous data protection (CDP) is a supplemental backup technology that allows IT staff to recover the maximum amount of data in the absolute minimum time. CDP addresses three key issues of data backup: backup window, recovery point objective (RPO) and recovery time objective (RTO). Downtime occurs any time that a production system is taken offline for backup. The "backup window" is the amount of time needed to perform a system backup while the system is unavailable. The RPO refers to the required "freshness" of a backup (the closest restore point available), and the RTO indicates the amount of time needed to restore a system from an established backup.

CDP reduces time windows

Many enterprise IT departments can experience a backup window, RPO and RTO exceeding several hours. CDP technology can potentially shorten these time objectives dramatically. When properly implemented, CDP can move all three of these times toward zero. For example, a CDP system works to monitor and record transactions continuously, so there is no need for regular interruptive backups. With no downtime for backups, the backup window becomes zero. With each transaction recorded and logged, administrators can locate a precise point in time (usually just prior to the failure) from which to restore, significantly reducing the RPO. Once a recovery point is selected, CDP can restore the system (or just individual files) from disk-based media, trimming tremendous time from the RTO. Vendors generally agree that CDP can offer far better RPO (and typically better RTO) than conventional backup tactics.

Some data recovery may still be needed

Experts agree that CDP is certainly not a panacea for every organization or deployment. A move to CDP must be based on a thorough understanding of corporate needs and IT objectives. "It's all about requirements," says W. Curtis Preston, vice president of data protection at GlassHouse Technologies Inc.. "The requirements that push people to CDP are essentially a backup window of zero, an RPO of zero and an RTO of zero -- or anything close to those numbers." He explains that not all companies experience such stringent requirements. "Most companies don't have RPOs and RTOs, and backup windows that are so tight. They can backup all night long and nobody cares. As long as they recover within 24 hours (or 72 hours) the company isn't going to fire anyone." The problem is that with an RPO of 24 hours, the last backup can be up to 24 hours old. With an ever increasing number of mission-critical business deployments, such wide RPOs are under tremendous pressure. Preston adds, "There are some applications like Oracle and Exchange and SQL Server that we back up regularly throughout the day because a 24 hour RPO is just not enough." For extremely aggressive backup windows, or applications that refuse to accept downtime for recovery (or cannot tolerate any data loss), CDP may be the ideal solution.

It's important to point out that any data changes or transactions that occur between a fault event and its discovery are not protected by CDP, so some amount of data recovery may still be required even when CDP is implemented. Jerome M. Wendt, an independent storage analyst, explains further: "CDP should not be confused with backup – though it complements backup technologies – and a few backup products are already incorporating this feature functionality into their product offering." Wendt notes that CDP is often the first line of defense for data recovery, quickly returning application files and database logs to a "known good" point. But any file or data changes that took place after the selected CDP restore point may still require separate restoration.

Don't discard other backup practices

While CDP offers some powerful benefits for 24/7 IT operations, Brian Babineau, analyst at Enterprise Strategy Group, says that CDP is not meant to stand alone – it is one layer in a multi-layer protection scheme. "The CDP server itself needs to be protected," he says. "So when I'm capturing changes and sending the changes to a local storage system, that's another system that needs to be backed up because there's critical information on there." CDP is not intended to replace existing backup tactics, but instead provide a new layer of protection that can improve IT service to enterprise users.

Babineau says that agent deployment and maintenance are potential roadblocks to CDP adoption. "Many CDP solutions require that a collector [agent] reside on the file servers or laptops," he says. "They collect information and then send it back to a centralized repository [a CDP server] that organizes it in case it needs to be restored. System administrators are not fond of adding processor overhead (a.k.a. agents) to their devices for manageability and performance reasons." Babineau cites CDP bandwidth requirements as another possible impediment to adoption. Although CDP data typically requires little network bandwidth, he says that some amount of user control is needed. "Some of the features coming out of the CDP vendors will toggle how much network bandwidth they [CDP products] can actually consume."

Users may also face configuration and interoperability issues. Wendt says, "Network-based solutions such as Alacritus Software [now owned by Network Appliance Inc.] and Revivio take time to configure (e.g., LUN masking and zoning), require multi-pathing software on servers, additional ports on the SAN, and will be confronted with the rigid interoperability and data integrity requirements of data centers." Many of these issues are further complicated by corporate politics involved in controlling and managing a large and diverse IT infrastructure.

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  • Introduction
  • CDP: An overview
  • CDP: Strengths and weaknesses
  • CDP: The vendors
  • CDP: User perspectives
  • CDP: Future directions
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