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Evaluating High-Availability (HA) vs. Fault Tolerant (FT) Solutions

October 8, 2010

“When evaluating how to increase availability and reduce downtime for your deployments, solutions can commonly be categorized as either a ‘High Availability’ solution or a ‘Fault Tolerant’ solution. In this blog I thought I would take a moment to discuss pros and cons of each…”

Read the full MDSN Blog Article Here….

“HA vs. FT
So which solution is right for you? Well, the initial and obvious conclusion most instantly come to is that ‘no’ downtime is better than ‘some’ downtime, so FT must be preferred over HA! Zero downtime is also the ultimate IT utopia which we all strive to achieve, which is goodness. Also FT is pretty cool from a technology perspective, so that tends to get the geek in all of us excited and interested.

However, it is important to understand they protect against different types of scenarios… and the key aspect to understand is what are the most important to you and your business requirements. It is true that FT solutions provide great resilience to hardware faults, such as if you walk up and yank the power cord out of the back of the server… the secondary mirror will take over with zero client downtime. However, remember that FT solutions are running a common operating system across those systems. In the event that there is a software fault (such as a hang or crash), both machines are affected and the entire solution goes down. There is no protection from software fault scenarios and at the same time you are doubling your hardware and maintenance costs. At the end of the day while a FT solution may promise zero downtime, it is in reality only to a small set of failure conditions. With a loosely coupled HA solution such as Failover Clustering, in the event of a hang or blue screen from a buggy driver or leaky application. Then the application will failover and recover on another system….

…While a loosely coupled system such as Failover Clustering cannot deliver zero downtime for hardware failures, it does protect against a wider range of failures up and down the stack including hardware, OS, and even provides application health monitoring. HA solutions can also reduce downtime for other scenarios, such as patching….

At the end of the day, there is no right or wrong answer. It boils down to you evaluating what your individual most common sources of downtime are, then deploying a solutions which helps mitigate them. Going back and analyzing the root cause of your sources of downtime over the last year is a good place to start, then you can come up with a strategy on what solution best mitigates them. Additionally, the business requirements vary for each deployment, so the service level agreement (SLA) you need to achieve, and what the acceptable levels of downtime for the failure conditions you need to protect against are ultimately up to you… “

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