While they should only represent a small percentage of your environment, mission critical applications need the most attention when designing a protection process for them. Most of these applications can only be down for a few minutes and can only lose a few minutes of data. Since most backup applications, even with incremental forever technologies, only protect data every 15 to 30 minutes, mission critical applications need something more.
Mirroring or Replication?
Generally, there are two types of protection available for rapid protection of mission critical applications. The first is synchronous mirroring. With this protection strategy, as new data is written to the storage system it is also written to a second storage system. The application is not given an acknowledgement of that write until both storage systems record it. That means the second storage system, while not in the same data center, has to be fairly close by; typically, within the metropolitan area. That also requires a very high-speed connection between the two storage systems and it means that the second storage system, generally speaking, has to be as fast as the primary storage system. In most cases it is identical.
All of the requirements for a synchronous mirror add up to a very expensive way to keep a perfect copy of data. For a few organizations that can’t tolerate even a minute of downtime, there is no other choice. The expense of being down cost justifies the investment in synchronous mirroring.
There is one other problem with using synchronous mirroring to protect mission critical applications; lack of distance. As we’ve seen over the last few months, major disasters strike a broad range of geography. In most cases, the disaster can impact an entire metropolitan area.
Replication – Mission Critical Protection for the Rest of Us
Replication is the asynchronous copying of data from a primary storage system to a secondary storage system. Since the copy is asynchronous, the application does not have to wait for acknowledgement of the write being accepted on both storage systems. As long as the primary storage system responds with the acknowledgment the application proceeds. The secondary system eventually catches up. Most replication solutions have now mastered the concept of write order fidelity, so if the same block continuously gets updated it knows the order in which to write those blocks.
The advantage of replication is the secondary site can be much further away from the primary site, even in the cloud. The speed of the network, while still important, is not as critical as it is with synchronous mirroring. Replication also enables that secondary storage system to be different in brand and capabilities than the primary. That system still needs to have production level performance, but maybe not as much as the primary.
The negative for replication is data loss of a few seconds, potentially even minutes, is likely if there is a unexpected disaster. Essentially the organization may lose the data that is “on-the-wire.” In reality, many disasters are not unexpected. Hurricanes, floods, and tornados often give IT at least a few minutes to shutdown the application in order to make a final clean sync. If there is a sudden outage then IT may need to re-index or rebuild some data.
The advantage though is replication as a data protection mechanism is far more affordable than the synchronous mirroring option. The savings in network bandwidth and using a tier 2 storage system is tremendous. Plus, in most cases, organizations using a synchronous mirror will still then replicate the data once more to get the distance needed to survive a major disaster.
Hitting the Data Protection Sweet Spot
Most organizations will need some form of replication to protect their mission critical applications. Others may need to add synchronous mirroring to drive out those few minutes of downtime that replication may expose. In both cases, these protection strategies should be augmented with backup and recovery solutions that provide an extra layer of protection as well as long term retention of data.
Replication solutions are typically available in two forms; software that can leverage almost any storage hardware, and storage vendor supplied that only works within a particular vendor’s solution set. We’ll cover this in more detail in our next blog. But, for now check out our on demand webinar, “Hitting Your Data Protection Sweet Spot”.