SCSI Specs & Comparisons - answers (long) (fwd)
Martin Mokrejs
mmokrejs at prfdec.natur.cuni.cz
Tue Feb 10 03:08:30 PST 1998
Hello,
it's not related to solving problems on FreeBSD or Linux, but it might
help you.
Martin
---------- Forwarded message ----------
Date: Fri, 06 Feb 1998 10:52:44 -0500 (EST)
From: Dan Kirkpatrick <dkirk at suhep.phy.syr.edu>
To: decstation-managers at ornl.gov, alpha-osf-managers at ornl.gov
Cc: alan at nabeth.cxo.dec.com, tpb at zk3.dec.com, dkirk at suhep.phy.syr.edu
Subject: SCSI Specs & Comparisons - answers (long)
Followup-To: poster
Thank you all who helped me with my SCSI questions...
especially:
"Alan Rollow - Dr. File System's Home for Wayward Inodes."
<alan at nabeth.cxo.dec.com>
"Dr. Tom Blinn, 603-884-0646" <tpb at zk3.dec.com>
the best two sites I found were:
SCSI Introduction & Comparisons
http://www.network.com/Products/Connectivity/TechBrief/SCSI/SCSIintro.html
Connecting wide & narrow scsi devices
http://www.adaptec.com/support/configuration/connect.html
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>From alan at nabeth.cxo.dec.com Fri Feb 6 09:45:00 1998
Date: Thu, 5 Feb 1998 11:01:12 -0700
From: "Alan Rollow - Dr. File System's Home for Wayward Inodes."
<alan at nabeth.cxo.dec.com>
To: dkirk at suhep.phy.syr.edu
Subject: Re: SCSI questions
>Can anyone give me the speed differences between SCSI, SCSI-2, Fast SCSI,
>Fast SCSI-2, and Ultra-Wide SCSI?
"SCSI" by itself doesn't offer any clue as to what speeds are
available because it refer to any particular version of an
ANSI standard that defines that the available speeds are.
SCSI-2 is a standard that offers two speed choices. The default
speed runs the bus clock at 5 Mhz. The other, optional speed,
runs the bus clock at 10 Mhz. For any given data transfer,
the device and host adapter (or two devices) will negociate
for the speed they want to use. Cable length plays an
important part what speeds are supported. The total cable
length, including internal device cables for single-ended
SCSI running "Fast" is 3 meters. Hopefully devices will
detect the signal degredation on longer busses and fall back
to a safe speed. If not, you can expect data corruption and
errors. Hopefully, the devices will detect the errors, but
it is easy not to.
"Fast SCSI" almost certainly refers to devices/adapters that
support the fast option (10 Mhz). It is probably the same as
"Fast SCSI-2", but vendors were supporting the option long before
the specification was a standard. A particular vendor may use
the difference to indicate a pre-spec device and a post-spec
device. Or they may simply drop the -2 to print few characters.
"Ultra-Wide SCSI" refers to one of:
o The Wide32 option of SCSI-2
o Ultra-Fast transfers and the Wide16 option of SCSI-2
SCSI-2 offers three options for the width of data transfer; 8 bit
(the default), 16 bit (commonly used) and 32 bit (rare). Controllers
and adapters that support Fast/Wide transfers are common. Vendors
could chose to build Wide/Slow SCSI that used a clock speed of
5 Mhz, but transfered 16 bits per tick, but I doubt anybody does.
Wide does require extra signal lines to carry the extra 8 bits
of data and probably some extra signal lines. When wide is
involved you'll have different cables and connectors. Wide32
simply transfer 32 bits per clock tick. There may be devices
and specialized adapters that support this, but I've only
seen it on SCSI analyzers.
"Ultra" is the term given to what will be a feature of SCSI-3;
20 Mhz data transfers. SCSI-3 isn't a standard yet, but as with
SCSI-2 vendors are already starting to implement the most useful
features. In general application the maximum length of a single-
ended bus running Ultra speeds is 1.5 meters. Through the work
done by Bill Ham of Digital Equipment Corporation, it is
possible to run longer if you're running a single point-to-point
connect (host adapter to a single device at the end). All the
cables, connectors, etc, between have to up to a high standard
but it can be done. You can also use repeater chips between
these long segments.
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>From tpb at zk3.dec.com Fri Feb 6 09:45:19 1998
Date: Thu, 05 Feb 98 14:31:28 -0500
From: "Dr. Tom Blinn, 603-884-0646" <tpb at zk3.dec.com>
To: Dan Kirkpatrick <dkirk at suhep.phy.syr.edu>
Subject: SCSI standards web page is http://www.symbios.com/x3t10
Dan, in a message earlier today to alpha-osf-managers you asked some
questions about SCSI; I passed your query along to one of the talented
engineers in our group who does a lot of work on the SCSI subsystem who I
figured could point to the place where the answers would be, and he gave me
the pointer to the SCSI standards web page (above) as well as provided an
explanation/overview. I've included that below.
In the latest issue of the Digital Technical Journal (I don't know if we
have it on our external web site, but you could probably get a copy from
your Digital account manager or I could get one for you), there is a good
article on parallel SCSI that might also fill in some of the gaps. I have
not read the article yet, just took the issue home the other night, but it
looks interesting.
Hope this helps.. Not my work, I don't know this stuff that well, but it
really seems to hit the key points you were asking about.
Tom
Dr. Thomas P. Blinn, UNIX Software Group, Digital Equipment Corporation
110 Spit Brook Road, MS ZKO3-2/U20 Nashua, New Hampshire 03062-2698
Technology Partnership Engineering Phone: (603) 884-0646
Internet: tpb at zk3.dec.com Digital's Easynet: alpha::tpb
ACM Member: tpblinn at acm.org PC at Home: tom at felines.mv.net
Worry kills more people than work because more people worry than work.
Keep your stick on the ice. -- Steve Smith ("Red Green")
My favorite palindrome is: Satan, oscillate my metallic sonatas.
-- Phil Agre, pagre at ucsd.edu
Opinions expressed herein are my own, and do not necessarily represent
those of my employer or anyone else, living or dead, real or imagined.
------- Forwarded Message
Date: Thu, 5 Feb 1998 12:18:22 -0500
Subject: Re: Is there a web site somewhere where this is explained?
(I thought I'd only take a moment, but oh well. but this is actually useful.. :)
The SCSI standards web page is http://www.symbios.com/x3t10
This page should cover it somewhere.
Trying to sort out terminology relative to "speed":
Real "SCSI" is SCSI-1, which was only 8-bit with async transfers (I think).
However, SCSI-1 is so antique that SCSI is now synonymous with SCSI-2
Relative to SCSI/SCSI-2, there is asynchronous (slow) and
synchronous (fast) transfers.
Async (slow) transfers require a 1:1 REQ/ACK link handshake
per byte. As this handshake was based on how fast the chips
could setup/latch lines, transfers were usually between
3-5 MB/sec. Please note this is for "more recent" hardware.
Older scsi devices could be much slower than this.
(note: theoretically, async can match/better the speed of
sync with infinitely fast chips)
Sync (fast) transfers allowed devices to "buffer" REQ/ACK
transitions. The devices would negotiate on how fast they
could sample/latch transitions (period) and how many they
could buffer (offset). For SCSI-2, the fastest period supported
was 100ns, thus a maximum speed of 10 MB/sec.
Note: the above was defined per 8-lines of data. Thus, the
rates given cover 8-bit (narrow) scsi buses. The standard
defined up to 4 8-bit segments, allowing for a 32-bit bus
and a max of 40 MB/sec. In reality, nobody to create reliable
cables/connectors beyond 16-bits, thus the word "WIDE" is
equates to 16-bit scsi buses.
SCSI-3 has extended the SCSI-2 spec by allowing faster synchronous
periods. Thus, if the period is twice as fast (50ns), the bus
supports 20 MB/sec, if 4x as fast (25ns) 40MB/sec, etc.
Obviously, as the clocks get faster, distance on the cables get
lower. Thus, there's a new transceiver type, LVD (Low Voltage
Differential) that tries to extend the cable lengths on these
fast buses back to the values we're used to from SCSI-2 (up to
25 meters).
This leaves the following (relevant) table entries:
Max
Speed Xfr Type Bus Width NickName(s)
(MB/s)
-------------------------------------------------------------------------
1-5 MB/s Async 8-bit (narrow) SCSI-1, SCSI
1-5 MB/s Async 16-bit (wide) sometimes "wide" SCSI
10 MB/s Sync-10 8-bit Fast SCSI, SCSI-2, Fast SCSI-2
20 MB/s Sync-10 16-bit Fast Wide SCSI
Fast Wide SCSI-2
(usually shortened to Wide SCSI)
20 MB/s Sync-20 8-bit ULTRA, ULTRA SCSI (sometimes SCSI-3)
40 MB/s Sync-20 16-bit ULTRA, ULTRA Wide SCSI
40 MB/s Sync-40 8-bit ULTRA2, ULTRA2 SCSI
80 MB/s Sync-40 16-bit ULTRA2, ULTRA2 Wide SCSI
Relative to mixing wide and narrow devices, narrow devices just don't see
the additional data lines. As Parallel SCSI devices establish a point to
point connection (e.g. selection/reselection in scsi terms) prior to
transfering data, the two devices can transfer data as fast as they
negotiate, independent of what else is on the bus. However, all wide
devices must be able to see the additional data lines. Thus, always
cable the wide devices first, then put the narrow devices on the end of
the chain. Caution - you can really confuse things if you have 2+ wide
segments on the same scsi bus. ALWAYS run one wide segment and one narrow
segment. This way, the wides will run wide, and the narrows (and narrow to
wide) will run narrow.
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And one last note from me...
When thinking about computing/transfer times... I once heard (From a
Sun performance monitoring course) that the same process running on the
following, when put into perspective.
Memory = 1 sec.
Network = 2 weeks
Disk(swap) = 1 year
I don't have the specifics on what speeds of each they were talking about.
This must have been with 10mbps net, and 5 or 10 mb/s SCSI.
As far as Network vs. SCSI...
Standard Ethernet:10 Mbps (mega BITS per sec.)=1.25 mb/sec (megaBYTES/sec)
Fast Ethernet: 100 Mbps = 12.5 mb/sec
Compare that with local disk speeds:
old SCSI-1 = 5 mb/sec
Fast or Wide SCSI (SCSI-2) = 10 mb/sec
Ultra or Fast Wide SCSI = 20 mb/sec
Ultra Wide SCSI (SCSI-3) = 40 mb/sec
Hope this helps others!
- Dan Kirkpatrick - dkirk at suhep.phy.syr.edu -
Systems Administrator
Physics Department
Syracuse University, Syracuse, NY
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