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I am running tests on 2 different types of iscsi adapter cards on linux. The test method is to call rsync to copy a folder with large amount of data files from the data center to a local hard drive. The wireshark shows that one test transfers TCP packets of 12 to 14 KBytes in the "length" column, the other only shows 1KBytes. Both of them show TCP windows size 64KBytes. Could you provide any information to explain why the second card transmits such small packet?

asked 14 Jan '14, 20:01

rsync's gravatar image

accept rate: 0%

"one test transfers TCP packets of 12 to 14 KBytes in the "length" columumn "

This is only possible if Segmentation Offload is enabled on the NIC. So the other card might not support it.

You can check with ethtool -k eth0 whether TCP segmentation offload is supported.

Is the MSS option in the 3-way handshake the same over both interfaces?

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answered 14 Jan '14, 22:55

mrEEde's gravatar image

accept rate: 20%

Thank you for the reply, mrEEde.

You are right about the tcp segmentation offload setting. The first card has it enabled, but the second on was disabled. Following you suggestion, I turned on the segmentation Offload on the second card. I can see the TCP flow rate getting better, but the rsync performance seems not changes much. Also wireshark still shows the frame length is about 1K for the second card. I assume that the frame length had contributed the rsync performance difference between the 2 cards.

The MTU in both cases is set to 1500. And "ethtool -k" displays the same offload settings on both cards. I had not found a way to check the MSS yet. Any other settings could make such a difference in frame length?

(15 Jan '14, 10:45) rsync

Without a trace it is all guessing. Can you provide a sample trace including the 3-way handshake for both interfaces to

(15 Jan '14, 22:31) mrEEde
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question asked: 14 Jan '14, 20:01

question was seen: 1,456 times

last updated: 15 Jan '14, 22:31

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