********************************RDMA Network Block Device (RNBD)******************************** Introduction------------ RNBD (RDMA Network Block Device) is a pair of kernel modules(client and server) that allow for remote access of a block device onthe server over RTRS protocol using the RDMA (InfiniBand, RoCE, iWARP)transport. After being mapped, the remote block devices can be accessedon the client side as local block devices. I/O is transferred between client and server by the RTRS transportmodules. The administration of RNBD and RTRS modules is done viasysfs entries. Requirements------------   RTRS kernel modules Quick Start----------- Server side:  # modprobe rnbd_server Client side:  # modprobe rnbd_client  # echo "sessname=blya path=ip:10.50.100.66 device_path=/dev/ram0" > \            /sys/devices/virtual/rnbd-client/ctl/map_device   Where "sessname=" is a session name, a string to identify the session  on client and on server sides; "path=" is a destination IP address or  a pair of a source and a destination IPs, separated by comma.  Multiple  "path=" options can be specified in order to use multipath  (see RTRS  description for details); "device_path=" is the block device to be  mapped from the server side. After the session to the server machine is  established, the mapped device will appear on the client side under  /dev/rnbd<N>.  RNBD-Server Module Parameters============================= dev_search_path--------------- When a device is mapped from the client, the server generates the pathto the block device on the server side by concatenating dev_search_pathand the "device_path" that was specified in the map_device operation. The default dev_search_path is: "/". dev_search_path option can also contain %SESSNAME% in order to providedifferent device namespaces for different sessions.  See "device_path"option for details. ============================Protocol (rnbd/rnbd-proto.h)============================ 1. Before mapping first device from a given server, client sends anRNBD_MSG_SESS_INFO to the server. Server responds withRNBD_MSG_SESS_INFO_RSP. Currently the messages only contain the protocolversion for backward compatibility. 2. Client requests to open a device by sending RNBD_MSG_OPEN message. Thiscontains the path to the device and access mode (read-only or writable).Server responds to the message with RNBD_MSG_OPEN_RSP. This containsa 32 bit device id to be used for  IOs and device "geometry" relatedinformation: side, max_hw_sectors, etc. 3. Client attaches RNBD_MSG_IO to each IO message send to a device. Thismessage contains device id, provided by server in his rnbd_msg_open_rsp,sector to be accessed, read-write flags and bi_size. 4. Client closes a device by sending RNBD_MSG_CLOSE which contains only thedevice id provided by the server. =========================================Contributors List(in alphabetical order)=========================================Danil Kipnis <danil.kipnis@profitbricks.com>Fabian Holler <mail@fholler.de>Guoqing Jiang <guoqing.jiang@cloud.ionos.com>Jack Wang <jinpu.wang@profitbricks.com>Kleber Souza <kleber.souza@profitbricks.com>Lutz Pogrell <lutz.pogrell@cloud.ionos.com>Milind Dumbare <Milind.dumbare@gmail.com>Roman Penyaev <roman.penyaev@profitbricks.com>Swapnil Ingle <ingleswapnil@gmail.com>