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Troubleshooting File Server Networking Issues in Windows Server 2012 R2

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In this blog post, I’m sharing a few troubleshooting tips related to File Server Networking in Windows Server 2012 R2.

For each one, I first describe the issue that is commonly reported, followed by a quick explanation of the root cause of the issue and finally a way to solve it.

Let me know if those are helpful and feel free to share your own issues related to File Server Networking using the comments.

 

1. Make sure your network interfaces are RSS-capable

 

Issue:

  • Certain 10GbE NICs won’t perform as well as others
  • Multichannel might not aggregate multiple 10GbE in certain configurations

Cause

  • Some 10GbE NICs show as non-RSS capable. Without RSS, SMB uses 1 TCP connection.
  • If mixing RSS and non-RSS NICs, Multichannel will use only the RSS-capable NICs

Solution (both are complete solutions, choose one)

 

2. Use Multiple VNICs on the host

 

Issue

  • When using a VNIC on the host, I cannot achieve maximum performance.
  • This is noticeable when using 10GbE NICs RSS-capable NICs and/or NIC Teaming.

Cause

  • The VNIC on the host is not RSS capable. Without RSS, SMB uses only 1 TCP connection.

Solution

 

3. SMB prefers slow NIC to faster VNICs

 

Issue

  • When using a VNIC on the host, SMB prefers a slower physical NIC to the VNIC.
  • This is important when using 10GbE NICs RSS-capable NICs and/or NIC Teaming.

Cause

  • The VNIC on the host is not RSS capable and the physical NIC is RSS capable.
  • SMB will always prefer RSS NICs to non-RSS NICs, even if at slower speeds.

Solution (both are complete solutions, choose one)

  • Disable the RSS-capability of the physical NIC
  • Use SMB Multichannel Constraints to prefer the VNICs

 

4. More TCP connections for SMB inside a VM

 

Issue

  • In Windows Server 2012, I got 1 TCP connection per VMNIC. In R2 I now get 4.
  • Why has this changed? Is there a problem?

Cause

  • With the new Virtual RSS feature, VMNICs now report themselves as RSS-capable.

Solution

  • This is by design. Enjoy the increased performance...

 

5. More connections to a Scale-Out File Server

 

Issue

  • Windows Server 2012 R2 creates more connections to Scale-out File Servers.
  • Why has this changed? Is there a problem?

Cause

  • Windows Server 2012 used one set of connections per Scale-Out File Server.
  • Windows Server 2012 R2 uses one set of connections per Share on a SOFS,
    when this helps avoid server-side redirection (typical case: Mirrored Storage Spaces)

Solution

  • This is by design. Enjoy the increased performance...

 

6. Use multiple subnets when deploying SMB Multichannel in a cluster

 

Issue

  • When using multiple NICs on the same subnet in a cluster, only one is used

Cause

  • Cluster networking won’t use more than one NIC per subnet
  • You can confirm by using the cmdlet Get-SmbServerNetworkInterface

Solution

 

7. Update firmware and driver for your NICs

 

Issue

  • I’m using an RDMA NIC, but SMB reports the NIC as not RDMA capable.

Cause

  • NICs with older firmware or driver might not report themselves correctly.

Solution

 

8. How much traffic needs to pass between the SMB Client and Server before Multichannel actually starts?

 

Issue

  • SMB3 always starts with single TCP/IP then moves to multiple TCP/IP or RDMA.
  • Concern with timing of initial handshake and speed of transition to faster behavior.

Cause

  • SMB Multichannel is used to discover RSS and RDMA capabilities.
  • For server SKUs, Multichannel starts on the first read or write operation.
  • For client SKUs, Multichannel won’t start unless you’re doing some amount of work.

Solution (both are complete solutions, choose one)

 

9. Can I use SMB3 storage without RDMA?

 

Issue

  • Concern around performance of SMB3 without RDMA NICs

Cause

  • We have talked so much about RDMA...

Solution (both items contribute to the solution)

 

10. Is it possible to run SMB Direct from within a VM?

 

Issue

  • Running file server or SQL Server in a VM
  • Desire to use RDMA networking from the guest

Cause

  • Windows Server 2012 and Windows Server 2012 R2 cannot do RDMA to the guest

Solution (both are complete solutions, choose one)

 

11. Use Client Access network for CSV Traffic

 

Issue

  • Limited performance when accessing a scale-out file server in Windows Server 2012
  • Happens when client hitting non-owner node and redirection is required

Cause

  • The client access network is high speed RDMA but the cluster network is not
  • Redirection is happening over cluster network only (usually a 1GbE NIC)

Solution (both are complete solutions, choose one)

 

12. Single file copy performance

 

Issue

  • Limited performance when copying a single large file to a scale-out file server
  • Using a 10GbE connection, can only achieve less than 150MB/sec throughput

Cause

  • File extension done in 1MB increments, serialized in write-trough mode
  • This leads to loss of the asynchronous nature of SMB2/3

Solution (both items contribute to the solution)

Comments

  • Anonymous
    January 01, 2003
    Great post!
  • Anonymous
    January 25, 2014
    In this post, I'm providing a reference to the most relevant content related to Windows Server 2012
  • Anonymous
    January 26, 2014
    it is always pleasure when reading your articles as they are pure technical and on the right theme.Regarding multichannel I have a question. We have several Hyper-V clusters on all of them im trying to restrict SMB Multichannel for CSV traffic. First I configured CSV network with lowest cluster network metric. After I configured SMB constraints but CSV traffic was going thru fe80:: IPv6 address pointed as Server Name which was found with Get-SmbMultichannelConnection. We found that it is the Microsoft cluster virtual adapter. For the rest SMB traffic the constraints work perfectly but not for the CSV. Disabling SMB multichannel for client and server still does not restrict traffic to pass only the lowest metric adapter.My question is if it is still possible to restrict CSV traffic to pass only specific adapters and how to restrict it?
  • Anonymous
    January 30, 2014
    Hi Jose. You write that "Cluster networking won’t use more than one NIC per subnet" - is this even if we team our NICs in a way that should allow for the use of multiple NICs?
  • Anonymous
    March 30, 2014
    In this post, I'm providing a reference to the most relevant content related to Windows Server 2012
  • Anonymous
    July 07, 2014
    In this post, I'm providing a reference to the most relevant content related to Windows Server 2012