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Chapter 4. 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:

network diagram MTU
View larger image
network diagram MTU

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.

If your network requires a different Maximum Transmission Unit (MTU) than the default, you can configure this setting in the corresponding NetworkManager connection profile.

Jumbo frames are network packets with a payload of between 1500 and 9000 bytes. All devices in the same broadcast domain have to support those frames.

Prerequisites

  • All devices in the broadcast domain use the same MTU.
  • You know the MTU of the network.
  • You already configured a connection profile for the network with the divergent MTU.

Procedure

  1. Optional: Display the current MTU: 

    # ip link show
...
3: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:74:79:56 brd ff:ff:ff:ff:ff:ff
...
    Copy to Clipboard Toggle word wrap

  2. Optional: Display the NetworkManager connection profiles: 

    # nmcli connection show
NAME     UUID                                  TYPE      DEVICE
Example  f2f33f29-bb5c-3a07-9069-be72eaec3ecf  ethernet  enp1s0
...
    Copy to Clipboard Toggle word wrap

  3. Set the MTU in the profile that manages the connection to the network with the divergent MTU: 

    # nmcli connection modify Example mtu 9000
    Copy to Clipboard Toggle word wrap

  4. Reactivate the connection: 

    # nmcli connection up Example
    Copy to Clipboard Toggle word wrap

Verification

  1. Display the MTU setting: 

    # ip link show
...
3: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:74:79:56 brd ff:ff:ff:ff:ff:ff
...
    Copy to Clipboard Toggle word wrap

  2. Verify that no host on the transmission paths fragments the packets:

    • On the receiver side, display the IP reassembly statistics of the kernel: 

      # nstat -az IpReasm*
#kernel
IpReasmTimeout 0 0.0
IpReasmReqds 0 0.0
IpReasmOKs 0 0.0
IpReasmFails 0 0.0
      Copy to Clipboard Toggle word wrap

      If the counters return 0, packets were not reassembled.

    • On the sender side, transmit an ICMP request with the prohibit-fragmentation-bit: 

      # ping -c1 -Mdo -s 8972 destination_host
      Copy to Clipboard Toggle word wrap

      If the command succeeds, the packet was not fragmented.

      Calculate the value for the -s packet size option as follows: MTU size - 8 bytes ICMP header - 20 bytes IPv4 header = packet size

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