AWS Storage Explained: S3 vs EBS vs EFS, and When to Use Which

AWS offers three storage services: S3, EBS, and EFS. While they may look similar, they serve entirely different purposes, and using the wrong one can result in complicated architectures and unexpected costs. However, once you understand the key difference between them, deciding which one to use becomes straightforward. The main distinction lies in how your application accesses the storage.

EBS (Elastic Block Store) is a virtual hard drive attached to a single EC2 instance. Your operating system treats it just like a local disk, enabling you to read and write data as you would on a traditional drive. EBS is the ideal choice for the boot volume of an EC2 instance and for databases or applications that run on that instance and require fast, low-latency block access. If your application needs a dedicated disk, then EBS is the right choice.

EFS (Elastic File System) is a managed network file system that allows multiple instances to mount the same file system simultaneously, accessing the same files. It can automatically scale and spans Availability Zones. EFS is the go-to option when multiple servers need to share the same files. This shared file system is perfect for workloads such as serving identical content on a fleet of web servers, creating a shared home directory, or managing a content management system across instances.

However, it is more expensive per GB than EBS and has network-file-system latency, making it unsuitable for high-performance databases. Use EFS when several instances need to access and collectively manage the same set of files.

S3 (Simple Storage Service) is a completely different kind of storage compared to EBS and EFS. Instead of a disk or a file system, S3 stores objects (files plus metadata) in buckets and allows access to them through an HTTP API. Your application interacts with S3 using SDK calls, not traditional filesystem reads. S3 provides virtually unlimited scale, exceptional durability, and is significantly cheaper per GB compared to EBS and EFS.

The objects stored in S3 can be accessed from anywhere over the network, not tied to a specific instance. You cannot browse through folders or directories like you would with a filesystem; instead, you request individual objects using their unique key. S3 is best suited for backing up data, storing static assets (images, videos, downloads), creating data lakes, handling logs, or hosting static websites.

If your application stores files that are accessed as objects rather than being edited in place, then S3 is the most cost-effective and scalable solution.

A helpful rule to determine which storage service to use is to ask two questions: "Who needs to access this data, and how?" If one instance needs a fast local disk for its operating system, database, or application, then EBS is the right choice. If multiple instances require access to the same files in real-time, use EFS. Lastly, if you are storing files that an application reads and writes as objects over an API (such as assets, backups, logs, or data), then S3 is the optimal option.

In most cases, the answer to these questions will be clear once you frame them this way. A common mistake is using EBS or EFS for situations that would be better suited for S3, which can lead to stateful applications, scalability issues, and increased costs. Moreover, EBS gp3 volumes are currently cheaper and more efficient than the older gp2 volumes.

In summary, S3, EBS, and EFS cater to distinct needs, and understanding the primary difference between them allows you to make the right choice every time. For most situations, starting with S3 is the best and most cost-effective choice.