"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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