Understanding Lustre Networking (LNet)

This chapter introduces Lustre networking (LNet). It includes the following sections:

Introducing LNet

In a cluster using one or more Lustre file systems, the network communication infrastructure required by the Lustre file system is implemented using the Lustre networking (LNet) feature.

LNet supports many commonly-used network types, such as InfiniBand and IP networks, and allows simultaneous availability across multiple network types with routing between them. Remote direct memory access (RDMA) is permitted when supported by underlying networks using the appropriate Lustre network driver (LND). High availability and recovery features enable transparent recovery in conjunction with failover servers.

An LND is a pluggable driver that provides support for a particular network type, for example ksocklnd is the driver which implements the TCP Socket LND that supports TCP networks. LNDs are loaded into the driver stack, with one LND for each network type in use.

For information about configuring LNet, see Configuring Lustre Networking (LNet).

For information about administering LNet, see Part III: Administering Lustre.

Key Features of LNet

Key features of LNet include:

  • RDMA, when supported by underlying networks
  • Support for many commonly-used network types
  • High availability and recovery
  • Support of multiple network types simultaneously
  • Routing among disparate networks

LNet permits end-to-end read/write throughput at or near peak bandwidth rates on a variety of network interconnects.

Lustre Networks

A Lustre network is comprised of clients and servers running the Lustre software. It need not be confined to one LNet subnet but can span several networks provided routing is possible between the networks. In a similar manner, a single network can have multiple LNet subnets.

The Lustre networking stack is comprised of two layers, the LNet code module and the LND. The LNet layer operates above the LND layer in a manner similar to the way the network layer operates above the data link layer. LNet layer is connectionless, asynchronous and does not verify that data has been transmitted while the LND layer is connection oriented and typically does verify data transmission.

LNets are uniquely identified by a label comprised of a string corresponding to an LND and a number, such as tcp0, o2ib0, or o2ib1, that uniquely identifies each LNet. Each node on an LNet has at least one network identifier (NID). A NID is a combination of the address of the network interface and the LNet label in the form:*address*@*LNet_label*.

Examples:

192.168.1.2@tcp0
10.13.24.90@o2ib1

In certain circumstances it might be desirable for Lustre file system traffic to pass between multiple LNets. This is possible using LNet routing. It is important to realize that LNet routing is not the same as network routing. For more details about LNet routing, see Configuring Lustre Networking (LNet)

Supported Network Types

The LNet code module includes LNDs to support many network types including:

  • InfiniBand: OpenFabrics OFED (o2ib)
  • TCP (any network carrying TCP traffic, including GigE, 10GigE, and IPoIB)
  • RapidArray: ra
  • Quadrics: Elan