Maximum MTU doesn’t mean Best Performance

MTU (maximum transmission unit) is the largest protocol data unit that the layer can transmit which is different from packet size. You need to use the same value in all network device during transmission.

Here is a test result with different MTU in QNAP TS-112 and Lenovo: ThinkCentre Edge 72z. Both are connected directly.

Speed comparison on different MTU in QNAP TS-112.

Although the higher MTU may bring more data in each unit, not all unit may reach destination without error. The cost for resend is increasing, too. Therefore, it’s not a good idea to set the value to its maximum, you need to test to find the best value for you. Usually we will leave it to 1500.

A larger MTU brings greater efficiency because each packet carries more user data while protocol overheads, such as headers or underlying per-packet delays, remain fixed; the resulting higher efficiency means a slight improvement in bulk protocol throughput. A larger MTU also means processing of fewer packets for the same amount of data. In some systems, per-packet-processing can be a critical performance limitation.

However, this gain is not without some downside. Large packets can occupy a slow link for some time, causing greater delays to following packets and increasing lag and minimum latency. For example, a 1500-byte packet, the largest allowed by Ethernet at the network layer (and hence over most of the Internet), ties up a 14.4k modem for about one second.

Large packets are also problematic in the presence of communications errors. Corruption of a single bit in a packet requires that the entire packet be retransmitted. At a given bit error rate, larger packets are more likely to be corrupted. Retransmissions of larger packets takes longer. Despite the negative effects on retransmission duration, large packets can still have a net positive effect on end-to-end TCP performance… Wiki: Maximum transmission unit

Reference

1. Wiki: Maximum transmission unit
2. 鳥哥的 Linux 私房菜：第二章、基礎網路概念：2.2.5 MTU 最大傳輸單位
3. OpenSuSE Forums: How do you set the Network Card MTU for Jumbo Frames?
4. ServerFault: What can I do to optimize NFS copy speed on a 1GBit home network?
5. whirlpool: Jumbo Frames worth it?
6. EMC Community: MTU中 jumbo frame 性能探讨
7. QNAP: TS-112
8. Lenovo: ThinkCentre Edge 72z
9. Connect NAS to your Computer Directly