Disable Large Receive feature on Linux Logical Partitions (LPAR) with VIOS - IBM BladeCenter

Resolving The Problem

Source

RETAIN tip: H196947

Symptom

Numerous Transmission Control Protocol (TCP) retransmits, Duplicate Acknowledgments, and Out of Order Segments are seen when network traces are examined for Linux Logical Partitions (LPARs) contained in the Virtual I/O Server (VIOS), resulting in poor network performance.

Affected configurations

The system may be any of the following IBM servers:

• BladeCenter JS12, type 7998, any model
• BladeCenter JS21, type 7988, any model
• BladeCenter JS21, type 8844, any model
• BladeCenter JS22, type 7998, any model
• BladeCenter JS23, type 7778, any model
• BladeCenter JS43, type 7778, any model

This tip is not software specific.

This tip is not option specific.

Solution

This behavior will be corrected in a future release of Integrated Management Module (IMM).

The target date for this release is scheduled for second quarter 2010.

The file will be available by selecting the appropriate machine type on the 'Software and device drivers IBM System x' web page, at the following URL:

Workaround

Disable Large Receive Offload (LRO)/TCP Segment Aggregation off on the real adapter explicitly. This can be done using the "smit ethernet" configuration menu.

Complete these steps to disable LRO/TCP Segment Aggregation:

1. Use "smit ethernet" fastpath and select the Adapter.
   
2. Select the option Change / Show Characteristics of an Ethernet Adapter.
   
3. Select the adapter on which you wish to disable LRO.
   
4. Under the Enable receive TCP segment aggregation option, select "No" and press Enter.
   

Note: If an EtherChannel is configured, then the LRO must be disabled on the real adapter explicitly. To resolve this, the EtherChannel must be detached from the Shared Ethernet Adapter (SEA) to disable LRO on the real adapter and then added back.

Remove the adapter from the EtherChannel configuration to change this configuration option and then re-add the SEA to the EtherChannel.

For example:
Use this procedure to remove an EtherChannel or Link Aggregation:

1. Type smitty chinet and select the interface belonging to the EtherChannel.
   
2. Change the Current STATE attribute to detach, and press Enter.
   
3. On the command line, type the following:
   
   

   smitty etherchannel

4. Select Remove an EtherChannel and press Enter.
   
5. Select the EtherChannel that you want to remove and press Enter.
   Use chdev or the smit configuration menu to disable the large_receive setting from the appropriate adapter.
   

Additional information

TCP Segment Aggregation, also known as LRO, enables the coalescing of received packets into a larger packet before passing them to upper Open System Interconnection (OSI) layers for enhanced network performance.

LRO, which is not a supported configuration option for Linux LPARs, is enabled by default on the Hardware Ethernet Adapter (HEA).