Dr. I Doctor
Dr. I Doctor's Informational Juggernaut
February 1, 2008
Jumbo Frames on Virtual Ethernet Interfaces
Dear Doctor,
Our System i includes a couple of Linux LPAR partitions that we use to collect data from other systems; the data is then manipulated nightly in a very I/O intensive process. I discovered that I could speed the nightly process by a factor of four by simply changing the default Ethernet frame size of the virtual LAN interfaces from 1,500 to 9,000 bytes (so-called "jumbo" frames). Even though our System i has 100BaseT physical interfaces, I figured inter-Linux traffic won't use those, so I went for the maximum frame size. All went well the first night, but the next day users complained that the Linux applications had very slow network performance. I discovered that switching the frame size back to 1,500 bytes restored daytime performance, but now nighttime processing is slow again. Can't I have my cake and eat it too?
Gentle User,
You can have your cake and eat it too, even the jumbo-sized cake you want to consume at night. The trick is in adapting your virtual network to the physical network that you users traverse when using Linux applications during the day.
As you've seen, bumping the frame size can really speed network performance on the virtual network. The reason you encountered problems the next day is that your physical Ethernet, running at only 100 Mbps, isn't able to process jumbo frames. So it fragments them into convenient 1,500-byte chunks. Alas, all this fragmenting activity takes time and memory, both in i5/OS and in the destination host systems, which is why your users saw degraded performance.
You can circumvent this problem by creating two virtual networks, or VLANs: one for inter-LPAR traffic and one for communication with the outside world. Dr. I Doctor recommends restoring the 1,500-byte frame size for your existing VLAN and then creating a second VLAN for jumbo packet traffic, configuring the Linux VLAN interfaces with 9,000-byte frames. Take care to use a completely separate IP subnet for VLAN2. Your nightly Linux applications will now use this subnet for inter-LPAR communications. Devices on your physical LAN won't know how to reach this subnet, and thus will never traverse the jumbo-frame interfaces. Performance will be optimized for each VLAN.
Posted by mbeckman at February 1, 2008 4:44 PM
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