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From: guineau@wjg.enet.dec.com (W. John Guineau)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: SCSI II specs
Keywords: Big, mean, fast & shiny controllers
Message-ID: <1990Sep20.125810@wjg.enet.dec.com>
Date: 20 Sep 90 17:04:13 GMT
References: <1232@tardis.Tymnet.COM> <3769@ruuinf.cs.ruu.nl> <14418@cbmvax.commodore.com>
Sender: newsdaemon@shlump.nac.dec.com
Reply-To: guineau@wjg.enet.dec.com
Organization: Digital Equipment Corporation, Marlboro, MA
Lines: 47

In article <1232@tardis.Tymnet.COM>, jms@tardis.Tymnet.COM (Joe Smith) writes:

[stuff deleted] 

|> 
|> I've heard that from a software point of view, SCSI-II is mainly the
enhanced
|> set of commands that Dave mentions.  But from a hardware point of view,
|> SCSI-II has two options, which can be combined.
|> 
|>   1) Just the new commands (including better connect/reconnect) = 5M
bytes/sec
|>      (this is copatible with SCSI-1 devices)
|>   2) Fast SCSI = 10 megabytes per second
|>   3) Wide SCSI = 16 bit transfers at 5 million per second = 10 megabytes/sec
|>      (SCSI-I and SCSI-II devices can co-exist on the 16-bit bus)
|>   4) Fast and wide = 16 bit transfers at 10 M = 20 megabytes per second.
|> 

SCSI II also specifies a 32 bit data path option. Couple that with FAST
and you get 40MB/s. Both 16 and 32 bit require a second cable (termed the 
B cable). I think there are actually 2 or 3 cabling schemes defined in the
latest spec revision.

|> At Sun-Expo, several companies announced plans for 16-bit SCSI-II.
|> No single disk produces 20 megabytes per second, but some companies are
|> working on disk clusters that act like a single, large, fast disk.

Yup, they're called arrays. There is a paper from Berkeley describing
various array architectures (RAID levels | RAID = Redundant Array using 
Inexpensive Disks). RAID 3 and 5 are the most common. RAID 3 is optimum for 
getting lots of KB/s while RAID 5 gives lots of IO/s.

|> -- 
|> Joe Smith (408)922-6220 | SMTP: jms@tardis.tymnet.com or
jms@gemini.tymnet.com
|> BT Tymnet Tech Services | UUCP: ...!{ames,pyramid}!oliveb!tymix!tardis!jms
|> PO Box 49019, MS-C41    | BIX: smithjoe | 12 PDP-10s still running! "POPJ
P,"
|> San Jose, CA 95161-9019 | humorous dislaimer: "My Amiga speaks for me."
|> 


--
W. John Guineau   			guineau@wjg.enet.dec.com
Digital Equipment Corporation
Marlboro MA. 01752