Storage devices may be connected to computers in one of several ways. Devices directly attached to a single server are called Direct Attach Storage (DAS). Devices shared over a general purpose, usually IP based network are known as Network Attached Storage (NAS). Devices using a network that has been developed especially for storage use a Storage Area Network (SAN). NAS devices access data at the file level, while DAS and SAN devices access data at the block level. SAN storage typically offers higher performance than NAS devices, but at a much higher cost. Each of these attachment methods has several different standards or protocols and each protocol offers various advantages and disadvantages. Some of these protocols have been "converged" to use common command sets, interfaces, and cabling.
SCSI: Small Computer System Interface (SCSI) is a computer bus interface and a set of standards for physically connectors and data transfer between computers and data storage devices. The SCSI standards defines protocols, commands, and electrical and optical interfaces. Several generations of SCSI have been developed after first appearing in the mid 1980s. SCSI commands are still used in newer standards such as FC, and SAS.
SATA: Serial ATA (SATA or Serial Advanced Technology Attachment) is a computer bus interface for connecting Host Bus Adapters (HBA's) to storage devices such as disks and optical drives. Serial ATA was designed to replace the older AT attachment standard (also known as EIDE). SATA offers several advantages over the older parallel ATA (PATA) interface, including smaller cables, lower cost, hot swapping, faster data transfer, and more efficiency through an optional I/O queuing protocol.
SAS: Serial Attached SCSI (SAS) is a computer bus used to move data to and from computer storage devices. SAS depends on a point-to-point serial protocol that replaces the parallel SCSI bus technology, and it uses standard SCSI commands. SAS offers backwards-compatibility with second-generation SATA drives. SAS combines high-end features of fiber channel (such as multi-initiator support and full duplex communication) and the physical interfaces of SATA.
FC: Fibre Channel, or FC, is a high-speed storage networking technology. Fibre Channel was primarily used in the supercomputer field, but now, has become the standard connection type for storage area networks (SAN) in enterprise storage. Despite its name, Fibre Channel can use both twisted pair copper wire and fiber-optic cables. common FC speeds are 1,2,4,8, with 10 and 16 Gbps emerging.
Traditional Ethernet using TCP/IP was never designed to handle data storage traffic. Issues such as latency, dropped packets and collisions, and small data payload size made using Ethernet for storage device data transfer challenging. As traditional network speed and reliability increases, new standards are emerging which allow data traffic to be encapsulated and sent over shared, or, "coverged" LAN, WAN, and Internet links. These include NAS, iSCSI, and FCoE:
NAS: Network-attached storage (NAS) is a dedicated data storage appliance connected to a computer network providing file level data access to clients. NAS not only operates as a file server, but is specialized for this task either by its hardware, software, or configuration. NAS systems typically provide access to files using network file sharing protocols such as NFS, SMB/CIFS, or AFP.
iSCSI: Internet Small Computer System Interface (iSCSI), an Internet Protocol (IP)-based storage networking standard for linking data storage facilities. By carrying SCSI commands over IP networks, iSCSI is used to facilitate data transfers over intranets and to manage storage over long distances. iSCSI can be used to transmit data over local area networks (LANs), wide area networks (WANs), or the Internet and can enable location-independent data storage and retrieval. The protocol allows clients (called initiators) to send SCSI commands (CDBs) to SCSI storage devices (targets) on remote servers. It is a Storage Area Network (SAN) protocol, allowing organizations to consolidate storage into data center storage arrays while providing hosts (such as database and web servers) with the illusion of locally-attached disks. Unlike traditional Fibre Channel, which requires special-purpose cabling, iSCSI can be run over long distances using existing network infrastructure.
FCoE: Fibre Channel over Ethernet (FCoE) is an encapsulation of Fibre Channel frames over Ethernet networks. This allows Fibre Channel to use 10 Gigabit Ethernet networks or higher speed while using the Fibre Channel protocol. FCoE was meant to integrate with existing Fibre Channel networks and management software. Many data centers use Ethernet for TCP/IP networks, and Fibre Channel for storage area networks (SANs). With FCoE, Fibre Channel becomes another network protocol running on Ethernet, alongside traditional Internet Protocol (IP) traffic. FCoE operates directly above Ethernet in the network protocol stack, in contrast to iSCSI which runs on top of TCP and IP. As a consequence, FCoE is not routable at the IP layer, and will not work across routed IP networks. FCoE reduces cables, switches, HBAs, and cost in these "converged" networks.
Storage Standards Links:
http://www.incits.org/ InterNational Committee for Information Technology Standards (NCITS)
http://www.ansi.org American National Standards Institute (ANSI)
http://www.ietf.org/mail-archive/web/ietf-announce/current/msg04239.html [IP Storage (iSCSI)
http://www.t11.org T11 Technical Committee (Fibre Channel, HIPPI, IPI, SBCON)
http://fcoe.com/ T11 FCoE Information
http://www.t13.org T13 Technical Committee (ATA & ATAPI)
http://www.scsita.org SCSI Trade Association (STA)
http://www.sffcommittee.com Small Form Factor (SFF) Committee
http://www.serialata.org Serial ATA Working Group
http://www.fibrechannel.org Fibre Channel Association (a trade association)
http://www.1394ta.org/ [IEEE 1394 Trade Association
http://www.cmpcmm.com/cc/standards.html Computer and Communications Standards
http://www.ieee-sssc.org/ IEEE Storage System Standards Committee
http://www.ietf.org Internet Engineering Task Force (IETF)
Storage Standards are defined by various committees of the American National Standards Institute (ANSI).
The T10 Committee is responsible for SCSI and related Storage Interfaces.
The T11 Committee is responsible for Fibre Channel and related Interfaces.
The T13 Committee is responsible for ATA and related Interfaces.
The InterNational Committee on Information Technology Standards (INCITS) operates under the American National Standards Institute (ANSI). Voluntary standards are developed by a consensus of industry groups as part of INCITS. INCITS develops Information Processing System standards, while ANSI approves and and publishes them. ANSI also serves as the representative for the United States on Joint Technical Committee – 1 (JTC-1) of the International Standards Organization (ISO) and the International Electrotechnical Commission (IEC). The following chart from ANSI, illustrates these relationships.