SCSI - Wikipedia. Single Ended Parallel SCSI icon/logo. Small Computer System Interface (SCSI, SKUZ- ee). The SCSI standards define commands, protocols, electrical and optical interfaces. SCSI is most commonly used for hard disk drives and tape drives, but it can connect a wide range of other devices, including scanners and CDdrives, although not all controllers can handle all devices.
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A Performance Comparison of NFS and iSCSI for IP-Networked Storage Peter Radkov, Li Yin, Pawan Goyal, Prasenjit Sarkar and Prashant Shenoy Dept. PowerVM QuickSheet Version: 1.0.0 Date: 3/15/10 Concepts CPU virtualization, LPAR capability, and virtual networking are capabilities provided by the System P hypervisor (not VIOS.) VIOS (Virtual I/O Server) is a specialized.
The SCSI standard defines command sets for specific peripheral device types; the presence of . SASI controller boards were typically the size of a hard disk drive and were usually physically mounted to the drive's chassis. SASI, which was used in mini- and early microcomputers, defined the interface as using a 5. SCSI (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.
The official comp.periphs.scsi FAQ. The Eternal Question Should I buy SCSI or just go with EIDE/ATA? This has to be the most commonly asked question regarding SCSI! DeveloperWorks; Technical topics; Linux; Technical library; Tour the Linux generic SCSI driver. Dive into the Linux generic SCSI driver API and surface with a usage example.
Almost a full day was devoted to agreeing to name the standard . Apple started using Parallel ATA (also known as IDE) for its low- end machines with the Macintosh Quadra 6. Power Macintosh G3 in 1. Apple dropped on- board SCSI completely (in favor of IDE and Fire.
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Wire) with the (Blue & White) Power Mac G3 in 1. PCI controller card as an option on up to the Power Macintosh G4 (AGP Graphics) models. Commodore included a SCSI interface on the Amiga 3. T systems and it was an add- on to previous Amiga 5. Starting with the Amiga 6. Commodore switched to the IDE interface. Atari included SCSI interface 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. Modern SCSI. 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, and improved fault isolation. The primary reason for 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. Almost all RAID subsystems on servers have used some kind of SCSI hard disk drives for decades (initially Parallel SCSI, recently SAS and Fibre Channel), though a number of manufacturers offer SATA- based RAID subsystems as a cheaper option. Moreover, SAS offers compatibility with SATA devices, creating 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 M. PCIe gaining popularity as SATA can bottleneck modern solid- state drives. Interfaces. The first was parallel SCSI (also called SCSI Parallel Interface or SPI), which uses a parallelbus design. Since 2. 00. 5, 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.
With the advent of SAS and SATA drives, provision for parallel SCSI on motherboards was discontinued. Its standardization started as a single- ended 8- bit bus in 1. MB/s, and evolved into a low- voltage differential 1. MB/s. The last SPI- 5 standard from 2. MB/s speed which failed to be realized. Parallel SCSI specifications include several synchronous transfer modes for 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. Other SCSI interfaces. Gbit/s. 8b. 10b. 31. MB/s (2,5. 17 Mbit/s)4. SCSI Express. SCSI over PCIe (SOP)T1. INCITS 4. 89serial.
GT/s (PCIe 3. 0)1. MB/s. External cables are typically shielded (but may not be), with 5. These connections are hot- pluggable and are usually implemented with optical fiber. Serial attached SCSI. This protocol can run over any RDMA- capable physical transport, e.
Infini. Band or Ethernet when using Ro. CE or i. WARP. USB Attached SCSI. The ADI standard specifies the use of RS- 4. The second- generation ADT- 2 standard defines i. ADT, use of the ADT protocol over IP (Internet Protocol) connections, such as over Ethernet. The Automation/Drive Interface .
The SCSI command architecture was originally defined for parallel SCSI buses but has been carried forward with minimal change for use with i. SCSI and serial SCSI.
Other technologies which use the SCSI command set include the ATA Packet Interface, USB Mass Storage class and Fire. Wire SBP- 2. In SCSI terminology, communication takes place between an initiator and 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 0.
Check Condition), or 0. When 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 4 categories of SCSI commands: N (non- data), W (writing data from initiator to target), R (reading data), and B (bidirectional). There are about 6. SCSI commands in total, with 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 . A typical LBA equates to 5. 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 a 2. LBA address. The Read(1. Read(1. 2), Read Long, Write(1.
Write(1. 2), and Write Long commands all contain a 3. LBA address plus various other parameter options.
The capacity of a . Read and write operations on 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. Device identification. On earlier models a physical jumper or switch controls the SCSI ID of the initiator (host adapter).
On modern host adapters (since about 1. I/O to the adapter sets the SCSI ID; for example, the adapter often contains a BIOS program that runs when the computer boots up and that program has menus that let 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 1. The SCSI ID of a device in a drive enclosure that has a backplane is set either by jumpers or by 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 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 is typically for a CD- ROM drive. 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 is a . The SCSI ID, WWN, etc.
The SSA initiator (normally the host computer through the 'host adaptor') . For i. SCSI, because of 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. Device Type. Even CD- ROMs are not handled by all controllers.
Device Type 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 . The initiator can communicate with the enclosure using a specialized set of SCSI commands to access power, cooling, and other non- data characteristics. See also. Smithsonianchips. A+ Complete Study Guide (2nd Edition). Alameda, CA, USA: l Sybex. The lun command is used to create and manage luns.
If a LUN ID is not specified, the smallest number .
SCSI Enclosure Services - Wikipedia. Most recent SCSIenclosure products support a protocol called SCSI Enclosure Services (SES).
The initiator can communicate with the enclosure using a specialized set of SCSI commands to access power, cooling, and other non- data characteristics. SES devices. The disk- drive then communicates with the enclosure by some other method, the only commonly used one being Enclosure Services Interface (ESI). In fault- tolerant enclosures, more than one disk- drive slot has ESI enabled to allow SES communications to continue even after the failure of any of the disk- drives. The definition of the ESI protocols is owned by an ANSI committee and defined in their specifications ANSI SFF- 8.
SFF- 8. 04. 5. Standalone enclosure services enclosures have a separate SES processor which occupies its own address on the SCSI bus. The protocol for this uses direct SCSI commands. An enclosure can be fault- tolerant by containing two SES processors. SES commands. Some universal SCSI commands such as Inquiry are also used with standalone enclosure services to perform basic functions such as initial discovery of the devices. SES elements. There are many different element codes defined to cover a wide range of devices. The most common SES elements are power supply, cooling fan, temperature sensor, and UPS.
The SCSI command protocols assume that there may be more than one of each device type so they must be each given an 8- bit address. When an SES controller is interrogated for the status of an SES element, the response includes a 4- bit element status code. The most common element status codes are: 1h=OK, 2h=critical, 3h=warning, 5h=not installed. SES element thresholds.
This allows an enclosure to detect and report unacceptable environmental conditions. The SCSI standard allows for two different threshold levels, noncritical (warning) and critical. Also, each threshold has a minimum and maximum value. So for example the threshold values for the 1. Minimum critical - 1. Minimum noncritical - 1.
Maximum noncritical - 1. Maximum critical - 1.
Threshold values are set by a Send Diagnostic command to the Threshold Out diagnostic page (0. Threshold values can be interrogated by a Receive Diagnostic Results command to the Threshold In diagnostic page (0. PCV bit set to one. Subenclosures. The subenclosure with address 0. SCSI attached enclosure services.
It allows a host computer to communicate with the enclosure to access its power, cooling, and other non- data characteristics. The host computer communicates with the disks in the enclosure via a SCSI interface which may be Parallel SCSI, FC- AL, SAS, or SSA.
One of the disk devices located in the enclosure is set up to allow SCSI Enclosure Services (SES) communication through a logical unit. The disk- drive then communicates with the SES processor in the enclosure, usually via Enclosure Services Interface (ESI), or a protocol called DSI for SSA enclosures. The data sent over the ESI or DSI interface is simply the contents of a SCSI command and the response to that command.
Implemented commands.