第 4 章 Improving the throughput of large amounts of contiguous data streams

Improving throughput for large, contiguous data streams is beneficial because it reduces overhead by transmitting more data per packet.

According to the IEEE 802.3 standard, a default Ethernet frame without Virtual Local Area Network (VLAN) tag has a maximum size of 1518 bytes. Each of these frames includes an 18 bytes header, leaving 1500 bytes for payload. Consequently, for every 1500 bytes of data the server transmits over the network, 18 bytes (1.2%) Ethernet frame header are overhead and transmitted as well. Headers from layer 3 and 4 protocols increase the overhead per packet further.

Consider employing jumbo frames to save overhead if hosts on your network often send numerous contiguous data streams, such as backup servers or file servers hosting numerous huge files. Jumbo frames are non-standardized frames that have a larger Maximum Transmission Unit (MTU) than the standard Ethernet payload size of 1500 bytes. For example, if you configure jumbo frames with the maximum allowed MTU of 9000 bytes payload, the overhead of each frame reduces to 0.2%.

Depending on the network and services, it can be beneficial to enable jumbo frames only in specific parts of a network, such as the storage backend of a cluster. This avoids packet fragmentation.

4.1. Considerations before configuring jumbo frames

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Depending on your hardware, applications, and services in your network, jumbo frames can have different impacts. Decide carefully whether enabling jumbo frames provides a benefit in your scenario.

Prerequisites for jumbo frames:

All network devices on the transmission path must support jumbo frames and use the same Maximum Transmission Unit (MTU) size. In the opposite case, you can face the following problems:

• Dropped packets.
• Higher latency due to fragmented packets.
• Increased risk of packet loss caused by fragmentation. For example, if a router fragments a single 9000-bytes frame into six 1500-bytes frames, and any of those 1500-byte frames are lost, the whole frame is lost because it cannot be reassembled.

In the following diagram, all hosts in the three subnets must use the same MTU if a host from network A sends a packet to a host in network C:

Benefits of jumbo frames:

• Higher throughput: Each frame contains more user data while the protocol overhead is fixed.
• Lower CPU utilization: Jumbo frames cause fewer interrupts and, therefore, save CPU cycles.

Drawbacks of jumbo frames:

• Higher latency: Larger frames delay packets that follow.
• Increased memory buffer usage: Larger frames can fill buffer queue memory more quickly.