The software costs are higher in active/active mode because any system that’s running in active mode must have licensed software. When the system is in dark disaster-recovery mode, the second system does not require paid licenses for database cores, for example, because only one set is live at a time. The fact that the two systems are staying in synch does not affect costs at all.
In synchronous replication, there needs to be reliable network connections between the two servers. There is also extra labor involved in having to manage another site constantly.
The negative with asynchronous replication involves losing some data between the downtime and when the server was last updated. It can be set up, though, for automatic failover.
Anand Hariharan, vice president of product at Webscale Networks, says this is essentially the concept of cold/warm/hot backup servers. The pros and cons fall into two groups: service-level agreement and cost. Recovery point objective (RPO) and recovery time objective (RTO) define the SLA a vendor will provide to inform the user of the acceptable length of time data could be lost in the event of an outage and how fast they will restore services.
“Naturally, with a hot backup, or an active/active architecture, there is zero downtime and a perfect replica of data, so from an SLA perspective, this is a very favorable path to take as it ensures that critical data isn’t lost and critical applications continue to function without interruption,” Hariharan says. “The downside here is of course cost. Maintaining two systems that are always running is essentially twice the cost, whether these costs be related to running replica architectures in a private data center, paying a managed hosting provider to perform the same task in an offsite location, or the cost of running double the instances in the cloud. In some of these scenarios, and depending on the size of the deployment, there is likely also a headcount consideration, where the additional technical staff required to managed twice the systems will also cause a steep increase in costs.”
Given an average (and increasing) rate of $7,900 per minute (Ponemon Institute), downtime creates a potentially huge cost for enterprises, both in immediate business and long-term reputation.
Other costs include servers at a collocation site. They have the superficial attraction of saving money by distributing infrastructure costs over many users, but a closer look reveals that those savings aren’t actualized, according to a ScaleArc White Paper. The collocation company still charges for any unused resources, including dark ones that might someday be activated into full use. Yet enterprises can't reduce the amount of resources dedicated to the secondary site because all information from the primary server must be backed up to the co-lo secondary.
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