[glossary]
Glossary
--------
Here are some common terms used in discussing Lustre, POSIX semantics,
and the protocols used to implement them.

[glossary]
Object Storage Device (OSD)::
The OSD is the storage on a server that holds objects or metadata. The
two kinds of file system available for OSDs are ldiskfs and ZFS.

Client::
A Lustre client is a computer taking advantage of the services
provided by MDSs and OSSs to assemble a POSIX-compliant file system
with its namespace and data storage capabilities.

Distributed Lock Manager::
The distributed lock manager (DLM, often referred to as the Lustre
Distributed Lock Manager, or LDLM) is the service that enforces a
consistent (cache-coherent) view of the data objects in the file
system.

early reply::
A partial message reply returned in response to a request message
that allows the target to indicate to the requester to increase its
timeout value without otherwise indicating the request has progressed.

export::
The term for the per-client shared state information held by a
server. See <<struct-obd-export>>.

extent:: A range of offsets in a file.

import::
The term for the per-target shared state information held by a
client. See <<struct_obd_import>>.

Inodes and Extended Attributes::
Metadata about a Lustre file is encoded in the extended attributes of
an inode on the MDT. Some of that information establishes the stripe
layout of the file. The size of the stripe layout information may be
different for each file. The amount of space reserved for layout
information is a small as possible given the maximum stripe count on
the file system. Clients, servers, and the distributed lock manager
will all need to be aware of this size, which is communicated in the
'ocd_max_easize' field of the <<struct-obd-connect-data>> structure.

LNet::
A lower level protocol employed by PtlRPC to abstract the mechanisms
provided by various hardware centric protocols, such as TCP or
Infiniband.

The Log Subsystem::

Fixme:
The log subsystem (LOG) is something I have no idea about right now.

Logical Object Volume::
The logical object volume (LOV) layer abstracts the targets on a server.

Management Server::
The Management Server (MGS) maintains an understanding of the other
servers, targets, and client in the Lustre file system. It holds their
identities and keeps track of disconnections and reconnections in
order to support imperative recovery.

Metadata Server::
A metadata server (MDS) is a computer responsible for running the
Lustre kernel services in support of managing the POSIX-compliant name
space and the indices associating files in that name space with the
locations of their corresponding objects. As of Lustre 2.4 there can be
multiple MDSs in a single Lustre file system.

Metadata Target::
A metadata target (MDT) is the service provided by an MDS that
mediates the management of name space indices on the underlying file
system hardware. As of Lustre 2.4 there can be multiple MDTs on an MDS.
An MDT can be served by different MDS nodes at different times, but
will only be available from a single MDS at any given time.

Object Based Disk::
Object Based Disk (OBD) is the term used for any target, MDT or OST.

Object Storage Server::
An object storage server (OSS) is a computer responsible for running
Lustre kernel services in support of managing bulk data objects on the
underlying storage. There can be multiple OSSs in a Lustre file
system.

Object Storage Target::
An object storage target (OST) is the service provided by an OSS that
mediates the placement of data objects on the specific underlying file
system hardware. There can be multiple OSTs on a given OSS.  An OST
may be served by different OSS nodes at different times, but will
only be available from one OSS at any given time.

protocol::
An agreed upon formalism for communicating between two entities, such
as between two servers or between a client and a server.

PtlRPC::
The protocol (or set of protocols) implemented via RPCs that is (are)
employed by Lustre to communicate between its clients and servers.

Remote Procedure Call::
A mechanism for implementing operations involving one computer acting
on the behalf of another (RPC).

server::
A computer that provides a service. For example, management (MGS),
Metadata (MDS), or object storage (OSS) services in support of a
Lustre file system.

target::
Storage available to be served, such as an OST or an MDT. Also the
service being provided.

UUID:: A universally unique identifier.