An application writes uploaded customer files to a local NVMe instance store volume on its EC2 instance, alongside its temporary scratch data. The instance was stopped for a maintenance change and started again an hour later. The uploaded files are gone; the operating system and the application, which live on the root EBS volume, are intact. What explains this?
AWS Certified Cloud Practitioner (CLF-C02), objective 3. Cloud technology and services hard
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The options
Correct Instance store volumes are ephemeral: their data does not survive stopping or terminating the instance
Correct. An instance store is physically attached to the host the instance is running on. Stopping the instance releases that host, so the data goes with it. A reboot keeps the same host and therefore keeps the data, which is exactly why this failure mode hides for months.
Not correct The instance store volume was not encrypted, so AWS discarded it
Wrong. Encryption protects data from being read by the wrong party; it has no bearing on whether the data persists. AWS never deletes your data because it is unencrypted.
Not correct Starting the instance moved it to a different Availability Zone, leaving the volume behind
Wrong. A stopped instance starts again in the same Availability Zone, and its EBS volumes follow it. That would not be the mechanism even if it did move.
Not correct The root volume filled up and the operating system deleted the oldest files
Wrong. A full disk produces write errors at the time of the write, and no Linux or Windows system deletes user data to make room. It also would not explain why only the instance store's contents vanished.
Why
Two kinds of disk attach to an EC2 instance and they behave nothing alike. Instance store is local to the physical host: the fastest option, included in the instance price, and lost the moment the instance stops or is terminated. EBS is network-attached: it persists independently of the instance, can be detached and reattached, and can be snapshotted. Use instance store for caches, scratch space and data that is replicated elsewhere; use EBS for anything you would miss.
Where this comes from
- Cited
- AWS exam guide task statement 3.3
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