I have a SQL Server (2008 R2) WSFC on Windows Server 2008 R2 on which I am experiencing some storage latency between the OS and SAN. The SAN admin has noted that the SAN is experiencing very little pressure on the LUNs associated with the server noting that the highest response time is 8ms.

However, the OS and SQL Server is not getting that quick enough. I found a few questions referring to TCP Chimney Offloading and some issues that it may cause so I've been exploring the configuration on the server.

Following this guide, I see that at the OS level, Chimney Offload State is automatic

Querying active state...

TCP Global Parameters
Receive-Side Scaling State          : enabled
Chimney Offload State               : automatic
NetDMA State                        : enabled
Direct Cache Acess (DCA)            : disabled
Receive Window Auto-Tuning Level    : normal
Add-On Congestion Control Provider  : ctcp
ECN Capability                      : disabled
RFC 1323 Timestamps                 : disabled

However, the settings on the adapters in Device Manager show TCP Connection Offload (IPv4) as "Disable."

OS: Windows Server 2008 R2 Enterprise Edition SP1 Network Adapters: HP NC551i Dual Port FlexFabric 10Gb Converged Network Adapter.

There are 8 of these FlexFabric adapters (which are the ones that I checked for chimney offload) but only 3 are enabled (one of which is to a "Private" network for the cluster). There is also an HP Network Team to a "Public" network.

What are the implications, if any, of having the chimney offload "automatic" on the OS but disabled at the card?

  • 1
    More CPU. THe problm is not offloading unless you run CPU load problems. I would run a proer IO Test..... And check for queue lengths. – TomTom Dec 13 '13 at 18:25
  • 9 times out of 10, fiddling with the offload / TCP parameters and changing them from their default values, makes stuff worse, not better. – Tom O'Connor Dec 13 '13 at 20:36
  • @TomO'Connor thanks. i'm just wondering if there is any adverse impact to having it disabled on the adapter but enabled on the OS. – swasheck Dec 13 '13 at 21:23

As per Microsoft KB951037 to which you linked:

TCP Chimney Offload can be enabled or disabled in the following two locations:

- The operating system
- The advanced properties page of the network adapter

TCP Chimney Offload will work only if it is enabled in both locations.

By default, TCP Chimney Offload is disabled in both these locations. However,
OEM installations may enable TCP Chimney Offload in the operating system, in
the network adapter, or in both the operating system and the network adapter.

So having offloading disabled in any one place has the effect of disabling the function.

For the 'automatic' setting I believe one must consult the manual, it is instinctively unclear how the combo 'enabled-automatic' would operate for instance.

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