S3 availability and durability — AWS S3 Solutions Architect Exam Topic
What is the durability of RRS?
What is the availability of RRS?
you will be able to answer this frequent AWS SA exam topic by understanding the reason and not just by memorizing which you will soon forget.
Short note on comparison between availability and durability between S3 tiers. Asking actual decimal number of availability and durability of S3 storage tiers are frequently asked question in Solutions Architect exam.
I drew up this chart because I could not find any documentation online that has S3 OneZone-IA and RRS(Reduced Redundancy Storage) in a single chart. This might be because RRS is going to be replaced by OneZone-IA but still there is a possibility for outdated question to pop up in the exam so I thought I’d rather put this into one chart to take a peek right before the exam.
This information is gathered from below three documentations of AWS. What I was confused when solving some example questions is that they do not specify whether they are asking for ‘designed for’ durability / availability or ‘SLA(service level agreement’ durability / availability of each storage tier.
It seems like unless the question specifies otherwise, the answer should be based on ‘designed for’ durability and availability.
Above is a great blog post about data availability and durability. Durability of data is associated with redundancy. so it is understandable that RRS(Reduced Redundancy Storage)is the only storage tier that has relatively lower Durability. Availability is about system uptime. S3 Standard and S3 RRS only differs in redundancy of data. Therefore, it makes sense for the two to have same “designed for” availability of 99.99%. S3 Standard-IA and S3 One Zone-IA each has 99.9% and 99.5% ‘designed for’ availability which are gradually lower than S3 Standard and S3 RRS. Having redundancy in one zone must have lower total redundancy which makes the difference between Standard-IA availability and OneZone-IA reasonable.
So set the baseline of Durability as 99.999999999%, Availability as 99.99% and calculate durability and availability of each storage tier based on their performance vs cost compromises. you will be able to infer rest of the numbers.
