Re: iSCSI: Re: iSCSI & Linked Commands

[Mark Mokryn <mark@sangate.com>]

Doug,

Whether or not a device uses relative addressing has nothing to do with
command linking. A LOCATE command moves the head, and a following READ
reads the data. Using command linking, relative addressing, etc. does
not protect the sender of the above commands from another command, sent
by that initiator or any other, which may move the head, and thus cause
the original READ to get the data from the wrong place. There is no
protection between the first and any subsequent commands under command
linking. Thus, the proper way to use LOCATE and READ is with
RESERVE/RELEASE. I assume that it was in order to settle this confusion,
that FC-TAPE decided to ban linked commands.

-mark

Douglas Otis wrote:
> 
> Stephen,
> 
> Unlike random access devices, sequential access devices operate with
> relative addressing.  For random access devices, this is a seldom used
> option.  There is a requirement to bind commands together to ensure order of
> execution on these devices.  By popular, you mean not sequential?
> 
> Doug
> 
> > Julian,
> >
> > > According to your logic no FCP implementation can use linked commands?
> > > Is this true for all OS's?  Is it a verified fact or foloklor?
> >
> > In my experience it's fact.  I have never used a SCSI stack which both
> > supported AND used linked commands.  Like some others here, I always
> > assumed AIX might :^) Ralph has pointed out that T10 is well aware
> > that the feature is not popular.  There are other ways of
> > accomplishing the same thing that are less likely to blow up in your
> > face.
> >
> > > Is it so also for the new MS StorPort driver?
> >
> > I don't know, but I'd be really surprised if they did use linked
> > commands.  You have to be pretty nuts to rely on a feature that's not
> > even exercised by most SCSI implementations.
> >
> > Steph
> >

-- 
Mark Mokryn      SANgate Systems Inc.      mark@SANgate.com
Phone: +972-9-8919821                Mobile: +972-54-270030
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