RPC 8: OST CONNECT - Client connection to an OST
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[[ost-connect-rpc]]

When a client initiates a connection to a specific target on an OSS,
it does so via an OST_CONNECT RPC ('pb_opc' = 8).

.OST_CONNECT Request Packet Structure
image::ost-connect-request.png["OST_CONNECT Request Packet Structure",height=50]

//////////////////////////////////////////////////////////////////////
The ost-connect-request.png diagram resembles this text art:

       OST_CONNECT:
      --request--------------------------------------------------
      | ptlrpc_body | target_uuid | client_uuid | lustre_handle |
      -----------------------------------------------------------
      | obd_connect_data |
      --------------------
//////////////////////////////////////////////////////////////////////

'ptlrpc_body'::
RPC descriptor. See <<struct-ptlrpc-body>>. In a connect message the
'ptlrpc_body' field 'pb_handle', which is a <<struct-lustre-handle>>,
is 0. That 'lustre_handle' is distinct from the one mentioned
below.

'target_uuid'::
A string identifying the target. See <<struct-obd-uuid>>. The client
learns the UUID strings for the targets via an interaction with the
MGS.

'client_uuid'::
A string with the client's own UUID. This is also a
<<struct-obd-uuid>>. The target sets the 'exp_client_uuid' field of
its 'eport' structure to this value.

'lustre_handle'::
See <<struct-lustre-handle>>. This 'lustre_handle' is distinct from
the 'pb_handle' field in the 'ptlrpc_body'.  This 'lustre_handle'
provied a unique 'cookie' to identify this client for this connection
attempt. After a disconnect, a subsequent connect RPC will have a
different value. The target preserves this cookie in the 'exp_handle'
field of its 'obd_export' structure for this client. In that way it
can distinguish between a resent connection request and an entirely
new connection request.

'obd_connect_data'::
See <<struct-obd-connect-data>>.

.OST_CONNECT Reply Packet Structure
image::ost-connect-reply.png["OST_CONNECT Reply Packet Structure",height=50]

//////////////////////////////////////////////////////////////////////
The ost-connect-reply.png diagram resembles this text art:

       OST_CONNECT:
      --reply---------------------------
      | ptlrpc_body | obd_connect_data |
      ----------------------------------
//////////////////////////////////////////////////////////////////////

'ptlrpc_body'::
RPC descriptor. See <<struct-ptlrpc-body>>. The 'pb_handle' field has
the value, $C_t$, uniquely identifying that target. The value $C_t$ is
preserved in the 'imp_remote_handle' field of the client's import
structure for this target. See <<struct-obd-import>>.

'obd_connect_data'::
See <<struct-obd-connect-data>>.