| Introduction (continued)
Definitions
Block Oriented Device
(BOD). A storage device's operating mode in which its space is managed as an
ordered set of fixed length data blocks or sectors. This is the mode used almost
universally today. This is the term that will be used to refer to a traditional data
storage architecture, as opposed to the proposed Object Oriented Device (OOD).
Device or
Storage Device. A secondary storage unit that can store or retrieve data. Discs,
tapes, CD-ROMs and array subsystems are examples of storage devices.
Network Attached
Storage (NAS or SAN). One or more storage devices having peer connections
with two or more computer systems. This is also called Storage Area Networking (SAN).
Network Attached
Secure Discs (NASD). The term used by Carnegie Mellon University for its research
on network attached storage. The NSIC project sponsoring the CMU research.
Object. The
smallest visible unit of capacity allocation on a device operating in Object Oriented
Device mode. An object on such a storage device consists of an ordered set of sectors
associated with a unique identifier. Data is referenced by the identifier and an offset
into the object. Conceptually similar to a file, it is allocated and placed on the media
by the device itself, while the operating system manages its files and metadata in these
object constructs, instead of managing sectors of data, as it does in most current
architectures.
Object File System (OFS
or FFS). A filing system consisting of a single level list of the
objects for each partition on the device by which an Object Oriented Device manages its
objects. This is also called a Flat File System (FFS).
Object Oriented Device (OOD). A storage device operating in a mode in which data is organized
and accessed as objects rather than as an ordered sequence of sectors.
Requester.
A computer system. An element in a cluster or network of systems which submits a request
for action on a storage device. The term Requester is used as a general description for
systems including both clients and servers, as either could be directly connected to NAS
and impose workloads on it. A client whose workload goes to a server which in turn submits
I/O requests to storage is not a requester; its server is, as it is the element actually
doing I/O with the NAS.
Sector. The
smallest unit of data exchange with the device. A device will always read or write
multiples of the sector size even in OOD mode. The difference between sectors in an OOD
and in a BOD is that sector addressing in an OOD is relative to the first sector of an
object, while sector addressing in a BOD is relative to the first sector on the device.
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