Remote Direct Memory Access

This page introduces Remote Direct Memory Access (RDMA), describes the advantages of RDMA over TCP/IP, documents RDMA system requirements, and lists commands you can use to disable RDMA.

What is RDMA?

TCP/IP communication uses copy operations that involve user-kernel context switching, user-kernel memory copies, Linux kernel interrupt processing, and kernel packet processing. TCP/IP suffers from two major problems:

  • TCP/IP consumes significant CPU cycles and memory resources
  • TCP/IP has large end-to-end latency

Remote Direct Memory Access (RDMA) mitigates these major problems by copying data directly between virtual memory buffers on two different machines, resulting in lower latency, higher throughput, and smaller CPU footprint.

RDMA transfers do not involve the CPU, and there are no context switches. Transfers occur in parallel with other system operations.

The following diagram compares TCP and RDMA operations:

Figure 1. TCP/IP vs RDMA Communication

Supported RDMA Type

There are two kinds of RDMA protocols in existence - RDMA over Converged Ethernet (RoCE) and iWARP. HPE Ezmeral Data Fabric supports only RoCE.

When HPE Ezmeral Data Fabric Uses RDMA

HPE Ezmeral Data Fabric uses RDMA when it needs to transfer data between:

  • Fileclient (Java Client, FUSE, NFS) and MFS
  • NFS clients and NFS gateway
  • MFS instances

RDMA System Requirements

To benefit from RDMA, your system needs to have a Network Interface Card (NIC) that supports RDMA. HPE Ezmeral Data Fabric is tested with Mellanox cards, but any NIC that supports RDMA should work. Ensure that you have Infiniband support installed. To install Infiniband support, run:

On CentOS:
yum -y groupinstall "Infiniband Support" 
To determine whether your NIC supports RDMA, run:
ibv_devinfo | grep "PORT_ACTIVE"

If the command returns the active ports, then your NIC(s) support(s) RDMA.

For example:
# ibv_devinfo | grep "PORT_ACTIVE"
                        state:                  PORT_ACTIVE (4)
                        state:                  PORT_ACTIVE (4)
Optionally, to determine the interfaces with RDMA support:
  1. Run:
    The output returns the Infiniband devices. For example:
    device                 node GUID
        ------              ----------------
        mlx4_0              040973ffffd661f0
        mlx4_1              b88303ffff9e5440
  2. Run:
    ls /sys/class/infiniband/<Infiniband_Device_Name>/device/net/
    to determine the RDMA NIC. For example:
    ls /sys/class/infiniband/mlx4_0/device/net/
      eno5d1  ib0
    Here, the NIC is eno5d1.
  3. To confirm that this NIC exists, run:
    ip a | grep <NIC>
    For example:
    ip a | grep eno5d1
      6: eno5d1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
        inet brd scope global noprefixroute eno5d1
        inet scope global eno5d1:~m0

RDMA is automatically enabled only when the NICs/nodes are RDMA capable. HPE Ezmeral Data Fabric automatically uses TCP/IP when the system does not support RDMA.

Disabling RDMA

By default, RDMA is automatically enabled and functional on all nodes and clients that support RDMA. To disable RDMA, use any of the following options:

To Disable RDMA Perform the Following Task
On all nodes in the cluster Set /opt/mapr/bin/maprcli config save -values '{"support.rdma.transport":"0"}'. With this option, clients can have RDMA enabled but will revert to TCP because RDMA is disabled cluster wide.
On a local node for MFS, clients, and NFS server In the /opt/mapr/conf/ file on that node, set export MAPR_RDMA_SUPPORT=false
Only for a local MFS node (and NOT for any client) In the /opt/mapr/conf/mfs.conf file on that node, set mfs.listen.on.rdma=0
For communication between an NFS server and MFS only In the /opt/mapr/conf/nfsserver.conf file on that NFS node, uncomment NfsRdmaToMfs=0
For the clients (FUSE, Hadoop) only Set the property fs.mapr.disable.rdma.transport to true in the /opt/mapr/hadoop/hadoop-<version>/etc/hadoop/core-site.xml file.

NFS Port for RDMA Communication

By default, NFS servers use port 20049 to communicate with NFS clients using RDMA. To change this port, set the NfsRdmaPort parameter in /opt/mapr/conf/nfsserver.conf to the desired port. For example:
NOTE Setting this port to 0 causes NFS servers to use TCP to communicate with NFS clients.

NFS Mount With RDMA

To mount an NFS server with RDMA support on an NFS client, use the following command:
mount -o vers=3,proto=rdma,port=20049 <NFSserver IP>:<directory> <mount point>

RDMA Specific Commands

You can use the following mrconfig commands to display RDMA information: