> We define availability as the fraction of “qualified” RPCs the service answers successfully within latency thresholds: 5 seconds for a Safe request, and 15 seconds for a Recent request as leader re-election in Spanner may take up to 10 seconds. ... To compute availability, we aggregate success ratios over 90-day windows averaged across clusters. Figure 5 shows Zanzibar’s availability as measured by these probers. Availability has remained above 99.999% over the past 3 years ofoperation at Google. In other words, for every quarter, Zanzibar has less than 2 minutes of global downtime and fewer than 13 minutes when the global error ratio exceeds 10%.
Basically, they're counting by number of requests. That's fairly typical for Google, who in their SRE book point out that measuring only total outages is a poor indicator of actual user experiences. Imagine if you had an electric company that had frequent brownouts and rolling blackouts but bragged about never having a total blackout. You'd be fairly unimpressed.
Google SREs also make the point that beyond five nines, your efforts are rendered moot by reliability issues you cannot control. Mostly network issues. If you have 99.99999% reliability but the mobile data network only has 99.99%, you've wasted a lot of money on something most folks will never notice.
> We define availability as the fraction of “qualified” RPCs the service answers successfully within latency thresholds: 5 seconds for a Safe request, and 15 seconds for a Recent request as leader re-election in Spanner may take up to 10 seconds. ... To compute availability, we aggregate success ratios over 90-day windows averaged across clusters. Figure 5 shows Zanzibar’s availability as measured by these probers. Availability has remained above 99.999% over the past 3 years ofoperation at Google. In other words, for every quarter, Zanzibar has less than 2 minutes of global downtime and fewer than 13 minutes when the global error ratio exceeds 10%.
Basically, they're counting by number of requests. That's fairly typical for Google, who in their SRE book point out that measuring only total outages is a poor indicator of actual user experiences. Imagine if you had an electric company that had frequent brownouts and rolling blackouts but bragged about never having a total blackout. You'd be fairly unimpressed.
Google SREs also make the point that beyond five nines, your efforts are rendered moot by reliability issues you cannot control. Mostly network issues. If you have 99.99999% reliability but the mobile data network only has 99.99%, you've wasted a lot of money on something most folks will never notice.