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Understanding the Nines - Reliability & Fault Tolerance | System Design Interview Prep | Repovive

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System Design Interview Prep22 sections · 691 units23h total

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Understanding the Nines

Availability math

Availability is measured in "nines." Each additional nine is $10$ x harder to achieve.

Nines	Uptime	Downtime/Year
$99$ %	$2$ nines	$3.65$ days
$99.9$ %	$3$ nines	$8.76$ hours
$99.99$ %	$4$ nines	$52.6$ minutes
$99.999$ %	$5$ nines	$5.26$ minutes

$3$ nines is typical for most web services. $4$ nines requires significant investment. $5$ nines is rare and expensive.

Calculate: (total time - downtime) / total time × $100$ %