Diagnosing and Optimizing SMB Performance Between Two M1 Macs

Table of Contents

• 1. Diagnosing the Problem
• 2. Diagnosing SMB-Specific Issues
• 3. Optimizing macOS for SMB
• 4. Testing Performance After Optimization
• 5. Expected Improvements

1. Diagnosing the Problem

1. Check Network Speed

   • Use iperf3 to measure network throughput between the two Macs.

     # On Mac B (SMB Server)
     iperf3 -s
     
     # On Mac A (Client)
     iperf3 -c <Mac B's IP>

   • If the speed is close to 1Gbps, the network is performing as expected.

   • If it’s significantly lower (e.g., below 500 Mbps), there may be a bottleneck in the network hardware or cabling.

2. Check Latency with ping

   • Use ping to measure round-trip time between Mac A and Mac B.

     ping -c 10 <Mac B's IP>

   • A response time below 1ms is expected.

   • If latency is higher, check for network congestion or interference.

3. Test File Transfer Speed

   • Copy a large file using rsync to measure SMB performance.

     rsync --progress /Volumes/SharedDrive/testfile.bin ~/Desktop/

   • A well-performing SMB connection on a 1Gb network should achieve speeds close to 110 MB/s.

2. Diagnosing SMB-Specific Issues

SMB Version

Check which SMB version is being used.

smbutil statshares -a

• If the SMB version is 2.1, it may not be negotiating SMB 3 properly.

Forcing SMB 3

• Modify /etc/nsmb.conf to enforce SMB 3.

  [default]
  protocol_vers_map=6

• Restart SMB services:

  sudo launchctl stop com.apple.smbd
  sudo launchctl start com.apple.smbd

3. Optimizing macOS for SMB

Finder Metadata Indexing

• Finder generates thumbnails and previews, which can slow down SMB access.

• Disable animations and metadata caching:

  defaults write com.apple.finder DisableAllAnimations -bool true
  killall Finder

Opportunistic Locking

• Disabling OpLocks can improve performance. Add to /etc/nsmb.conf:

  [default]
  oplocks=no

• Restart SMB services:

  sudo launchctl stop com.apple.smbd
  sudo launchctl start com.apple.smbd

Spotlight Indexing

• Prevent Spotlight from indexing SMB shares:

  sudo mdutil -i off /Volumes/SharedDrive

• Permanently exclude the SMB share from indexing:

  echo "/Volumes/SharedDrive" >> ~/.metadata_never_index

Disabling SMB Signing

SMB signing adds security but reduces performance. It is enabled by default on macOS, which can impact file transfer speeds ^[1].

1. Check if SMB signing is enabled:

   sysctl -a | grep net.smb.fs.kern.signed

   • If it returns 1, signing is enabled.

2. Disable SMB signing:

   [default]
   signing_required=no

3. Restart SMB services:

   sudo launchctl stop com.apple.smbd
   sudo launchctl start com.apple.smbd

4. Testing Performance After Optimization

• Re-run iperf3 to verify network speeds.

• Use cp or rsync to test file transfer speeds.

• If performance is still slow, consider using NFS instead of SMB for macOS-to-macOS file sharing ^[2].

5. Expected Improvements

• Enabling SMB 3: 10–20% improvement in speed.

• Disabling SMB signing: Up to 2× faster transfers.

• Disabling Finder previews: Faster browsing of network shares.

• Preventing Spotlight indexing: No lag when accessing files.

• Using rsync instead of Finder: More consistent transfer speeds.

After applying these optimizations, SMB performance should be significantly improved, reducing latency and maximizing transfer speeds across the 1Gb network.

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1. Apple Support - SMB Signing

2. Apple Stack Exchange - SMB vs NFS