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SAA-020
An application on EC2 needs to access a shared file system. When multiple EC2 instances across multiple AZs need simultaneous access to the same files, which storage is the most appropriate?
Choices
Explanation and answer
Read the explanation and correct answer (spoiler — try answering first)
Correct answer: D. Amazon EFS
Explanation
Amazon EFS is a managed NFS file system that supports shared access from multiple EC2 instances. It suits shared file access across multiple AZs.
Per-choice explanations
A. Amazon EBS
Incorrect. EBS is block storage that is typically attached to an EC2 instance in a single AZ.
B. Amazon S3 Glacier Deep Archive
Incorrect. Glacier Deep Archive is for long-term archives and is not suitable for frequent shared access.
C. Instance store
Incorrect. Instance store is ephemeral storage, not a shared file system.
D. Amazon EFS
Correct. It is suited to shared file storage.
Practical Use
Amazon EFS is a fully managed file system that can be mounted concurrently from multiple EC2 instances using NFSv4. By creating a mount target in each AZ, multiple EC2 instances across multiple AZs can read and write the same file tree simultaneously. The file system itself is redundant within the Region and remains available even during an AZ failure. It fits naturally with requirements such as shared content for web servers, CI/CD workspaces, data volumes for containers, and dataset sharing for machine learning. Alternatives are EBS and S3. EBS is block storage attached to a single EC2 instance, and even with the Multi-Attach feature it requires being in the same AZ and using a cluster file system, so it is not suited to general cross-AZ sharing. S3 is object storage operated by key-based APIs rather than a file system API (POSIX-compatible), so existing applications that use "file open, read, write" cannot use it as-is. The strength of EFS is enabling cross-AZ sharing for existing applications that expect a POSIX file system, with little to no modification. On the other hand, choosing the wrong performance mode (General Purpose or Max I/O) or throughput mode (Bursting, Provisioned, or Elastic) can cause latency or throughput to fall short of requirements, so the design must reflect the workload characteristics. On SAA-C03, when keywords such as "shared file access from multiple EC2," "simultaneous access across multiple AZs," or "managed NFS" appear, EFS is the correct answer. Distractors include EBS Multi-Attach (limited to a single AZ and requires a cluster file system), "mount S3 to share" (API mismatch and different consistency model), and instance store (ephemeral storage that is lost when the EC2 stops).