transport/rdma: Add rdma connection manager documentation

Change-Id: I15ad9cafbe607074ddb9b36f9e879c8421f6eae2
BUG: 1086743
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-on: http://review.gluster.org/7901
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Niels de Vos <ndevos@redhat.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/doc/features/rdmacm.md b/doc/features/rdmacm.md
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+++ b/doc/features/rdmacm.md
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+## Rdma Connection manager ##
+
+### What? ###
+Infiniband requires addresses of end points to be exchanged using an out-of-band channel (like tcp/ip). Glusterfs used a custom protocol over a tcp/ip channel to exchange this address. However, librdmacm provides the same functionality with the advantage of being a standard protocol. This helps if we want to communicate with a non-glusterfs entity (say nfs client with gluster nfs server) over infiniband.
+
+### Dependencies ###
+* [IP over Infiniband](http://pkg-ofed.alioth.debian.org/howto/infiniband-howto-5.html) - The value to *option* **remote-host** in glusterfs transport configuration  should be an IPoIB address
+* [rdma cm kernel module](http://pkg-ofed.alioth.debian.org/howto/infiniband-howto-4.html#ss4.4)
+* [user space rdmacm library - librdmacm](https://www.openfabrics.org/downloads/rdmacm)
+
+### Limitations ###
+* Because of bug [890502](https://bugzilla.redhat.com/show_bug.cgi?id=890502), we've to probe the peer on an IPoIB address. This imposes a restriction that all volumes created in the future have to communicate over IPoIB address (irrespective of whether they use gluster's tcp or rdma transport).
+ 
+* Currently client has independence to choose b/w tcp and rdma transports while communicating with the server (by creating volumes with **transport-type tcp,rdma**). This independence was a by-product of our ability to use the tcp/ip channel - transports with *option transport-type tcp* - for rdma connection establishment handshake too. However, with new requirement of IPoIB address for connection establishment, we loose this independence (till we bring in [multi-network support](https://bugzilla.redhat.com/show_bug.cgi?id=765437) - where a brick can be identified by a set of ip-addresses and we can choose different pairs of ip-addresses for communication based on our requirements - in glusterd).
+
+### External links ###
+* [Infiniband Howto](http://pkg-ofed.alioth.debian.org/howto/infiniband-howto.html)