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From: chugins@hpcupt1.cup.hp.com (Chris Hugins)
Newsgroups: comp.periphs.scsi
Subject: Re: WARM SWAP
Message-ID: <67660002@hpcupt1.cup.hp.com>
Date: 26 Apr 91 17:34:04 GMT
References: <22249@shlump.nac.dec.com>
Organization: Hewlett Packard, Cupertino
Lines: 27

Some mid-range computers do allow peripherals to be powered on and off, and
even replaced without interrupting other users from performing their
tasks.  One example is the tape drive which may be seldom used.  Another
is that of a disk containing a private data set which may be moved from
machine to machine.  Not to mention the replacement/repair of a bad
peripheral.

Some database and manufacturing shops do not allow any (ANY) downtime.
Turning off the computer is not an option.

SCSI is not just for desk-tops anymore.  Unfortunately, the design of
the Small Computer System Interface (even "2") has not fully become
cognizant of this fact.

The ability to power-on and off devices without corruption of data 
across a common bus with other devices is important.  It is not 
completely clear if SCSI allows this, due to "noise" generated on
the bus at power-on (and OFF!) of scsi peripherals.  Some are "quieter"
than others, dependent upon the method of filtering at the line-drivers.

To remove/replace peripherals/cables on SCSI (even on a quiesced bus) 
is extremely risky.  Basically, it's "Do you feel lucky, punk?"

Maybe with SCSI-3....

Chris T. Hugins
chugins@hpisoa2.hp.com