High availability with the Distributed Replicated Block Device

"The Distributed Replicated Block Device (DRBD) provides a networked version of data mirroring, classified under the redundant array of independent disks (RAID) taxonomy as RAID-1. Let's begin with a quick introduction to high availability (HA) and RAID, and then explore the architecture and use of the DRBD.

"Introducing high availability

"High availability is a system design principle for increased availability. Availability, or the measure of a system's operational continuity, is commonly defined as a percentage of uptime within the span of a year. For example, if a given system is available 99% of the time, then its downtime for a year is measured as 3.65 days. The value 99% is usually called two nines. Compare this to five nines (99.999%), and the maximum downtime falls to 5.26 minutes per year. That's quite a difference and requires careful design and high quality to achieve.

"One of the most common implementations for HA is redundancy with failover. In this model, for example, you can define multiple paths to a given resource, with the available path being used and the redundant path used upon failure. Enterprise-class disk drives illustrate this concept, as they provide two ports of access (compared to one access port for consumer-grade drives)."

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