Storage choices drive both performance and cost, the second and fourth exam domains. Choosing S3 Standard when Glacier would do can multiply a bill; choosing EBS when EFS was needed can break a multi-instance design.

The exam constantly asks you to match a durability, access pattern, and cost requirement to exactly one storage service and class.

Amazon S3 is object storage with eleven nines (99.999999999%) of durability by design. Its storage classes trade retrieval speed and cost: S3 Standard (frequent access), S3 Standard-IA and One Zone-IA (infrequent, lower storage cost + retrieval fee), S3 Intelligent-Tiering (auto-moves objects by access pattern), and the Glacier family — Glacier Instant Retrieval, Glacier Flexible Retrieval, and Glacier Deep Archive — for archival at the lowest cost with longer retrieval times.

Lifecycle rules automatically transition objects between classes and expire them, so you don't overpay for cold data.

Block and file storage differ fundamentally. EBS is a network block volume attached to one EC2 instance (in one AZ). EFS is an elastic NFS file system that many instances across AZs can mount concurrently. S3 is not a filesystem at all — it's accessed over HTTP APIs.

A fleet of EC2 instances across three AZs must share a common set of files with POSIX semantics and read/write concurrency. Which storage?

The answer is EFS, because it can be mounted by many instances across AZs simultaneously. EBS is wrong — a standard EBS volume attaches to a single instance in a single AZ. S3 is wrong — it lacks POSIX file semantics.

Now flip it: archive compliance logs accessed maybe once a year, retrieval within 12 hours acceptable, lowest cost. That's S3 Glacier Deep Archive, ideally reached via a lifecycle rule that transitions logs there after 90 days.

Storage overlaps heavily with cost optimization — lifecycle policies and Intelligent-Tiering are core cost-saving tools. It also touches security: S3 supports encryption at rest (SSE-S3, SSE-KMS) and access control via bucket policies and Block Public Access.

Durability versus availability is a recurring distinction: S3 Standard offers 11 nines durability but 99.99% availability; One Zone-IA keeps the same durability within one AZ but loses data if that AZ is destroyed. The exam loves this trade-off.