andrewsr@remus.rutgers.edu (Rich Andrews) (03/11/91)
Hello All! What is the difference between SCSI and SCSI-II? (My guess: SCSI is 16bit and SCSI-II is 32bit). Is it worth it to get SCSI-II over SCSI? How much of a price difference are we talking about? How much of a performance improvement? (NOTE: I will be plugging it into a 32bit machine and I want the fastest (but affordable) setup). I am considering $850 for a 210MB Quantum SCSI at the moment. Thanks in advance, -Rich -- // Rich | "have you ever noticed that opinion without knowledge is \X/ Andrews | always a shabby sort of thing?" - Socrates
neese@adaptx1.UUCP (03/12/91)
>/* ---------- "SCSI vs. SCSI-II" ---------- */ > >What is the difference between SCSI and SCSI-II? > >(My guess: SCSI is 16bit and SCSI-II is 32bit). Nope. SCSI-1 is 8Bit and so is SCSI-2. SCSI-2 allows extending to a wider bus, but it was the cable and connector are still in the air. Should be resolved in SCSI-3. >Is it worth it to get SCSI-II over SCSI? How much of a price >difference are we talking about? How much of a performance >improvement? Potentially, there is performance to be had with SCSI-2. But all the performance *options* in SCSI-2 may not be implemented in any particular SCSI-2 device. FAST SCSI (i.e. > 5MBytes/sec), tag queuing and such things are all optional in the SCSI-2 spec. A device can be SCSI-2 and not implement any of these types of options. In this case, you get nothing more than a better implementation of the SCSI command set. One of the biggest reasons to be concerned about a SCSI-2 implementation is in the areas other than disk drives. It was in this area SCSI-1 failed to deal appropriately with these devices. This is what I get excited about when I think of SCSI-2. Finally, a defination for tape, CD-ROM, WORM, magneto-optical, juke-boxes, printers, communications devices, and processors. >(NOTE: I will be plugging it into a 32bit machine and I want the >fastest (but affordable) setup). > >I am considering $850 for a 210MB Quantum SCSI at the moment. Good choice. Quantum has historically had one of the best implementations of the SCSI bus/command set. Roy Neese Adaptec Senior SCSI Applications Engineer UUCP @ neese@adaptex
jlohmeye@donald.WichitaKS.NCR.COM (John Lohmeyer) (03/25/91)
In article <Mar.10.14.46.39.1991.26822@remus.rutgers.edu> andrewsr@remus.rutgers.edu (Rich Andrews) writes: > >What is the difference between SCSI and SCSI-II? > About 400 pages :-). A somewhat shorter answer was contained in the January issue of Computer Technology Review. My thanks to Dal Allan and CTR for permission to post that article electronically: =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-=-=-=-=-=-=-=-=-=-= The following article was written by Dal Allan of ENDL in April 1990. It was published nine months later in the January 1991 issue of "Computer Technology Review". While it appeared in the Tape Storage Technology Section of CTR, the article is general in nature and tape-specific. In spite of the less than timely publication, most of the information is still valid. It is reprinted here with the permission of the author. If you copy this article, please include this notice giving "Computer Technology Review" credit for first publication. ------------------------------------------------------------------------------ What's New in SCSI-2 Scuzzy is the pronunciation and SCSI (Small Computer System Interface) is the acronym, for the best known and most widely used ANSI (American National Standards Institute) interface. Despite use of the term "Small" in its name, everyone has to agree that Scuzzy is large - in use, in market impact, in influence, and unfortunately, in documentation. The standards effort that began with a 20-page specification in 1980 has grown to a 600 page extravaganza of technical information. Even before ANSI (American National Standards Institute) published the first run of SCSI as standards document in 1986, ASC (Accredited Standards Committee) X3T9.2 was hard at work on SCSI-2. No technical rationale can be offered as to why SCSI-1 ended and SCSI-2 began, or as to why SCSI-2 ended and SCSI-3 began. The justification is much more simple - you have to stop sometime and get a standard printed. Popular interfaces never stop evolving, adapting, and expanding to meet more uses than originally envisaged. Interfaces even live far beyond their technological lifespan. SMD (Storage Module Drive) has been called technically obsolete for 5 years but every year there are more megabytes shipped on the SMD interface than the year before. This will probably continue for another year or so before the high point is reached, and it will at least a decade before SMD is considered to be insignificant. If SCSI enhancements are cut off at an arbitrary point, what initiates the decision? Impatience is as good an answer as any. The committee and the market get sick of promises that the revision process will "end soon," and assert pressure to "do it now." The SCSI-3 effort is actively under way right now, and the workload of the committee seems to be no less than it was a year ago. What is pleasant, is that the political pressures have eased. There is a major difference between the standards for SCSI in 1986 and SCSI- 2 in 1990. The stated goal of compatibility between manufacturers had not been achieved in SCSI in 1986 due to a proliferation of undocumented "features." Each implementation was different enough that new software drivers had to be written for each device. OEMs defined variations in hardware that required custom development programs and unique microcode. Out of this diversity arose a cry for commonality that turned into CCS (Common Command Set), and became so popular that it took on an identity of its own. CCS defined the data structures of Mode Select and Mode Sense commands, defect management on the Format command and error recovery procedures. CCS succeeded because the goals were limited, the objectives clear and the time was right. CCS was the beginning of SCSI-2, but it was only for disks. Tape and optical disks suffered from diversity, and so it was that the first working group efforts on SCSI-2 were focused on tapes and optical disks. However, opening up a new standards effort is like lifting the lid on Pandora's Box - its hard to stay focused on a single task. SCSI-2 went far beyond extending and consolidating CCS for multiple device types. SCSI-2 represents three years of creative thought by some of the best minds in the business. Many of the new features will be useful only in advanced systems; a few will find their way into the average user's system. Some may never appear in any useful form and will atrophy, as did some original SCSI features like Extended Identify. Before beginning coverage of "what's new in SCSI-2," it might be well to list some of the things that aren't new. The silicon chips designed for SCSI are still usable. No new features were introduced which obsolete chips. The cause of silicon obsolescence has been rapid market shifts in integrating functions to provide higher performance. Similarly, initiators which were designed properly, according to SCSI in 1986, will successfully support SCSI-2 peripherals. However, it should be pointed out that not all the initiators sold over the last few years behaved according to the standard, and they can be "blown away "by SCSI-2 targets. The 1986 standard allows either initiators or targets to begin negotiation for synchronous transfers, and requires that both initiators and targets properly handle the sequence. A surprisingly large percentage of SCSI initiators will fail if the target begins negotiation. This has not been as much of a problem to date as it will become in the future, and you know as well as I do, that these non-compliant initiators are going to blame the SCSI-2 targets for being "incompatible." Quirks in the 1986 standard, like 4 bytes being transferred on Request Sense, even if the requested length was zero have been corrected in SCSI-2. Initiators which relied on this quirk instead of requesting 4 bytes will get into trouble with a SCSI-2 target. A sincere effort has been made to ensure that a 1986-compliant initiator does not fail or have problems with a SCSI-2 target. If problems occur, look for a non-compliant initiator before you blame the SCSI-2 standard. After that little lecture, let us turn to the features you will find in SCSI-2 which include: o Wide SCSI: SCSI may now transfer data at bus widths of 16 and 32 bits. Commands, status, messages and arbitration are still 8 bits, and the B-Cable has 68 pins for data bits. Cabling was a confusing issue in the closing days of SCSI-2, because the first project of SCSI-3 was the definition of a 16- bit wide P-Cable which supported 16-bit arbitration as well as 16-bit data transfers. Although SCSI-2 does not contain a definition of the P-Cable, it is quite possible that within the year, the P-Cable will be most popular non-SCSI-2 feature on SCSI-2 products. The market responds to what it wants, not the the arbitrary cutoffs of standards committees. o Fast SCSI: A 10 MHz transfer rate for SCSI came out of a joint effort with the IPI (Intelligent Peripheral Interface) committee in ASC X3T9.3. Fast SCSI achieves 10 Megabytes/second on the A-Cable and with wider data paths of 16- and 32-bits can rise to 20 Megabytes/second and even 40 Megabytes/second. However, by the time the market starts demanding 40 Megabytes/second it is likely that the effort to serialize the physical interface for SCSI-3 will attract high-performance SCSI users to the Fiber Channel. A word of caution. At this time the fast parameters cannot be met by the Single Ended electrical class, and is only suitable for Differential. One of the goals in SCSI-3 is to identify the improvements needed to achieve 10 MHz operation with Single Ended components. o Termination: The Single Ended electrical class depends on very tight termination tolerances, but the passive 132 ohm termination defined in 1986 is mismatched with the cable impedance (typically below 100 ohms). Although not a problem at low speeds when only a few devices are connected, reflections can cause errors when transfer rates increase and/or more devices are added. In SCSI-2, an active terminator has been defined which lowers termination to 110 ohms and is a major boost to system integrity. o Bus Arbitration, Parity and the Identify Message were options of SCSI, but are required in SCSI-2. All but the earliest and most primitive SCSI implementations had these features anyway, so SCSI-2 only legitimizes the de facto market choices. The Identify message has been enhanced to allow the target to execute processes, so that commands can be issued to the target and not just the LUNs. o Connectors: The tab and receptacle microconnectors chosen for SCSi-2 are available from several sources. A smaller connector was seen as essential for the shrinking form factor of disk drives and other peripherals. This selection was one of the most argued over and contentious decisions made during SCSI-2 development. o Rotational Position Locking: A rose by any other name, this feature defines synchronized spindles, so than an initiator can manage disk targets which have their spindles locked in a known relative position to each other. Synchronized disks do not all have to be at Index, they can be set to an offset in time relative to the master drive. By arraying banks of synchronized disks, faster transfer rates can be achieved. o Contingent Allegiance: This existed in SCSI-1, even though it was not defined, and is required to prevent the corruption of error sense data. Targets in the Contingent Allegiance state reject all commands from other initiators until the error status is cleared by the initiator that received the Check Condition when the error occurred. Deferred errors were a problem in the original SCSI but were not described. A deferred error occurs in buffered systems when the target advises Good Status when it accepts written data into a buffer. Some time later, if anything goes wrong when the buffer contents are being written to the media, you have a deferred error. o Extended Contingent Allegiance (ECA): This extends the utility of the Contingent Allegiance state for an indefinite period during which the initiator that received the error can perform advanced recovery algorithms. o Asynchronous Event Notification (AEN): This function compensates for a deficiency in the original SCSI which did not permit a target to advise the initiator of asynchronous events such as a cartridge being loaded into a tape drive. o Mandatory Messages: The list of mandated messages has grown: +----------------------+--------------------------+-------------------+ | Both | Target | Initiator | +----------------------+--------------------------+-------------------| | Identify | Abort | Disconnect | | | | | | Message Reject | No Operation | Restore Pointer | | | | | | Message Parity Error | Bus Device Reset | Save Data Pointer | | | | | | | Initiator Detected Error | | +----------------------+--------------------------+-------------------+ o Optional messages have been added to negotiate wide transfers and Tags to support command queueing. A last-minute inclusion in SCSI-2 was the ability to Terminate I/O and receive the residue information in Check Condition status (so that only the incomplete part of the command need be re-started by the initiator). o Command Queueing: In SCSI-1, initiators were limited to one command per LUN e.g. a disk drive. Now up to 256 commands can be outstanding to one LUN. The target is allowed to re-sequence the order of command execution to optimize seek motions. Queued commands require Tag messages which follow the Identify. o Disk Cacheing: Two control bits are used in the CDB (Command Descriptor Block) to control whether the cache is accessed on a Read or Write command, and some commands have been added to control pre-fetching and locking of data into the cache. Users do not have to change their software to take advantage of cacheing, however, as the Mode Select/Mode Sense Cache page allows parameters to be set which optimize the algorithms used in the target to maximize cache performance. Here is another area in which improvements have already been proposed in SCSI-3, and will turn up in SCSI-2 products shipping later this year. o Sense Keys and Sense Codes have been formalized and extended. A subscript byte to the Sense Code has been added to provide specifics on the type of error being reported. Although of little value to error recovery, the additional information about error causes is useful to the engineer who has to analyze failures in the field, and can be used by host systems as input to prognostic analysis to anticipate fault conditions. o Commands: Many old commands have been reworked and several new commands have been added. o Pages: Some method had to be found to pass parameters between host and target, and the technique used is known as pages. The concept was introduced in CCS and has been expanded mightily in SCSI-2. A number of new Common Commands have been added, and the opcode space for 10-byte CDBs has been doubled. o Change Definition allows a SCSI-2 initiator to instruct a SCSI-2 target to stop executing according to the 1986 standard, and provide advanced SCSI- 2 features. Most SCSI-2 targets will power on and operate according to the 1986 standard (so that there is no risk of "disturbing" the installed initiators, and will only begin operating in SCSI-2 mode, offering access to the advanced SCSI-2 capabilities, after being instructed to do so by the initiator using the Change Definition command. o The Mode Select and Mode Sense pages which describe parameters for operation have been greatly expanded, from practically nothing in 1986 to hundreds of items in SCSI-2. Whenever you hear of something being described as powerful and flexible tool, think complicated. Integrators are advised to be judicious in their selection of the pages they decide to support. o the Inquiry command now provides all sorts of interesting data about the target and its LUNs. Some of this is fixed by the standard, but the main benefit may be in the Vendor Unique data segregated into the special designation of Vital Product Data, which can be used by integrators as a tool to manage the system environment. o Select Log and Sense Log have been added so that the initiator can gather both historical (e.g. all Check Conditions) and statistical (e.g. number of soft errors requiring ECC) data from the target. o Diagnostic capabilities have been extended on the Read/Write Buffer and Read/Write Long commands. The ways in which the target can manage bad blocks in the user data space have been defined further and regulated to reduce inconsistencies in the 1986 standard. A companion capability to Read Defect Data permits the initiator to use a standard method to be advised of drive defect lists. o A new group of 12-byte command blocks has been defined for all optical devices to support the large volume sizes and potentially large transfer lengths. The Erase command has been added for rewritable optical disks so that areas on the media can be pre-erased for subsequent recording. Write Once disks need Media Scan, so that the user can find blank areas on the media. o New command sets have been added for Scanners, Medium Changers, and CD ROMs. All of this technical detail can get boring, so how about some "goodies" in SCSI-2 which benefit the common man and help the struggling engineer? First, and probably the best feature in SCSI-2 is that the document has been alphabetized. No longer do you have to embark on a hunt for the Read command because you cannot remember the opcode. In the 1986 standard, everything was in numeric sequence, and the only engineers who could find things easily were the microprogrammers who had memorized all the message and opcode tables. Now, ordinary people can find the Read command because it is in alphabetic sequence. This reorganization may sound like a small matter but it wasn't, it required a considerable amount of effort on the part of the SCSI-2 editors. It was well worth it. Another boon is the introduction for each device class of models which describe the device class characteristics. The tape model was the most needed, because various tape devices use the same acronym but with different meanings or different acronyms for the same meaning. The SCSI-2 tape model defines the terms used by SCSI-2, and how they correspond to the acronyms of the different tapes. For example, on a 9-track reel, End of Tape is a warning, and there is sufficient media beyond the reflective spot to record more data and a trailer. Not so on a 1/4" tape cartridge, End of Tape means out of media and no more data can be written. This sort of difference in terms causes nightmares for standardization efforts. So there it is, a summary of what is in SCSI-2. Its not scary, although it is daunting to imagine plowing through a 600-page document. Time for a commercial here. The "SCSI Bench Reference" available from ENDL Publications (408-867-6642), is a compaction of the standard. It takes the 10% of SCSI-2 which is constantly referenced by any implementor, and puts it in an easy- to-use reference format in a small handbook. The author is Jeff Stai, one of the earliest engineers to become involved with SCSI implementation, and a significant contributor to the development of both the 1986 standard and SCSI-2. SCSI-2 is not yet published as a standard, but it will be available later this year. Until then, the latest revision can be purchased from Global Engineering (800-854-7179). Biography Consultant and analyst I. Dal Allan is the founder of ENDL and publisher of the ENDL Letter and the "SCSI Bench Reference." A pioneer and activist in the development and use of standard interfaces, he is Vice Chairman of ASC X3T9.2 (SCSI) and Chairman of the SCSI-2 Common Access Method Committee. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= A couple notes: SCSI-2 is still available through Global Engineering Documents as mentioned in the article. The committee is revising it and expects to have revision 10d available in May. The CAM Committee has turned over the CAM project to X3T9.2. The latest version of the committee document is also available through Global Engineering Documents. Both of these documents are available electronically from the SCSI BBS (see .sig, below). And no, I can't post them here or email them--sorry. -- John Lohmeyer John.Lohmeyer@WichitaKS.NCR.COM NCR Corp. uunet!ncrlnk!ncrwic!donald!jlohmeye 3718 N. Rock Rd. Voice: 316-636-8703 Wichita, KS 67226 SCSI BBS 316-636-8700 300/1200/2400 24 hours