Path: utzoo!mnetor!tmsoft!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!uunet!npi!conroy
From: conroy@npi.com (Pat Conroy)
Newsgroups: comp.sys.dec
Subject: Re: big SCSI == LOST_DATA????
Message-ID: <1991Mar14.161018.4109@npi.com>
Date: 14 Mar 91 16:10:18 GMT
References: <1991Mar13.153825.22240@banana.fedex.com>
Organization: National Peripherals, Inc.
Lines: 31

In article <1991Mar13.153825.22240@banana.fedex.com> bill@banana.fedex.com (bill daniels) writes:
>Supposedly, the group 0 commands break down when referencing large addresses.
>My question is:  Does this address wraparound problem arise any time one
>addresses the last portions of a disk OR only if the disk is considered
>one large (read Ultrix "c") partition?

The group 0 read and write commands have 21 bits to address a logical block
on the disk.  Therefore, the last logical block that you can address is...
2,097,151.  At a 512 byte block-size that gives you 1,073,741,824 bytes that
you can address.  Since this restriction is at the SCSI interface level, 
you can't just repartition the drive into little chunks and access all of it.
You are stuck with 2**21 blocks per device, so partition accordingly.

The group 1 read-extended and write-extended commands have 4 bytes of address.
This address is either absolute, or in the case of linked commands, relative
to the last accessed block.  At a 512 byte block-size, this yeilds about
2.1TBytes of absolute addressable storage.  I don't think we'll see this passed
in the near future.

Opinion:  That "User data at risk using 1.2GB drives"  (I paraphrase) headline
is a gross overstatement of the problem.  The problem only occurs when you use
a 512byte block-size on the device (some OEMs use 1k), and only with some 1.2GB
disks (unformatted capacity).  For example, the Fujitsu 1.2 has slightly more
capacity than the Seagate 1.2.  This extra amount pushed the Fujitsu over that
2**21 limit.  The real story should have been "Disk Resellers Negligent in 
Peddling Disk Drives."  People should understand what they sell.

-- 
Pat Conroy			Mail: conroy@npi.com
Systems Engineer		Voice: 708-325-4151
National Peripherals, Inc.	Fax: 708-325-4230