S3 vs EFS in AWS
S3 and EFS are both storage services in AWS, but they operate on completely different interfaces. S3 is an HTTP API for immutable objects. EFS is a network file system you mount like a disk. Picking the wrong one forces either a code rewrite or an unnecessarily expensive storage bill.
The core distinction
S3 is object storage. You interact with it through an API: upload a file with a PUT request, download it with a GET, delete it with a DELETE. Objects are identified by a key (a path-like string) and are associated with a bucket. Once an object is written, you can’t modify part of it — you overwrite the whole object. There is no concept of file locking, directory traversal, or append operations. S3 is accessible from anywhere with an HTTPS request and correct credentials.
EFS is a network file system. You mount it on an EC2 instance, ECS container, or EKS pod using the NFS protocol, and it appears as a directory path on the filesystem (/mnt/efs). Applications interact with it using standard file system calls (open(), read(), write(), mkdir(), rename()). It supports POSIX permissions, file locking, append operations, and directory traversal. Multiple EC2 instances can mount the same EFS volume simultaneously and read/write concurrently.
The distinction matters for application compatibility: applications that use open("/data/config.json") can’t use S3 without code changes. Applications that make HTTP requests to download files can’t use EFS without NFS mounts.
S3 vs EFS comparison
| Dimension | S3 | EFS |
|---|---|---|
| Interface | HTTP API (AWS SDK, CLI, console) | POSIX file system via NFS mount |
| File locking | No | Yes — POSIX advisory locks |
| Concurrent writes | Last write wins per object | Full concurrent read/write from multiple clients |
| Storage cost | S3 Standard: $0.023/GB/month | EFS Standard: $0.30/GB/month |
| Access from Lambda | Native SDK — any Lambda function | Yes, via VPC mount — requires VPC configuration |
| Data mutability | Objects are immutable — overwrite the whole object | Files are mutable — append, overwrite portions |
| Global accessibility | Yes — accessible from any region or public internet | No — mounted within a VPC, regional |
| Directory structure | Simulated (key prefixes) — no real directories | Real POSIX directories |
| Max object/file size | 5 TB per object | File size limited by filesystem — effectively unlimited |
| Latency | Milliseconds (first byte) | Sub-millisecond for small files when warm |
When to use S3
Storing media, backups, and static assets. Images, videos, documents, backups, exports — any file that is stored and retrieved as a whole object belongs in S3. The cost advantage is significant: S3 Standard at $0.023/GB is 13x cheaper than EFS Standard at $0.30/GB.
Lambda code packages and container images. Lambda deployment packages and ECR container images are stored in S3 (ECR uses S3 under the hood). Application artifacts belong in S3.
Data lake foundation. S3 is the standard data lake storage layer in AWS. Redshift Spectrum, Athena, EMR, and Glue can all query data directly from S3 in Parquet, ORC, CSV, or JSON format. EFS has no equivalent integration with analytics services.
Static website hosting. S3 can host static websites (HTML, CSS, JavaScript) directly. Combined with CloudFront, this is a common pattern for high-performance static site delivery.
Webhook payloads and large Lambda inputs. When a payload exceeds Lambda’s 6MB invocation payload limit, store it in S3 and pass the S3 key instead. Lambda reads from S3 within the function.
Globally distributed access. S3 is accessible from any AWS region and from the public internet (with appropriate permissions). EFS is regional and requires NFS mount within a VPC.
When to use EFS
Shared storage between multiple EC2 instances. If you have an Auto Scaling Group of EC2 instances that need to share files — uploaded content, shared configuration, processed data that any instance might read — EFS provides the shared filesystem they can all mount simultaneously. S3 works for this in some scenarios, but applications that use file system calls require EFS.
Content management systems with file uploads. WordPress, Drupal, Magento, and similar CMS platforms write uploaded files to the local filesystem. When you scale to multiple instances, each instance must see the same files. EFS is the standard solution — all instances mount the same EFS filesystem and see the same uploads directory.
Legacy applications requiring a filesystem. Applications built before S3 existed, or built to run on-premises with a NAS, use standard file system calls. Migrating them to use the S3 API is a code change. Mounting EFS allows them to run unchanged — the application doesn’t know it’s using network storage.
Shared model weights for ML inference. Multiple ECS tasks or EKS pods serving ML inference can mount the same EFS volume containing model weights. Load the model from the shared filesystem rather than downloading it from S3 on every cold start. EFS latency for subsequent reads from in-memory page cache is sub-millisecond.
EFS is not appropriate for: high-performance random I/O workloads (use EBS io2), databases (use EBS or a managed database service), or any workload where the $0.30/GB cost can’t be justified vs S3’s $0.023/GB.
EFS vs EBS: a related comparison
EFS is often confused with EBS (Elastic Block Store). They’re both file systems you attach to EC2, but they’re different:
EBS (Elastic Block Store): A block storage volume attached to a single EC2 instance. Think of it as a virtual hard disk. Fast I/O, low latency, great for OS volumes, databases, and workloads that need single-instance attached storage. EBS Multi-Attach exists but has constraints (only io1/io2 volumes, same AZ, up to 16 instances).
EFS (Elastic File System): A network file system mounted over NFS that can be shared across thousands of instances across multiple AZs. Slower than EBS for single-instance workloads but enables the shared access that EBS can’t provide.
Rule of thumb: if you need one EC2 instance and fast disk access, use EBS. If you need multiple EC2 instances to share a file system, use EFS.
EFS pricing ($0.30/GB) vs EBS GP3 ($0.08/GB) is another consideration — EFS is significantly more expensive than EBS for single-instance workloads, making EBS the right choice when shared access isn’t needed.
Quick decision guide
- Choose S3 if: you’re storing objects (images, videos, backups, artifacts), your application can use the S3 API, you need global accessibility, or cost is a primary concern ($0.023/GB vs $0.30/GB for EFS).
- Choose EFS if: your application requires a POSIX filesystem (file locking, directory structure, standard file calls), multiple EC2 instances need to share files simultaneously, or you’re running a CMS or legacy app that writes to local disk.
- EFS vs EBS: EFS is for shared multi-instance access; EBS is for single-instance fast block storage. Use EBS for single-instance workloads, EFS only when you need shared access.
- The cost difference is real: EFS at $0.30/GB is 13x more expensive than S3 Standard. Use S3 wherever the API is compatible with your application.
Frequently asked questions
What is the difference between S3 and EFS?
S3 is object storage — you read and write files via an HTTP API (GET, PUT, DELETE). Objects are immutable once written. S3 is globally accessible and massively scalable. EFS is a network file system (NFS) — you mount it on EC2 instances or containers and use it like a local filesystem with a path structure, file locking, and simultaneous read/write from multiple instances. EFS is much more expensive per GB than S3.
Can multiple EC2 instances write to EFS at the same time?
Yes — this is EFS's main advantage over EBS. EFS is a shared file system that can be mounted simultaneously on thousands of EC2 instances, ECS tasks, or EKS pods. All of them can read and write concurrently. EBS volumes can only be attached to one EC2 instance at a time (unless you use EBS Multi-Attach, which has constraints).
Why would I use EFS when S3 is so much cheaper?
When your application requires a POSIX file system — one that supports file locking, directory structures, inode-based access, and read/write operations using standard file system calls. Applications built to use a local disk cannot use S3 without code changes. EFS provides the file system interface those applications expect, mounted over NFS to multiple instances simultaneously.