Connections Between Lustre Entities
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[[connection]]

The Lustre protocol is connection-based in that each two entities
maintain shared, coordinated state information. The most common
example of two such entities are a client and a target on some
server. The target is identified by name to the client through an
interaction with the management server. The client then 'connects' to
the given target on the indicated server by sending the appropriate
version of the various *_CONNECT message (MGS_CONNECT, MDS_CONNECT, or
OST_CONNECT) and receiving back the
corresponding *_CONNECT reply. The server creates an 'export' for the
connection between the target and the client, and the export holds the
server state information for that connection. When the client gets the
reply it creates an 'import', and the import holds the client state
information for that connection. Note that if a server has N targets
and M clients have connected to them, the server will have N x M
exports and each client will have N imports.

There are also connections between the servers: Each MDS and OSS has a
connection to the MGS, where the MDS (respectively the OSS) plays the
role of the client in the above discussion. That is, the MDS initiates
the connection and has an import for the MGS, while the MGS has an
export for each MDS. Each MDS creates an 'object storage proxy' (OSP)
connection to each OST, with an import on
the MDS and an export on the OSS. This connection supports requests
from the MDS to the OST to create and destroy data objects, to set
attributes (such as permission bits), and for 'statfs' information for
precreation needs.  Each OSS also connects to the first MDS to get
access to auxiliary services, with an import on the OSS and an export
on the first MDS.  The auxiliary services are: the File ID Location
Database (FLDB), the quota master service, and the sequence
controller. This connection for auxiliary services is a 'light weight
proxy' (LWP) connection in that it has no replay functionality and
consumes no space on the MDS for client data. Each MDS connects also
to all other MDSs for DNE needs.

Finally, for some communications the roles of message initiation and
message reply are reversed. This is the case, for instance, with
call-back operations. In that case the entity which would normally
have an import has, instead, a 'reverse-export' and the
other end of the connection maintains a 'reverse-import'. The
reverse-import uses the same structure as a regular import, and the
reverse-export uses the same structure as a regular export.

include::obd_connect_data.txt[]

include::import.txt[]

include::export.txt[]

include::timeouts.txt[]

include::eviction.txt[]

include::recovery.txt[]