Enabling TCP large send offload (LSO), on AIX, is generally considered a good thing. It helps with overall network performance.


Taking advantage of networking large-send large-receive



TCP large send offload

“This option improves the performance on 10 Gigabit Ethernet and faster adapters for workloads that manage data streaming (such as file transfer protocol (FTP), RCP, tape backup, and similar bulk data movement applications). The virtual Ethernet adapter and shared Ethernet adapter (SEA) devices are exceptions, where the large send offload option is disabled by default due to inter operability problems with the Linux or IBM i operating system. Enabling Large Send and other performance features can be done in AIX and virtual Ethernet adapter or SEA environments.”




So, is LSO enabled in my AIX partition?


No, it’s not!


# ifconfig en0


        inet netmask 0xffffff00 broadcast

         tcp_sendspace 262144 tcp_recvspace 262144 rfc1323 1


# netstat -p tcp | grep -i large

                0 large sends

                0 bytes sent using largesend

                0 bytes is the biggest largesend


Enable it now with chdev!


# chdev -l en0 -a mtu_bypass=on

en0 changed


# ifconfig en0


        inet netmask 0xffffff00 broadcast

         tcp_sendspace 262144 tcp_recvspace 262144 rfc1323 1


I can now see large send packet activity.


# netstat -p tcp | grep -i large

                1205 large sends

        19687632 bytes sent using largesend

        17376 bytes is the biggest largesend


netstat -p protocol