Skytap has found that by optimizing network settings within AIX for the conditions of a specific WAN, the throughput across the WAN can be greatly increased in some cases. Before changing a setting, the current setting should be recorded so it can be restored if a change has a negative effect.
Table of relevant AIX network settings
| SET PARAMETER (=VALUE) | GLOBAL STATUS (DEFAULT) | NIC STATUS (DEFAULT) | DEFINITION |
| # no -p -o tcprexmtthresh=x | # no -a | grep rex (3) | none | Specifies number of consecutive duplicate acks which causes TCP to go to fast retransmit phase |
| # no -p -o sack=x | # no -a | grep sack (0) | none | Enable selective acknowledgments AKA RFC2018 |
| # no -p -o sb_max=x | # no -a | grep sb (1048576) | none | Upper limit on the number of socket buffers queued to an individual socket, too large can use too much CPU |
| # no -p -o tcp_pmtu_discover=x | # no -a | grep pmtu_di (1) | none | Allows the protocol stack to determine the minimum MTU size on any network |
| # no -p -o pmtu_expire=x | # no -a | grep pmtu_ex (10) | none | PMTU table entry expiration |
| # no -p -o tcp_nodelayack=x | # no -a | grep node (0) | none | Prompts TCP to send an immediate acknowledgement, rather than the usual 200 ms delay |
| # chdev -P -l en0 -a rfc1323=x | # no -a | grep 1323 (0) | # ifconfig en0 | grep 1323 (1) | Value of 1 indicates that tcp_sendspace and tcp_recvspace can exceed 64 KB |
| # chdev -P -l en0 -a tcp_sendspace=x | # no -a | grep space (16384) | # ifconfig en0 | grep space (262144) | Specifies how much data sending application can buffer in the kernel before the application is blocked on a send call |
| # chdev -P -l en0 -a tcp_recvspace=x | # no -a | grep space (16384) | # ifconfig en0 | grep space (262144) | Specifies how many bytes of data receiving system can buffer in the kernel on the receiving sockets queue |
| # chdev -P -l en0 -a mtu_bypass=x | none | # lsattr -E -l en0 | grep byp (off) | Enable/Disable LARGESEND for virtual Ethernet AKA PLSO (true) |
| # chdev -P -l en0 -a mtu=x | none | # lsattr -E -l en0 | grep mtu (1500) | Maximum transmission unit size |
| # chdev -P -l en0 -a remmtu=x | none | # lsattr -E -l en0 | grep rem (576) | Max packet size for remote networks |
Skytap recommended base setting
The following AIX settings have been found to greatly increase WAN throughput across a WAN when both the transfer initiator and responder are AIX. These settings should be applied to both the initiator and responder for the most significant gains. It is important to note that the characteristics of every WAN are unique due to latency, number of hops, the specific version of AIX, and many other factors. These are a good base starting place, but for each unique WAN, further throughput increases may be gained by carefully increasing/decreasing/changing individual settings and retesting after each change.
| PARAMETERS |
| # no -p -o tcprexmtthresh=100 |
| # no -p -o sack=1 |
| # no -p -o sb_max=1048576 |
| # no -p -o tcp_pmtu_discover=1 |
| # no -p -o pmtu_expire=10 |
| # no -p -o tcp_nodelayack=1 |
| # chdev -P -l en0 -a rfc1323=1 |
| # chdev -P -l en0 -a tcp_sendspace=1048576 |
| # chdev -P -l en0 -a tcp_recvspace=1048576 |
| # chdev -P -l en0 -a mtu_bypass=off |
| # chdev -P -l en0 -a mtu=1240 |
| # chdev -P -l en0 -a remmtu=576 |
Copy and paste commands for gathering settings
no -a | grep rex
no -a | grep sack
no -a | grep sb
no -a | grep pmtu_di
no -a | grep pmtu_ex
no -a | grep node
ifconfig en0
lsattr -E -l en0
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