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    RE: iSCSI ACA requirement

    I don't understand all the concern about the ACA being a MUST, because
    this is only in the iSCSI PDU's and not in the SCSI INQUIRY DATA. The
    iSCSI target can emulate the local device supporting ACA. This is easily
    done by checking the status code on the returned command (something that
    must alreadly be done), and if it indicates a SCSI error issuing a
    SCSI REQUEST SENSE command. When the response for the SCSI REQUEST SENSE
    is returned, the target iSCSI implementation returns all the information
    in the iSCSI SCSI CMD Response message. This eliminates another round trip
    to retrieve the information. On the iSCSI initiator side the local SCSI
    layer receives the Check condition with valid request and sense data without
    the need of issuing the REQUEST SENSE with the round trip delay.
    Jeff Fellin
    > From Mon Feb  4 14:09 EST 2002
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    > Subject: RE: iSCSI ACA requirement
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    > I thought the outcome of the previous lengthy discussion on
    > ACA was to make it a SHOULD.  That is also the right requirement
    > level for Julian's concern that failure to use ACA may have
    > performance consequences.  I agree with Dave Peterson that
    > requiring ACA behavior from devices that have no intention
    > of supporting it other than a possible need for compliance
    > with the iSCSI spec is pointless.  Let's make ACA a SHOULD.
    > Thanks,
    > --David
    > ---------------------------------------------------
    > David L. Black, Senior Technologist
    > EMC Corporation, 42 South St., Hopkinton, MA  01748
    > +1 (508) 249-6449 *NEW*      FAX: +1 (508) 497-8500
    >         Cell: +1 (978) 394-7754
    > ---------------------------------------------------
    > -----Original Message-----
    > From: Julian Satran []
    > Sent: Saturday, February 02, 2002 2:22 AM
    > To:
    > Subject: Re: iSCSI ACA requirement
    > Dave, 
    > In fact we could make it SHOULD as ACA support is not anymore critical
    > provided that the right exception mechanism for busy etc. is in place and it
    > is now and does not use ACA(!).  However the effect on performance could be
    > more taxing than in the case of FCP - the network diameter supported is far
    > larger and the need to support commands in fly is more important than in the
    > case of FCP. 
    > Julo 
    > 	"Dave Peterson" <> 
    > Sent by: 
    > 02-02-02 00:17 
    >         To:        "Ips@Ece. Cmu. Edu" <> 
    >         cc:         
    >         Subject:        iSCSI ACA requirement 
    > I'm having heartburn with the statement in iSCSI rev 10 clause 9.2:
    > "As iSCSI can have many commands in-flight between initiator and target,
    > iSCSI mandates support for ACA."
    > My understanding of the above statement is that iSCSI target must indicate
    > support for NACA=1.
    > Requiring ACA is problematic and normally not necessary for implementations
    > for a variety of reasons.
    > Examples:
    > a. A small number of devices actually support NACA=1.
    > b. In practice FC applications do not require command ordering (i.e., use of
    > the Simple Queue Tag). If ordering is a consideration the application will
    > issue the command and wait for the response.
    > c. The FC/FCP standards do not require NACA=1.
    > d. Complicates bridging implementations
    >                 - bridge must proxy NACA=1 for a device that does not
    > support NACA=1
    >                 - bridge must maintain NACA behavior when the end device
    > does not support
    > NACA=1
    > I do understand the benefits to requiring NACA=1 especially when command
    > ordering and stateful operations are desired, but its not realistic at this
    > time, IMHO.
    > As such this use of ACA should be a SHOULD, not a must or mandate.
    > Dave


Last updated: Mon Feb 04 15:17:57 2002
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