AWS S3: IA vs One-Zone IA Storage Classes

Question

Amazon announced on April 2018 the availability of a new storage class, named One-Zone Infrequent Access, that complements the plain IA option by lowering the cost via the usage of only one AZ for storage.

Its site , advertises that all storage classes have the so-called 11 9s durability.

My question is how two options (IA vs One Zone IA) can have the same (ie 11 9s) durability, given that the later uses 1 AZ (vs multiple of the former)

Amazon S3 Standard, S3 Standard-IA, S3 One Zone-IA, and Amazon Glacier, are all designed for 99.999999999% durability. Amazon S3 Standard, S3 Standard-IA and Amazon Glacier distribute data across a minimum of three geographically-separated Availability Zones to offer the highest level of resilience to AZ loss. S3 One Zone-IA saves cost by storing infrequently accessed data with lower resilience in a single Availability Zone. Amazon S3 Standard-IA is a good choice for long-term storage of master data that is infrequently accessed. For other infrequently accessed data, such as duplicates of backups or data summaries that can be regenerated, S3 One Zone-IA provides a lower price point

ps. I think they are themselves implying that one zone has lower durability.

One Zone-IA saves cost by storing infrequently accessed data with lower resilience in a single Availability Zone

Answer 1

S3 One Zone IA is just as durable from an engineering standpoint as the other storage classes except in the event that the availability zone where your data is stored is destroyed.

S3 One Zone-IA offers the same high durability†, high throughput, and low latency of Amazon S3 Standard and S3 Standard-IA

...

† Because S3 One Zone-IA stores data in a single AWS Availability Zone, data stored in this storage class will be lost in the event of Availability Zone destruction.

https://aws.amazon.com/s3/storage-classes/

So, the claim being made appears to be that 1ZIA is using the same engineering design as other storage classes -- redundant storage media -- except that everything is physically in a single AZ rather than distributed across multiple zones... so it offers comparable durability... except for the case of a catastrophic event involving that AZ.

Answer 2

It seems that having one AZ instead of 2, does reduce the availability :

Amazon S3 One Zone-Infrequent Access (S3 One Zone-IA)

Same low latency and high throughput performance of S3 Standard

Designed for durability of 99.999999999% of objects in a single Availability Zone†

Designed for 99.5% availability over a given year

Backed with the Amazon S3 Service Level Agreement for availability

Supports SSL for data in transit and encryption of data at rest S3

Lifecycle management for automatic migration of objects to other S3 Storage Classes