Comment 17 for bug 1692538
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| bugproxy (bugproxy) wrote Comment bridged from LTC Bugzilla | #17 |
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------- Comment From <email address hidden> 2017-11-20 04:13 EDT-------
Hi Daniel,
When one enables largesend offload in AIX, MTU still stays at 1500 (when Jumbo frames are disabled) and 9000 (when Jumbo frames are enabled). So, when a workload pushes data > MTU sizes, AIX LPAR will send the bigger payload with an MSS value based on MTU. This MSS value is what the adapter uses later for segmenting.
By default AIX doesn't set MTU to 64k when largesend is enabled. In the scenario described in bnx2x driver issue, I suspect the end-user manually changed the MTU to a bigger value (~64k or so), otherwise we shouldn't be seeing this issue.
Now, coming back to your question on what will happen if user configures a bigger MTU value (say for example 64k), AIX LPAR will send the bigger payload to VIOS with MSS value ~64k, this will lead to physical NICs in VIOS drop the packet, leading to restransmissions from AIX LPAR. Eventually AIX LPAR will disable largesend offload for the specific connection, post certain number of retransmissions and then the data flow goes through fine. So, in the event of user misconfiguration, I agree there will be a performance impact.
This issue may happen in non-virtualized environment too, when the end-user sets a higher MTU than the one supported by the physical adapter. Here the driver/adapter may drop the packet, leading to retransmissions.
Regards,
Siva K