Dr. I Doctor
Dr. I Doctor's Informational Juggernaut
January 1, 2008
Achieving Gigabit Ethernet Benefits in i5/OS
Dear Doctor,
We recently upgraded our entire backbone infrastructure to Gigabit Ethernet. As part of the upgrade, we also added Gigabit Ethernet NICs to our Windows servers (our System i already had Gigabit NICs). As expected, network performance on the Windows servers improved dramatically; for example, our nightly backups complete in one-fifth of the time it used to take! Alas, our System i is seeing no speed improvements at all, even in large file transfers to the Gigabit-enabled Windows servers. We ran an SNMP monitor and are seeing no packet errors on the servers, System i, or intervening switches. What could be wrong?
Gentle User,
Dr. I Doctor believes you may be experiencing the ill effects of obsolete default setting in i (nee i5/OS) TCP/IP. The problem is that many of these defaults were chosen back when 128K bps was considered "high speed" Internet access. The particular setting you should check is the global TCP send and receive buffer sizes. The default for this is a measly 8,192 bytes, but for Gigabit performance you should increase this to at least 65,536 bytes.
Use the Change TCP/IP Attributes (CHGTCPA) command to display the current buffer sizes and make the necessary changes. You may have to experiment with size to achieve optimal results. Keep in mind that these values will be used as the buffer allocation for every TCP/IP socket opened. Thus changing from 8,192 to 65,536 bytes means that every socket will now consume 128K of memory rather than the previous 16K. If you run many simultaneous open sockets, this could affect your overall memory utilization and require a memory upgrade.
Posted by mbeckman at January 1, 2008 4:51 PM
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