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+****************************
+InfiniBand Transport (IBTRS)
+****************************
+
+IBTRS (InfiniBand Transport) is a reliable high speed transport library
+which provides support to establish optimal number of connections
+between client and server machines using RDMA (InfiniBand, RoCE, iWarp)
+transport. It is optimized to transfer (read/write) IO blocks.
+
+In its core interface it follows the BIO semantics of providing the
+possibility to either write data from an sg list to the remote side
+or to request ("read") data transfer from the remote side into a given
+sg list.
+
+IBTRS provides I/O fail-over and load-balancing capabilities by using
+multipath I/O (see "add_path" and "mp_policy" configuration entries).
+
+IBTRS is used by the IBNBD (Infiniband Network Block Device) modules.
+
+======================
+Client Sysfs Interface
+======================
+
+This chapter describes only the most important files of sysfs interface
+on client side.
+
+Entries under /sys/devices/virtual/ibtrs-client/
+================================================
+
+When a user of IBTRS API creates a new session, a directory entry with
+the name of that session is created.
+
+Entries under /sys/devices/virtual/ibtrs-client/<session-name>/
+===============================================================
+
+add_path (RW)
+-------------
+
+Adds a new path (connection) to an existing session. Expected format is the
+following:
+
+ <[source addr,]destination addr>
+
+ *addr ::= [ ip:<ipv4|ipv6> | gid:<gid> ]
+
+max_reconnect_attempts (RW)
+---------------------------
+
+Maximum number reconnect attempts the client should make before giving up
+after connection breaks unexpectedly.
+
+mp_policy (RW)
+--------------
+
+Multipath policy specifies which path should be selected on each IO:
+
+ round-robin (0):
+ select path in per CPU round-robin manner.
+
+ min-inflight (1):
+ select path with minimum inflights.
+
+Entries under /sys/devices/virtual/ibtrs-client/<session-name>/paths/
+=====================================================================
+
+
+Each path belonging to a given session is listed here by its destination
+address. When a new path is added to a session by writing to the "add_path"
+entry, a directory with the corresponding destination address is created.
+
+Entries under /sys/devices/virtual/ibtrs-client/<session-name>/paths/<dest-addr>/
+=================================================================================
+
+state (R)
+---------
+
+Contains "connected" if the session is connected to the peer and fully
+functional. Otherwise the file contains "disconnected"
+
+reconnect (RW)
+--------------
+
+Write "1" to the file in order to reconnect the path.
+Operation is blocking and returns 0 if reconnect was successful.
+
+disconnect (RW)
+---------------
+
+Write "1" to the file in order to disconnect the path.
+Operation blocks until IBTRS path is disconnected.
+
+remove_path (RW)
+----------------
+
+Write "1" to the file in order to disconnected and remove the path
+from the session. Operation blocks until the path is disconnected
+and removed from the session.
+
+Entries under /sys/devices/virtual/ibtrs-client/<session-name>/paths/<dest-addr>/stats/
+=======================================================================================
+
+Write "0" to any file in that directory to reset corresponding statistics.
+
+reset_all (RW)
+--------------
+
+Read will return usage help, write 0 will clear all the statistics.
+
+sg_entries (RW)
+---------------
+
+Data to be transferred via RDMA is passed to IBTRS as scatter-gather
+list. A scatter-gather list can contain multiple entries.
+Scatter-gather list with less entries require less processing power
+and can therefore transferred faster. The file sg_entries outputs a
+per-CPU distribution table for the number of entries in the
+scatter-gather lists, that were passed to the IBTRS API function
+ibtrs_clt_request (READ or WRITE).
+
+cpu_migration (RW)
+------------------
+
+IBTRS expects that each HCA IRQ is pinned to a separate CPU. If it's
+not the case, the processing of an I/O response could be processed on a
+different CPU than where it was originally submitted. This file shows
+how many interrupts where generated on a non expected CPU.
+"from:" is the CPU on which the IRQ was expected, but not generated.
+"to:" is the CPU on which the IRQ was generated, but not expected.
+
+reconnects (RW)
+---------------
+
+Contains 2 unsigned int values, the first one records number of successful
+reconnects in the path lifetime, the second one records number of failed
+reconnects in the path lifetime.
+
+rdma_lat (RW)
+-------------
+
+Latency distribution of IBTRS requests.
+The format is:
+ 1 ms: <CNT-LAT-READ> <CNT-LAT-WRITE>
+ 2 ms: <CNT-LAT-READ> <CNT-LAT-WRITE>
+ 4 ms: <CNT-LAT-READ> <CNT-LAT-WRITE>
+ 8 ms: <CNT-LAT-READ> <CNT-LAT-WRITE>
+ 16 ms: <CNT-LAT-READ> <CNT-LAT-WRITE>
+ ...
+ 65536 ms: <CNT-LAT-READ> <CNT-LAT-WRITE>
+ >= 65536 ms: <CNT-LAT-READ> <CNT-LAT-WRITE>
+ maximum ms: <CNT-LAT-READ> <CNT-LAT-WRITE>
+
+wc_completion (RW)
+------------------
+
+Contains 2 unsigned int values, the first one records max number of work
+requests processed in work_completion in session lifetime, the second
+one records average number of work requests processed in work_completion
+in session lifetime.
+
+rdma (RW)
+---------
+
+Contains statistics regarding rdma operations and inflight operations.
+The output consists of 6 values:
+
+<read-count> <read-total-size> <write-count> <write-total-size> \
+<inflights> <failovered>
+
+======================
+Server Sysfs Interface
+======================
+
+Entries under /sys/devices/virtual/ibtrs-server/
+================================================
+
+When a user of IBTRS API creates a new session on a client side, a
+directory entry with the name of that session is created in here.
+
+Entries under /sys/devices/virtual/ibtrs-server/<session-name>/paths/
+=====================================================================
+
+When new path is created by writing to "add_path" entry on client side,
+a directory entry with source address is created on server.
+
+Entries under /sys/devices/virtual/ibtrs-server/<session-name>/paths/<source-addr>/
+===================================================================================
+
+disconnect (RW)
+---------------
+
+When "1" is written to the file, the IBTRS session is being disconnected.
+Operations is non-blocking and returns control immediately to the caller.
+
+hca_name (R)
+------------
+
+Contains the the name of HCA the connection established on.
+
+hca_port (R)
+------------
+
+Contains the port number of active port traffic is going through.
+
+Entries under /sys/devices/virtual/ibtrs-server/<session-name>/paths/<source-addr>/stats/
+=========================================================================================
+
+When "0" is written to a file in this directory, the corresponding counters
+will be reset.
+
+reset_all (RW)
+--------------
+
+Read will return usage help, write 0 will clear all the counters about
+stats.
+
+rdma (RW)
+---------
+
+Contains statistics regarding rdma operations and inflight operations.
+The output consists of 5 values:
+
+<read-count> <read-total-size> <write-count> <write-total-size> <inflights>
+
+wc_completion (RW)
+------------------
+
+Contains 3 values, the first one is int, records max number of work
+requests processed in work_completion in session lifetime, the second
+one long int records total number of work requests processed in
+work_completion in session lifetime and the 3rd one long int records
+total number of calls to the cq completion handler. Division of 2nd
+number through 3rd gives the average number of completions processed
+in completion handler.
+
+==================
+Transport protocol
+==================
+
+Overview
+--------
+An established connection between a client and a server is called ibtrs
+session. A session is associated with a set of memory chunks reserved on the
+server side for a given client for rdma transfer. A session
+consists of multiple paths, each representing a separate physical link
+between client and server. Those are used for load balancing and failover.
+Each path consists of as many connections (QPs) as there are cpus on
+the client.
+
+When processing an incoming rdma write or read request ibtrs client uses memory
+chunks reserved for him on the server side. Their number, size and addresses
+need to be exchanged between client and server during the connection
+establishment phase. Apart from the memory related information client needs to
+inform the server about the session name and identify each path and connection
+individually.
+
+On an established session client sends to server write or read messages.
+Server uses immediate field to tell the client which request is being
+acknowledged and for errno. Client uses immediate field to tell the server
+which of the memory chunks has been accessed and at which offset the message
+can be found.
+
+Connection establishment
+------------------------
+
+1. Client starts establishing connections belonging to a path of a session one
+by one via attaching IBTRS_MSG_CON_REQ messages to the rdma_connect requests.
+Those include uuid of the session and uuid of the path to be
+established. They are used by the server to find a persisting session/path or
+to create a new one when necessary. The message also contains the protocol
+version and magic for compatibility, total number of connections per session
+(as many as cpus on the client), the id of the current connection and
+the reconnect counter, which is used to resolve the situations where
+client is trying to reconnect a path, while server is still destroying the old
+one.
+
+2. Server accepts the connection requests one by one and attaches
+IBTRS_MSG_CONN_RSP messages to the rdma_accept. Apart from magic and
+protocol version, the messages include error code, queue depth supported by
+the server (number of memory chunks which are going to be allocated for that
+session) and the maximum size of one io.
+
+3. After all connections of a path are established client sends to server the
+IBTRS_MSG_INFO_REQ message, containing the name of the session. This message
+requests the address information from the server.
+
+4. Server replies to the session info request message with IBTRS_MSG_INFO_RSP,
+which contains the addresses and keys of the RDMA buffers allocated for that
+session.
+
+5. Session becomes connected after all paths to be established are connected
+(i.e. steps 1-4 finished for all paths requested for a session)
+
+6. Server and client exchange periodically heartbeat messages (empty rdma
+messages with an immediate field) which are used to detect a crash on remote
+side or network outage in an absence of IO.
+
+7. On any RDMA related error or in the case of a heartbeat timeout, the
+corresponding path is disconnected, all the inflight IO are failed over to a
+healthy path, if any, and the reconnect mechanism is triggered.
+
+CLT SRV
+*for each connection belonging to a path and for each path:
+IBTRS_MSG_CON_REQ ------------------->
+ <------------------- IBTRS_MSG_CON_RSP
+...
+*after all connections are established:
+IBTRS_MSG_INFO_REQ ------------------->
+ <------------------- IBTRS_MSG_INFO_RSP
+*heartbeat is started from both sides:
+ -------------------> [IBTRS_HB_MSG_IMM]
+[IBTRS_HB_MSG_ACK] <-------------------
+[IBTRS_HB_MSG_IMM] <-------------------
+ -------------------> [IBTRS_HB_MSG_ACK]
+
+IO path
+-------
+
+* Write *
+
+1. When processing a write request client selects one of the memory chunks
+on the server side and rdma writes there the user data, user header and the
+IBTRS_MSG_RDMA_WRITE message. Apart from the type (write), the message only
+contains size of the user header. The client tells the server which chunk has
+been accessed and at what offset the IBTRS_MSG_RDMA_WRITE can be found by
+using the IMM field.
+
+2. When confirming a write request server sends an "empty" rdma message with
+an immediate field. The 32 bit field is used to specify the outstanding
+inflight IO and for the error code.
+
+CLT SRV
+usr_data + usr_hdr + ibtrs_msg_rdma_write -----------------> [IBTRS_IO_REQ_IMM]
+[IBTRS_IO_RSP_IMM] <----------------- (id + errno)
+
+* Read *
+
+1. When processing a read request client selects one of the memory chunks
+on the server side and rdma writes there the user header and the
+IBTRS_MSG_RDMA_READ message. This message contains the type (read), size of
+the user header, flags (specifying if memory invalidation is necessary) and the
+list of addresses along with keys for the data to be read into.
+
+2. When confirming a read request server transfers the requested data first,
+attaches an invalidation message if requested and finally an "empty" rdma
+message with an immediate field. The 32 bit field is used to specify the
+outstanding inflight IO and the error code.
+
+CLT SRV
+usr_hdr + ibtrs_msg_rdma_read --------------> [IBTRS_IO_REQ_IMM]
+[IBTRS_IO_RSP_IMM] <-------------- usr_data + (id + errno)
+or in case client requested invalidation:
+[IBTRS_IO_RSP_IMM_W_INV] <-------------- usr_data + (INV) + (id + errno)
+
+
+Contact
+-------
+
+Mailing list: "IBNBD/IBTRS Storage Team" <ibnbd@profitbricks.com>