AWS Storage Services Compared: S3 vs EBS vs EFS

Storage questions are a reliable fixture on the AWS Certified Cloud Practitioner (CLF-C02) exam. The exam falls under Domain 3: Cloud Technology and Services, which carries a 34% weighting — the heaviest of all four domains. That means understanding when to use Amazon S3, Amazon EBS, or Amazon EFS isn't optional; it's essential.

The good news: once you understand what each service is fundamentally designed for, choosing between them becomes straightforward. This guide breaks down all three, compares them side by side, and walks through the kinds of scenario-based questions you'll see on exam day.

Disclaimer: This site is an independent exam-prep resource and is not affiliated with, endorsed by, or sponsored by Amazon Web Services, Inc. or its affiliates.

The Big Picture: Three Different Storage Models

AWS offers three broad types of storage, and each of these services represents one of them:

• Object storage — Amazon S3
• Block storage — Amazon EBS
• File storage — Amazon EFS

Understanding why these categories exist is more useful than memorising product names.

Object Storage: Flat, Scalable, and Durable

In object storage, data is stored as individual objects — think of each file bundled with its metadata and a unique identifier. There's no folder hierarchy at the operating system level (though S3 supports prefixes that look like folders). Objects are retrieved via a URL or API, not a file path.

Block Storage: Fast, Low-Latency, Attached

Block storage breaks data into fixed-size blocks, each with its own address. Your operating system treats it like a physical hard drive — it can be formatted, partitioned, and mounted. This is what your laptop's SSD does, and it's what EC2 instances need to run their operating systems.

File Storage: Shared, Hierarchical, and Familiar

File storage sits between object and block. It uses a familiar directory structure and is accessed over a network using standard protocols. Multiple clients can connect to the same file system simultaneously — something block storage can't do natively.

Amazon S3 (Simple Storage Service)

Amazon S3 is AWS's object storage service and one of the oldest and most widely used services in the entire AWS portfolio.

Key Characteristics

• Virtually unlimited storage capacity — you don't provision a volume size in advance.
• Accessed via HTTP/HTTPS — through the AWS Management Console, SDKs, CLI, or direct URL.
• Highly durable — S3 is designed to deliver very high durability for stored objects (verify current durability SLA against AWS S3 documentation before publishing).
• Not attached to a specific EC2 instance — any authorised user, application, or service can access it from anywhere.
• Not a file system — you cannot mount S3 as a drive your EC2 operating system reads from natively.

S3 Storage Classes

S3 offers multiple storage classes, letting you balance cost against access frequency. From most to least frequently accessed:

1. S3 Standard — general-purpose, low latency, high availability.
2. S3 Intelligent-Tiering — automatically moves objects between tiers based on usage patterns.
3. S3 Standard-IA (Infrequent Access) — lower storage cost, but a retrieval fee applies.
4. S3 One Zone-IA — same as Standard-IA but stored in a single Availability Zone; lower cost, less resilience.
5. S3 Glacier Instant Retrieval — archival storage with millisecond retrieval.
6. S3 Glacier Flexible Retrieval — archival with retrieval times ranging from minutes to hours.
7. S3 Glacier Deep Archive — lowest cost; retrieval can take up to 12 hours.

When to Choose S3

S3 is the right answer when the question involves: - Storing images, videos, documents, or backups - Hosting static website assets - Data lake storage for analytics - Log archiving - Sharing files across the internet or between services without a shared file system

Amazon EBS (Elastic Block Store)

Amazon EBS provides block storage volumes that attach to EC2 instances. Think of it as the virtual hard drive for your virtual server.

Key Characteristics

• Attached to a single EC2 instance at a time (with some exceptions for Multi-Attach on specific volume types — verify current Multi-Attach availability in AWS EBS documentation before publishing).
• Provisioned in advance — you choose a volume size (e.g., 20 GB, 500 GB) when you create it.
• Persists independently of the instance — if you stop or terminate your EC2 instance, the EBS volume (and its data) can survive if configured to do so.
• Lives in a single Availability Zone — an EBS volume is tied to the AZ where it was created. To use it in another AZ, you'd create a snapshot and restore it there.
• Low latency — designed for workloads that require consistent, low-latency I/O, such as databases and boot volumes.

EBS Volume Types

EBS offers several volume types optimised for different workloads — general-purpose SSD (gp2/gp3), provisioned IOPS SSD (io1/io2), and magnetic (st1, sc1, standard). For the CLF-C02 exam, you don't need to memorise every volume type, but understanding that options exist for different performance needs is useful.

When to Choose EBS

EBS is the right answer when the question involves: - The root volume (OS disk) of an EC2 instance - A relational database running on EC2 that needs fast, consistent disk I/O - Any workload that requires a formatted, mountable file system on a single server

Amazon EFS (Elastic File System)

Amazon EFS is a managed network file system that multiple EC2 instances can access simultaneously.

Key Characteristics

• Shared access — dozens, hundreds, or even thousands of EC2 instances can mount the same EFS file system at the same time.
• Fully managed and elastic — capacity grows and shrinks automatically as you add or remove files. You don't provision a size in advance.
• Accessible across multiple AZs — unlike EBS, EFS is a regional resource (for standard storage class) and can be mounted by instances in different Availability Zones within the same region.
• Uses NFS protocol — it appears to your Linux EC2 instances as a standard network file system.
• Linux-only — EFS is designed for Linux workloads. For Windows shared file storage, AWS offers Amazon FSx (not covered on CLF-C02 at the same depth, but worth knowing exists).

When to Choose EFS

EFS is the right answer when the question involves: - A web application where multiple web servers need to read and write to a shared set of files - A content management system where editors on different servers upload media - Machine learning workflows sharing a large dataset across many compute nodes - Any scenario explicitly mentioning "shared file system" or "concurrent access from multiple instances"

Side-by-Side Comparison

How This Appears on the CLF-C02 Exam

The CLF-C02 exam won't ask you to recite specs from memory. Instead, it presents short scenarios and asks which service fits best. Here's how to think through them:

"A company needs to store customer-uploaded photos and serve them via a web application." → S3. It's object storage, cost-effective at scale, and accessible over HTTP.

"A developer is launching an EC2 instance running a MySQL database and needs persistent storage." → EBS. The database needs a mounted, low-latency block device attached to that specific instance.

"A company runs a fleet of 50 web servers that all need to read from the same configuration files." → EFS. Shared file access across multiple instances is the hallmark of EFS.

If you want to test your instincts on these scenario types right now, our practice questions include a dedicated set covering AWS storage services.

Before your exam, it's also worth checking your current knowledge level with our diagnostic quiz — it'll highlight any gaps across all four exam domains.

FAQ

What is the main difference between S3 and EBS?

S3 is object storage accessed via an API or URL — it's not mounted as a drive. EBS is block storage that attaches directly to a single EC2 instance and behaves like a physical hard disk. Use S3 for storing files, backups, and media at scale; use EBS when an EC2 instance needs a disk to run its operating system or database.

Can multiple EC2 instances use the same EBS volume at the same time?

Generally, no. Standard EBS volumes are attached to one EC2 instance at a time. EFS is the correct choice when you need shared, concurrent access across multiple instances.

Is S3 cheaper than EBS?

S3 storage generally costs less per GB than EBS, but the right choice is always based on the use case rather than price alone. EBS is necessary for block-level workloads (like running a database), where S3 simply isn't a substitute. For archiving or large-scale object storage, S3's various storage classes (especially Glacier) make it very cost-effective.

Do I need to choose an S3 capacity size in advance, like I do with EBS?

No. S3 scales automatically — you just upload objects and pay for what you use. With EBS, you provision a specific volume size (like 100 GB) when you create it, though you can modify the size later.

How does EFS differ from EBS for Linux workloads?

Both can be mounted on Linux EC2 instances, but EBS attaches to one instance at a time and lives in a single Availability Zone, while EFS is a shared network file system that can be mounted by many instances across multiple Availability Zones simultaneously.

Which storage service questions are most likely to appear on the CLF-C02 exam?

The exam focuses on scenario-based questions — matching a business requirement to the right service. You're most likely to see questions contrasting S3 (object, scalable, shared access via URL) with EBS (block, single instance, provisioned) and EFS (file, shared among many instances). Review the exam guide to see exactly how storage fits into the Domain 3 objectives.

Putting It All Together

AWS storage isn't complicated once you anchor each service to its storage model: S3 for objects, EBS for blocks, EFS for shared files. The CLF-C02 exam rewards this kind of conceptual clarity — not rote memorisation of gigabyte prices or IOPS limits.

When you see a storage question on exam day, ask yourself three things: 1. Does this need to be mounted as a disk on a single server? → EBS 2. Does this need to be shared across multiple servers simultaneously? → EFS 3. Is this about storing files, backups, or media that are accessed via the internet or APIs? → S3

With that mental model in place, you'll navigate storage scenarios with confidence.