Misplaced Pages

#741258

45-468: (Redirected from SCSI Architectural Model ) SCSI architectural model SCSI Application Layer SAL SCSI Transport Protocol Layer STPL SCSI Interconnect Layer SIL The SCSI architectural model provides an abstract view of the way that SCSI devices communicate. It is intended to show how the different SCSI standards are inter-related. The main concepts and terminology of

90-466: A parallel bus design. Since 2005, SPI was gradually replaced by Serial Attached SCSI (SAS), which uses a serial design but retains other aspects of the technology. Many other interfaces which do not rely on complete SCSI standards still implement the SCSI command protocol ; others drop physical implementation entirely while retaining the SCSI architectural model . iSCSI , for example, uses TCP/IP as

135-667: A service delivery subsystem . A SCSI device has one or more SCSI ports , and a SCSI port may have an optional SCSI port identifier (SCSI ID or PID ). A SCSI device can have an optional SCSI device name which must be unique within the SCSI domain in which the SCSI device has SCSI ports. This is often called a World Wide Name . Note that the "world" may only consist of a very small number of SCSI devices. A SCSI target consists of one or more logical units (LUNs), which are identified by logical unit numbers . A LUN may have dependent LUN s embedded within it. This can recur up to

180-427: A target . The initiator sends a command to the target, which then responds. SCSI commands are sent in a Command Descriptor Block ( CDB ). The CDB consists of a one byte operation code followed by five or more bytes containing command-specific parameters. At the end of the command sequence, the target returns a status code byte, such as 00h for success, 02h for an error (called a Check Condition), or 08h for busy. When

225-592: A "LUN number" or "LUN id". In modern SCSI transport protocols, there is an automated process for the "discovery" of the IDs. The SSA initiator (normally the host computer through the 'host adaptor') "walk the loop" to determine what devices are connected and then assigns each one a 7-bit "hop-count" value. Fibre Channel – Arbitrated Loop (FC-AL) initiators use the LIP (Loop Initialization Protocol) to interrogate each device port for its WWN ( World Wide Name ). For iSCSI, because of

270-403: A 21-bit LBA address. The Read(10), Read(12), Read Long, Write(10), Write(12), and Write Long commands all contain a 32-bit LBA address plus various other parameter options. The capacity of a "sequential access" (i.e. tape-type) device is not specified because it depends, amongst other things, on the length of the tape, which is not identified in a machine-readable way. Read and write operations on

315-508: A maximum nesting depth of four addressable levels. There are three type of SCSI ports: initiator ports, target ports and target/initiator ports. A SCSI device may contain any combination of initiator ports, target ports and target/initiator ports. SCSI distributed objects are considered to communicate in a three layer model: The highest level of abstraction is the SCSI Application Layer (SAL) where an initiator and

360-459: A much broader range of options for RAID subsystems together with the existence of nearline SAS (NL-SAS) drives. Instead of SCSI, modern desktop computers and notebooks typically use SATA interfaces for internal hard disk drives, with NVMe over PCIe gaining popularity as SATA can bottleneck modern solid-state drives . SCSI is available in a variety of interfaces. The first was parallel SCSI (also called SCSI Parallel Interface or SPI), which uses

405-410: A sequential access device begin at the current tape position, not at a specific LBA. The block size on sequential access devices can either be fixed or variable, depending on the specific device. Tape devices such as half-inch 9-track tape , DDS (4 mm tapes physically similar to DAT ), Exabyte , etc., support variable block sizes. On a parallel SCSI bus, a device (e.g. host adapter, disk drive)

450-604: A target are considered to communicate using SCSI commands sent via the SCSI application protocol . The SCSI Transport Protocol Layer (STPL) is where an initiator and a target are considered to communicate using a SCSI transport protocol . Examples of SCSI transport protocols are Fibre Channel , SSA , SAS , UAS , iSCSI and the SCSI Parallel Interface . The lowest level is the SCSI Interconnect Layer (SIL) where an initiator and

495-784: A target are considered to communicate using an interconnect . It consists of the services, signaling mechanism and interconnect subsystem used for the physical transfer of data from an initiator to a target. A SCSI task is represented by an I_T_L_Q nexus. This is where one I nitiator Port talks to one T arget Port, addressing one L UN and together they execute one task (identified by Q ). External links [ edit ] SCSI architectural model T10 Technical Committee – list of all SCSI standards Retrieved from " https://en.wikipedia.org/w/index.php?title=SCSI_architectural_model&oldid=738301156 " Category : SCSI SCSI Small Computer System Interface ( SCSI , / ˈ s k ʌ z i / SKUZ -ee )

SECTION 10

#1732790752742

540-453: A transport mechanism, which is most often transported over Gigabit Ethernet or faster network links. SCSI interfaces have often been included on computers from various manufacturers for use under Microsoft Windows , classic Mac OS , Unix , Amiga and Linux operating systems, either implemented on the motherboard or by the means of plug-in adaptors. With the advent of SAS and SATA drives, provision for parallel SCSI on motherboards

585-522: A unified standard. In October 1981, the two companies agreed to co-develop SASI and present their standard jointly with ANSI. Until at least February 1982, ANSI developed the specification as "SASI" and "Shugart Associates System Interface". However, the committee documenting the standard would not allow it to be named after a company. Almost a full day was devoted to agreeing to name the standard "Small Computer System Interface", which Boucher intended to be pronounced "sexy", but ENDL's Dal Allan pronounced

630-421: Is a "virtual" disk—a stripe set or mirror set constructed from portions of real disk drives. The SCSI ID, WWN, etc. in this case identifies the whole subsystem, and a second number, the logical unit number (LUN) identifies a disk device (real or virtual) within the subsystem. It is quite common, though incorrect, to refer to the logical unit itself as a "LUN". Sometimes, redundantly, the actual LUN may be called

675-427: Is a 5-bit field reported by a SCSI Inquiry Command ; defined SCSI Peripheral Device Types include, in addition to many varieties of storage device, printer, scanner, communications device, and a catch-all "processor" type for devices not otherwise listed. In larger SCSI servers, the disk-drive devices are housed in an intelligent enclosure that supports SCSI Enclosure Services (SES) . The initiator can communicate with

720-496: Is a protocol that specifies how to transport SCSI commands over a reliable RDMA connection. This protocol can run over any RDMA-capable physical transport, e.g. InfiniBand or Ethernet when using RoCE or iWARP . USB Attached SCSI allows SCSI devices to use the Universal Serial Bus . The Automation/Drive Interface − Transport Protocol (ADT) is used to connect removable media devices, such as tape drives, with

765-530: Is a set of standards for physically connecting and transferring data between computers and peripheral devices , best known for its use with storage devices such as hard disk drives . SCSI was introduced in the 1980s and has seen widespread use on servers and high-end workstations, with new SCSI standards being published as recently as SAS-4 in 2017. The SCSI standards define commands , protocols, electrical, optical and logical interfaces . The SCSI standard defines command sets for specific peripheral device types;

810-416: Is considered to be the "father" of SASI and ultimately SCSI due to his pioneering work first at Shugart Associates and then at Adaptec , which he founded in 1981. A SASI controller provided a bridge between a hard disk drive's low-level interface and a host computer, which needed to read blocks of data. SASI controller boards were typically the size of a hard disk drive and were usually physically mounted to

855-565: Is identified by a "SCSI ID", which is a number in the range 0–7 on a narrow bus and in the range 0–15 on a wide bus. On earlier models a physical jumper or switch controls the SCSI ID of the initiator ( host adapter ). On modern host adapters (since about 1997), doing I/O to the adapter sets the SCSI ID; for example, the adapter often contains a Option ROM (SCSI BIOS) program that runs when the computer boots up and that program has menus that let

900-404: Is typically for a CD-ROM drive. Note that a SCSI target device (which can be called a "physical unit") is sometimes divided into smaller "logical units". For example, a high-end disk subsystem may be a single SCSI device but contain dozens of individual disk drives, each of which is a logical unit. Further, a RAID array may be a single SCSI device, but may contain many logical units, each of which

945-435: Is used which is also of serial nature. SCSI is popular on high-performance workstations, servers, and storage appliances. Almost all RAID subsystems on servers have used some kind of SCSI hard disk drives for decades (initially Parallel SCSI, interim Fibre Channel, recently SAS), though a number of manufacturers offer SATA -based RAID subsystems as a cheaper option. Moreover, SAS offers compatibility with SATA devices, creating

SECTION 20

#1732790752742

990-742: The Macintosh Quadra 630 in 1994, and added it to its high-end desktops starting with the Power Macintosh G3 in 1997. Apple dropped on-board SCSI completely in favor of IDE and FireWire with the (Blue & White) Power Mac G3 in 1999, while still offering a PCI SCSI host adapter as an option on up to the Power Macintosh G4 (AGP Graphics) models. Sun switched its lower-end range to Parallel ATA (PATA) with introduction of their Ultra 5 and 10 low end workstations using CMD640 IDE controller and continued this trend with

1035-831: The Amiga 3000/3000T systems and it was an add-on to previous Amiga 500/2000 models. Starting with the Amiga 600/1200/4000 systems Commodore switched to the IDE interface. Atari included SCSI as standard in its Atari MEGA STE , Atari TT and Atari Falcon computer models. SCSI has never been popular in the low-priced IBM PC world, owing to the lower cost and adequate performance of ATA hard disk standard. However, SCSI drives and even SCSI RAIDs became common in PC workstations for video or audio production. Recent physical versions of SCSI‍—‌ Serial Attached SCSI (SAS), SCSI-over- Fibre Channel Protocol (FCP), and USB Attached SCSI (UAS)‍—‌break from

1080-423: The SCSI architectural model are: Only the externally observable behavior is defined in SCSI standards. The relationship between SCSI devices is described by a client-server service-delivery model. The client is called a SCSI initiator and the server is called a SCSI target . A SCSI domain consists of at least one SCSI device , at least one SCSI target and at least one SCSI initiator interconnected by

1125-516: The SCSI standards also include an extensive set of command definitions. The SCSI command architecture was originally defined for parallel SCSI buses but has been carried forward with minimal change for use with iSCSI and serial SCSI. Other technologies which use the SCSI command set include the ATA Packet Interface , USB Mass Storage class and FireWire SBP-2 . In SCSI terminology, communication takes place between an initiator and

1170-529: The controllers of the libraries (automation devices) in which they are installed. The ADI standard specifies the use of RS-422 for the physical connections. The second-generation ADT-2 standard defines iADT, use of the ADT protocol over IP (Internet Protocol) connections, such as over Ethernet . The Automation/Drive Interface − Commands standards (ADC, ADC-2, and ADC-3) define SCSI commands for these installations. In addition to many different hardware implementations,

1215-456: The drive's chassis. SASI, which was used in mini- and early microcomputers, defined the interface as using a 50-pin flat ribbon connector which was adopted as the SCSI-1 connector. SASI is a fully compliant subset of SCSI-1 so that many, if not all, of the then-existing SASI controllers were SCSI-1 compatible. In around 1980, NCR Corporation had been developing a competing interface standard by

1260-413: The enclosure using a specialized set of SCSI commands to access power, cooling, and other non-data characteristics. Peripheral A peripheral can be categorized based on the direction in which information flows relative to the computer: Many modern electronic devices, such as Internet-enabled digital watches , video game consoles , smartphones , and tablet computers , have interfaces for use as

1305-538: The faster serial SCSI (SAS) host adapters. The "small" reference in "small computer system interface" is historical; since the mid-1990s, SCSI has been available on even the largest of computer systems. Since its standardization in 1986, SCSI has been commonly used in the Amiga , Atari , Apple Macintosh and Sun Microsystems computer lines and PC server systems. Apple started using the less-expensive parallel ATA (PATA, also known as IDE ) for its low-end machines with

1350-429: The jumpers are typically located; the switch emulates the necessary jumpers. While there is no standard that makes this work, drive designers typically set up their jumper headers in a consistent format that matches the way that these switches implement. Setting the bootable (or first) hard disk to SCSI ID 0 is an accepted IT community recommendation. SCSI ID 2 is usually set aside for the floppy disk drive while SCSI ID 3

1395-475: The later Blade 100 and 150 entry level systems and did not switch to contemporary SATA interface even with the introduction of the Blade 1500 in 2003 while the higher end Blade 2500 released at the same time used Ultra320 Parallel SCSI-3. Sun moved to SATA and SAS interfaces with their last UltraSPARC-III based workstations in 2006 with the entry level Ultra 25 and mid-range Ultra 45. Commodore included SCSI on

SCSI architectural model - Misplaced Pages Continue

1440-638: The most commonly used being: Each device on the SCSI bus is assigned a unique SCSI identification number or ID. Devices may encompass multiple logical units, which are addressed by logical unit number (LUN). Simple devices have just one LUN, more complex devices may have multiple LUNs. A "direct access" (i.e. disk type) storage device consists of a number of logical blocks, addressed by Logical Block Address ( LBA ). A typical LBA equates to 512 bytes of storage. The usage of LBAs has evolved over time and so four different command variants are provided for reading and writing data. The Read(6) and Write(6) commands contain

1485-512: The name of BYSE. In the summer of 1981, NCR abandoned their in-house efforts in favor of pursuing SASI and improving on its design for their own computer systems. Fearing that their extension of the SASI standard would induce market confusion, however, NCR briefly cancelled their contract with Shugart. NCR's proposed improvements to the design of SCSI piqued the interest of Optimem, a subsidiary of Shugart, who requested that NCR and Shugart collaborate on

1530-579: The new acronym as "scuzzy" and that stuck. The NCR facility in Wichita, Kansas developed the industry's first SCSI controller chip, the NCR 5385, released in 1983. According to its developers, the chip worked the first time it was tested. A number of companies, such as Adaptec and Optimem, were early supporters of SCSI. By late 1990 at least 45 manufactures offered 251 models of parallel SCSI host adapters Today, such host adapters have largely been displaced by

1575-414: The operator choose the SCSI ID of the host adapter. Alternatively, the host adapter may come with software that must be installed on the host computer to configure the SCSI ID. The traditional SCSI ID for a host adapter is 7, as that ID has the highest priority during bus arbitration (even on a 16 bit bus). The SCSI ID of a device in a drive enclosure that has a back plane is set either by jumpers or by

1620-470: The parallel cable, and an asynchronous mode. The asynchronous mode is a classic request/acknowledge protocol, which allows systems with a slow bus or simple systems to also use SCSI devices. Faster synchronous modes are used more frequently. Internal parallel SCSI cables are usually ribbons , with two or more 50–, 68–, or 80–pin connectors attached. External cables are typically shielded (but may not be), with 50– or 68–pin connectors at each end, depending upon

1665-515: The presence of "unknown" as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements. The initial Parallel SCSI was most commonly used for hard disk drives and tape drives , but it can connect a wide range of other devices, including scanners and CD drives , although not all controllers can handle all devices. The ancestral SCSI standard, X3.131-1986, generally referred to as SCSI-1,

1710-589: The shift to serial interfaces is the clock skew issue of high-speed parallel interfaces, which makes the faster variants of parallel SCSI susceptible to problems caused by cabling and termination. The non-physical iSCSI preserves the basic SCSI paradigm , especially the command set, almost unchanged, through embedding of SCSI-3 over TCP/IP . Therefore, iSCSI uses logical connections instead of physical links and can run on top of any network supporting IP. The actual physical links are realized on lower network layers , independently from iSCSI. Predominantly, Ethernet

1755-409: The slot in the enclosure the device is installed into, depending on the model of the enclosure. In the latter case, each slot on the enclosure's back plane delivers control signals to the drive to select a unique SCSI ID. A SCSI enclosure without a back plane often has a switch for each drive to choose the drive's SCSI ID. The enclosure is packaged with connectors that must be plugged into the drive where

1800-806: The specific SCSI bus width supported. The 80–pin Single Connector Attachment (SCA) is typically used for hot-pluggable devices Fibre Channel can be used to transport SCSI information units, as defined by the Fibre Channel Protocol for SCSI (FCP). These connections are hot-pluggable and are usually implemented with optical fiber. Serial attached SCSI (SAS) uses a modified Serial ATA data and power cable. iSCSI (Internet Small Computer System Interface) usually uses Ethernet connectors and cables as its physical transport, but can run over any physical transport capable of transporting IP . The SCSI RDMA Protocol (SRP)

1845-518: The target returns a Check Condition in response to a command, the initiator usually then issues a SCSI Request Sense command in order to obtain a key code qualifier ( KCQ ) from the target. The Check Condition and Request Sense sequence involves a special SCSI protocol called a Contingent Allegiance Condition. There are four categories of SCSI commands: N (non-data), W (writing data from initiator to target), R (reading data), and B (bidirectional). There are about 60 different SCSI commands in total, with

SCSI architectural model - Misplaced Pages Continue

1890-456: The traditional parallel SCSI bus and perform data transfer via serial communications using point-to-point links. Although much of the SCSI documentation talks about the parallel interface, all modern development efforts use serial interfaces. Serial interfaces have a number of advantages over parallel SCSI, including higher data rates, simplified cabling, longer reach, improved fault isolation and full-duplex capability. The primary reason for

1935-905: The unlimited scope of the (IP) network, the process is quite complicated. These discovery processes occur at power-on/initialization time and also if the bus topology changes later, for example if an extra device is added. SCSI has the CTL (Channel, Target or Physical Unit Number, Logical Unit Number) identification mechanism per host bus adapter , or the HCTL (HBA, Channel, PUN, LUN) identification mechanism, one host adapter may have more than one channels. While all SCSI controllers can work with read/write storage devices, i.e. disk and tape, some will not work with some other device types; older controllers are likely to be more limited, sometimes by their driver software, and more Device Types were added as SCSI evolved. Even CD-ROMs are not handled by all controllers. Device Type

1980-481: Was discontinued. Initially, the SCSI Parallel Interface (SPI) was the only interface using the SCSI protocol. Its standardization started as a single-ended 8-bit bus in 1986, transferring up to 5 MB/s, and evolved into a low-voltage differential 16-bit bus capable of up to 320 MB/s. The last SPI-5 standard from 2003 also defined a 640 MB/s speed which failed to be realized. Parallel SCSI specifications include several synchronous transfer modes for

2025-806: Was published by the X3T9 technical committee of the American National Standards Institute (ANSI) in 1986. SCSI-2 was published in August 1990 as X3.T9.2/86-109, with further revisions in 1994 and subsequent adoption of a multitude of interfaces. Further refinements have resulted in improvements in performance and support for ever-increasing data storage capacity. SCSI is derived from the Shugart Associates System Interface (SASI), developed beginning 1979 and publicly disclosed in 1981. Larry Boucher

#741258