troubleshooting-efs

SKILL.md

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 aws CLI is available

You MUST check if amazon-efs-utils or nfs-utils is 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

Symptom
Category

"wrong fs type" or mount command fails
A: Missing NFS Client

Connection timed out (hangs 2+ min)
B: Network/Security Group

"access denied by server"
C: IAM/Permissions

Slow throughput or high latency
D: Performance

NFS server error on encrypted FS
E: Encryption/KMS

DNS name resolution fails
F: VPC DNS

2. Category A — Missing NFS Client

# Amazon Linux / RHEL / CentOS
sudo yum -y install amazon-efs-utils  # preferred (includes mount helper + TLS)
# OR
sudo yum -y install nfs-utils

# Ubuntu / Debian
sudo apt-get install nfs-common

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:

aws efs describe-mount-targets --file-system-id fs-ID --region REGION

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

Test connectivity:

nc -zv fs-ID.efs.REGION.amazonaws.com 2049

Note: These security group troubleshooting steps also apply to S3 Files. The only difference is S3 Files uses aws s3files list-mount-targets instead of aws 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:

aws efs describe-file-system-policy --file-system-id fs-ID --region REGION

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:

aws efs describe-file-systems --file-system-id fs-ID --region REGION --query 'FileSystems[0].ThroughputMode'

Burst credit exhaustion (Bursting mode only):

aws cloudwatch get-metric-statistics --namespace AWS/EFS --metric-name BurstCreditBalance --dimensions Name=FileSystemId,Value=fs-ID --period 3600 --statistics Average --start-time $(date -u -d '24 hours ago' +%Y-%m-%dT%H:%M:%S) --end-time $(date -u +%Y-%m-%dT%H:%M:%S)

If credits near zero, switch to Elastic throughput:

aws efs update-file-system --file-system-id fs-ID --throughput-mode elastic --region REGION

General Purpose vs Max I/O:

Check PercentIOLimit metric — 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.

aws ec2 describe-vpc-attribute --vpc-id vpc-ID --attribute enableDnsHostnames
aws ec2 describe-vpc-attribute --vpc-id vpc-ID --attribute enableDnsSupport

Both MUST be true. If not:

aws ec2 modify-vpc-attribute --vpc-id vpc-ID --enable-dns-hostnames Value=true
aws ec2 modify-vpc-attribute --vpc-id vpc-ID --enable-dns-support Value=true

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

sudo tar -czf /tmp/efs-logs.tar.gz /var/log/amazon/efs/ /etc/amazon/efs/efs-utils.conf

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 tls for 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

Additional Resources

EFS Troubleshooting

EFS Performance

EFS Mount Helper