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How to enable Ceph with RDMA


  • RDMA (Remote Direct Memory Access) is a mechanism which allow the host to accessing(read, write) memory on a remote host without interrupting CPU.
  • The advantage of RDMA
    1. Zero-copy
    2. Kernel bypass
    3. No CPU involvement`
  • With RDMA, our data can transfer without the involvement of the linux kernel network stack and provide hight performance, low latency, low CPU consumption.
  • This article focus on how to enable the ceph with RDMA, including how to install ceph and enable the RDMA function.


  • I introduce two ways to install the ceph with RDMA, one is use widly used tool ceph-deploy and the other is manually build the ceph.


  • If you use the ceph-deploy to install the ceph, you must make sure the source package you installed is configure with -DWITH_RDMA=ON.
  • You can use the argument --dev and -dev--commit to select the source packet form the official ceph build phase.
    • you can find those avaliabe repos in the ceph site
    • choose the one you want to install and clink it into the next page, you will see something like this Repos ceph > wip-jd-testing > da2c3dabdad80c01ec3d3258b51640cc0a93e842 > default
    • wip-jd-testing is for --dev and da2c3... is for --dev-commit.
    • use the following command to install the ceph from above repos.
  ceph-deploy install --dev=wip-jd-testing --dev-commit=da2c3dabdad80c01ec3d3258b51640cc0a93e842

Manually build

  • Refer to followings step to build the ceph with RDMA.
cd ceph
cd build
time make -j54
sudo make install
  • You can add any other options in command
  • And than you should install the ceph to you environment and set up the monitor/osd by yourself.

Enable RDMA

  • Before we enable the RDMA, there're something we need prepare for, including the RDMA environment, systemd config (if you need) and the ceph.conf

  • Before we enable the RDMA, we must setup the RDMA environment, you should install the NIC driver and validate RDMA functionalities

RDMA environment

Systemd config

  • If you want to use the systemd to manage the ceph daemons, you should modify the systemd config to make it support RDMA because of the default config will fail for some access permission problem.
    • You can wait the official PR and use the next version.
    • Refer to this PR to modfiy the systemd config by yourself, and you can use systemctl reload the systemd config if you need.


  • Modify the ms_type to async+rdma, which tell the ceph use the AsyncMessenger + RDMA as your message type.
  • You can use ms_cluster_type and ms_public_type to indicate the message type for your cluster network or public network.
  • Use the command ibdev2netdev to get your device name and use it for ms_async_rdma_device_name
  • If your want to use the port 2 in your NIC for RDMA, set the ms_async_rdma_port_num to 2.
  • You can also use ms_async_rdma_buffer_size, ms_async_rdma_send_buffers and ms_async_rdma_receive_buffers to set the memory you want to allocate for RDMA.
    • ms_async_rdma_send_buffers and ms_async_rdma_receive_buffers are how many work requestes for RDMA send/receive queue respectively.
    • ms_async_rdma_buffer_size is the size os a single registered buffer.
    • the total memory we allocate for each application is ms_async_rdma_buffer_size * (ms_async_rdma_send_buffers + ms_async_rdma_receive_buffers) and you can refer to here to know more about it.

Example ceph.conf


Update the ceph.conf for each node and restart all daemons, after that, the ceph cluster will use the RDMA for all public/cluster network. If you want ot make sure the RDMA works, you can use the following method to dump the RDMA packet and use the wireshark to open it.

1. echo "options mlx4_core log_num_mgm_entry_size=-1" || sudo -a tee /etc/modprobe.d/mlx4.conf
2. sudo /etc/init.d/openibd restart
3. ibdump