Troubleshooting EFS
Overview
Domain expertise for diagnosing and resolving Amazon EFS issues. Covers mount failures, NFS connectivity, IAM and POSIX permissions, throughput and performance, and encryption problems.
For authoritative guidance, see EFS Troubleshooting.
Common Tasks
0. Verify Dependencies
- You MUST verify
awsCLI is available - You MUST check if
amazon-efs-utilsornfs-utilsis installed on the instance - You MUST ONLY check for tool existence and version — MUST NOT execute destructive or mutating commands during verification
- You MUST inform the user if any required tools are missing
- You MUST respect the user's decision to abort if tools are unavailable
- You SHOULD explain what each step does and why before executing it
- You SHOULD display write commands and wait for user confirmation before executing
1. Classify the Issue
2. Category A — Missing NFS Client
3. Category B — Network/Security Group
Connection timeout is the #1 EFS mount failure — almost always security groups.
- Verify mount target exists in the instance's AZ:
-
Verify security groups — check BOTH directions:
- Mount target SG:
aws ec2 describe-security-groups --group-ids sg-MT— MUST have inbound TCP 2049 from compute SG - Compute SG: MUST have outbound TCP 2049 to mount target SG
- Quick fix:
aws ec2 authorize-security-group-ingress --group-id sg-MT --protocol tcp --port 2049 --source-group sg-COMPUTE
- Mount target SG:
-
Test connectivity:
Note: These security group troubleshooting steps also apply to S3 Files. The only difference is S3 Files uses
aws s3files list-mount-targetsinstead ofaws efs describe-mount-targets.
4. Category C — IAM/Permissions
"access denied by server" with -o iam:
- Check identity-based IAM policy has
elasticfilesystem:ClientMount - Check file system resource policy:
Note: IAM authorization is only enforced when a file system policy exists that requires it. Without a file system policy, any client in the VPC with port 2049 access can mount — even with -o iam. To enforce IAM, you MUST create a file system policy that denies anonymous access.
POSIX permission denied (not IAM):
- Check file/directory ownership:
ls -la /mnt/efs/ - Use access points to enforce UID/GID for consistent permissions
5. Category D — Performance
Check throughput mode:
Burst credit exhaustion (Bursting mode only):
If credits near zero, switch to Elastic throughput:
General Purpose vs Max I/O:
- Check
PercentIOLimitmetric — if consistently >80%, consider Max I/O - Note: performance mode is IMMUTABLE — must create new FS and migrate
6. Category E — Encryption/KMS
NFS server error on encrypted FS = KMS key issue.
- Verify key is enabled in KMS console
- Verify EFS service-linked role has KMS permissions
- If key deleted: cancel deletion if within grace period
7. Category F — VPC DNS
DNS resolution failure = VPC DNS settings disabled.
Both MUST be true. If not:
Troubleshooting
Mount hangs then times out
Most common cause: security group. Verify TCP 2049 is open between compute and mount target.
Auto-mount fails on reboot
/etc/fstab entry MUST include _netdev option to wait for network before mounting.
"nfs not responding" after reconnect
Old kernel bug with TCP port reuse. Update kernel or add noresvport mount option.
Enable Debug Logs
Set logging_level = DEBUG in /etc/amazon/efs/efs-utils.conf. Logs at /var/log/amazon/efs/mount.log.
Collect Logs for AWS Support
Security Considerations
- IAM authorization is only enforced when a file system policy exists — without one, any VPC client with port 2049 access can mount
- When troubleshooting access denied, verify both identity-based and resource-based policies
- Use
-o tlsfor encryption in transit — unencrypted NFS traffic is visible on the network - Restrict
/var/log/amazon/efs/access — logs may contain file system IDs and mount target IPs
