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SAA-C03 Practice Question 8: Shared File Storage Across AZs

AWS Certified Solutions Architect – Associate (SAA-C03) · Test 1 · question 8 of 10

Storage Architecturemedium

A content management application currently runs on a single Amazon EC2 Linux instance and stores uploaded documents on an attached io2 EBS volume mounted at /var/www/uploads. The team is moving it to an Auto Scaling group spread across three Availability Zones.

Every instance must be able to read and write the same POSIX directory, and the application code, which uses ordinary file system calls, cannot be changed. Which storage design should the architect choose?

Select one answer
Show answer and explanation

Answer:

  • D. Create an Amazon EFS file system with a mount target in each Availability Zone and mount it on every instance

Amazon EFS is a managed, elastic NFS file system designed to be mounted concurrently by many EC2 instances. You create one mount target per Availability Zone, mount it at /var/www/uploads, and the application keeps issuing the same POSIX file system calls it always has, no code change. It scales storage automatically as documents accumulate and is the standard answer whenever "shared file system across Availability Zones" appears in a scenario.

Option A is the trap, and it fails on the most important detail in the stem. EBS Multi-Attach requires all attached instances to be in the same Availability Zone, so it cannot span the three AZs of the Auto Scaling group at all. It is also capped at 16 Nitro instances, cannot be enabled during instance launch via RunInstances, and, decisively, standard file systems such as XFS and ext4 are not designed for simultaneous access by multiple servers. Mounting ext4 from several instances corrupts the file system; Multi-Attach expects a clustered file system.

Option B changes the storage semantics the application depends on. S3 is object storage; a FUSE shim makes it look like a directory but does not provide real POSIX semantics, no proper file locking, expensive partial writes and renames, and unsupported third-party software in the critical path of a production app.

Option C gives each instance its own private copy of the data as of the last snapshot. Nothing is shared: a document uploaded through one instance is invisible to the others and is lost when that instance scales in. Snapshots are a backup mechanism, not a sharing mechanism.

Reference: What is Amazon Elastic File System?

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