An Auto Scaling group of EC2 instances across three AZs must all read and write the same file system, which must remain available if one AZ fails. Which storage service fits?
A.A single EBS volume attached to all instances
EBS volumes attach to a single instance in one AZ (multi-attach exists only for specific io1/io2 use cases within one AZ).
B.Amazon EFS with mount targets in each AZ
Correct. EFS provides shared file access across AZs.
C.Instance store volumes on each instance
Instance store is ephemeral, per-instance, and lost on stop/termination.
D.An S3 bucket mounted as a block device
S3 is object storage, not a mountable block device.
Why this answer
EFS is a managed, multi-AZ, NFS-style file system that many instances across AZs mount concurrently — exactly the shared, highly available file storage described.
Correct answer: B · Last reviewed 2026-10-25
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