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|RPO and RTO|
The recovery of critical data in an environment is measured by two key parameters: recovery point objective (RPO) and recovery time objective (RTO). RPO measures the maximum acceptable age of data at the time of an outage. RTO is the maximum acceptable length of time to resume operation following an outage.
Continuous backup or more accurately, continuous data protection (CDP), changes the rules by using disk technology to continuously capture updates to data in real time or near real time. The result: Backup windows become irrelevant because backup is occurring all the time.
But the main benefit of this technology is not on the backup side of the equation. Rather than the single point-in-time access to data that traditional products provide, this technology promises any point-in-time continuous access. Reducing the time to recovery to near zero will be the real driver for adopting CDP products. As with any new technology, it is important to understand what it does, how it works and where it fits within an overall data management and protection strategy.
Approaching instant restore
The recovery of critical data in an environment is measured by two key parameters: recovery point objective (RPO) and recovery time objective (RTO). These concepts are critical to an understanding of CDP.
- RPO measures the maximum acceptable age of data at the time of an outage. A backup performed nightly would represent an RPO of approximately 24 hours because the worst-case scenario would be an outage during the backup.
- RTO is the maximum acceptable length of time to resume operations following an outage. This includes the time to restore from the backup media plus all additional time required for data integrity validation, system or application preparation, rolling forward data and so on (see "RPO and RTO"). So for example, a database outage that requires four hours to restore plus two hours of analysis and validation, plus one hour to roll-forward logs results in a seven-hour recovery time. If your RTO is four hours, then you've missed your objective by three hours.
Tolerance for such an outage varies substantially, depending on the nature of the business and the criticality of the data within that business. For most data, the traditional backup approach may be sufficient, but there are key applications and critical data that warrant special consideration. Much time and effort has been invested to ensure that split mirroring techniques provide this additional degree of protection.
Continuous Data Protection (CDP) lets you recover a volume almost instantly, dramatically reducing both RPO and RTO.
What happens if a shorter RPO is required? Our four-hour scenario requires a two- to six-fold increase in storage, depending on the number of fall-back split mirror copies desired. A two-hour scenario could potentially require up to a 12-fold increase (assuming a full 24-hour history is maintained), and may not even be achievable, given the required recovery activities and management complexity involved. This is where CDP can help.
CDP to the rescue
CDP adds the dimension of time to storage mirroring. Conceptually, a CDP solution comprises a base-level image or mirror of an existing volume augmented by a time-stamped log of each subsequent write operation. To be technically accurate, in most solutions the mirror is updated with each write transaction and the log maintains a history of the data blocks that have been modified, similar to the copy-on-write technique used during a snapshot. In any case, most CDP products have the ability to present a virtualized volume representing the contents as they existed--I/O by I/O--at any point in time from the beginning of the log.
The effect is to provide a much finer degree of granularity than traditional split mirror or snapshot technology. It's possible to view a volume as it existed minutes, or even seconds, earlier and to recover that volume almost instantly, dramatically reducing both RPO and RTO (see "CDP recovery"). To make this even more compelling, the disk requirement for this approach is also far less than traditional split mirroring. Instead of the six times the capacity needed for our four-hour split mirror with the ability to rollback 24 hours, a continuous backup solution requires a base mirror, plus enough space to maintain the log, typically resulting in a less than two times the requirement within a 24-hour period. Additionally, adopters of this technology may choose to retain the log information to allow rollback for greater periods of time, up to several days or possibly even weeks, depending on the rate of change.
This was first published in June 2004