Proposal to add a new modifier to SCSI Write commands. (fwd)

John Lohmeyer - SCSI jlohmeye
Tue Dec 10 12:52:12 PST 1996

* From the SCSI Reflector (scsi at, posted by:
* John Lohmeyer - SCSI <jlohmeye>
Another message send to the reflector administrative address instead
of to scsi at ...

Milton Scritsmier wrote:
From: Milton Scritsmier <milton at>
Subject: Re: Proposal to add a new modifier to SCSI Write commands.
To: owner-scsi at Symbios.COM
Date: Mon, 2 Dec 1996 13:36:43 -0700 (MST)
In-Reply-To: <96120214084546 at> from "Tom" at Dec 2, 96 02:08:46 pm
Reply-To: milton at (Milton Scritsmier)

Tom wrote:
> * From the SCSI Reflector (scsi at, posted by:
> * coughlan at (Tom)
> *
> >> The host gets an unrecovered read error,
> >> so it will not reassign the block.  
> No, this is written the way I meant it, though I agree this host behavior
> is not in the Standard, and is not required.  
> Generally, when a host gets an unrecovered read error it has no way to know
> what data was supposed to be in the block.  Host-initiated Reassignment is
> a bad thing to do in this case, because subsequent reads of the block will
> succeed, but the data they deliver will be garbage as far as the
> application is concerned.  The host should only Reassign when it knows what
> to put in the new block (that is, after a failed Write or a Recovered Read).
> Maybe your system works differently?

You exclude the case where the host can get the correct data from someplace
else. This would include such things as a mirror disk or from the other
disks in a RAID 5 redundancy group containing the disk with the read error.
While this may not be the case for many host systems, we do this in our
RAID disk array and I expect other disk array vendors do the same thing as

   Milton Scritsmier
   Array Technology, a division of EMC Corp.
   Boulder, CO

   milton at

John Lohmeyer               E-Mail: john.lohmeyer at
Symbios Logic Inc.           Voice: 719-533-7560 Fax: 719-533-7036
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