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|What is a continuous snapshot?|
In an earlier article (see The search for cost-effective disaster recovery), I described how to develop an application/data classification foundation (ADCF) that lays the groundwork for cost-effective DR. This foundation has six steps:
- Classify each application and its data into four categories:
- Mission critical
- Less critical
Remote mirroring provides data accessibility protection for an application using physically separate locations. While similar to mirroring within a RAID array, remote mirroring takes place over MAN or WAN distances. It's usually between storage arrays or storage appliances, and can be synchronous or asynchronous.
Synchronous remote mirroring is the highest possible level for DR RPO and RTO. The RPO is "zero" lost data, and the RTO is typically seconds to minutes. Synchronous remote mirroring does this by neither completing nor acknowledging the local write until the remote write is completed and acknowledged. Additional writes can't occur until each preceding write has been completed and acknowledged. This means local performance is directly related to the performance of the DR remote device; distance is the limiting factor. Remote synchronous mirroring is rarely deployed for circuit distances greater than 160km (100 miles).
With asynchronous remote mirroring, local writes are completed and acknowledged before the remote writes. Asynchronous remote mirroring is a "store-and-forward" technique that reduces I/Os and wait delays, allowing remote writes to fall behind the local writes. This means the RPO for lost data can range from seconds to minutes, and even hours in some cases. Asynchronous remote mirroring is most often utilized when the remote site is a long distance from the local site.
The primary advantage of both synchronous and asynchronous remote mirroring is the minimal (asynchronous) to zero (synchronous) risk exposure in losing data during a disaster. A secondary advantage is the potential for quick data recovery when a disaster occurs. Remote mirroring doesn't require server agents, and it provides heterogeneous server and application support.
Remote mirroring applications are often pricey, the equipment is usually expensive, and it typically requires at least twice the primary disk space and sometimes much more. However, when the lowest possible RPO and RTO are the requirement, remote mirroring is the answer.
Another disadvantage is that remote mirroring doesn't prevent a rolling disaster, data damage, corruption or accidental deletion. If data is corrupted, damaged or deleted at the primary site, it will also be at the DR site. Some asynchronous remote mirroring products timestamp each transaction and allow recovery to a point in time before the corruption or deletion occurred, but they're exceptions to the rule. This means procedures other than remote mirroring must also be implemented to allow for recovery of corrupted, damaged or deleted data. Other disadvantages include lack of support for heterogeneous arrays, no support for internal storage, and nearly no application and file information.
Less-expensive alternatives to remote mirroring can also provide the lowest possible RPO and RTO. They're generally continuous data protection (CDP) products and include time-based continuous snapshots, automated backup, replication of changed data and automated, generational-change distributed backup. They offer a lower TCO than remote mirroring, support heterogeneous storage and provide better rollback capabilities. But they usually require installing and managing agents.
This was first published in January 2005