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    Re: iSCSI: draft 7: iSCSI response and SCSI sense data



    David,
    
    It appears to me that generating a SCSI CHECK CONDITION is
    the right thing for iSCSI to do regardless of T10's eventual
    decision (ACA/persistent-UA (T10 proposal 00-359r1)/something
    totally different) on a mechanism to enforce ordering semantics.
    
    Robert Elliott pointed out a violation by iSCSI in trying to 
    additionally qualify ASC, ASCQ with its response codes.  I suggest 
    that dropping this "additional description" attempt in all these 
    cases is all iSCSI needs to do to meet SAM's expectations.
    
    Regards.
    -- 
    Mallikarjun 
    
    Mallikarjun Chadalapaka
    Networked Storage Architecture
    Network Storage Solutions Organization
    MS 5668 Hewlett-Packard, Roseville.
    cbm@rose.hp.com
    
    
    Black_David@emc.com wrote:
    > 
    > In my lousy Indiana Jones impression - "ACA, why does it
    > always have to be ACA?" ....
    > 
    > This whole notion that ACA is a fundamental coordination
    > mechanism for shared SCSI access to a target is not headed
    > in a good direction - see the recent message from Ralph Weber
    > expressing T10's concern that expansion of the usage of ACA is
    > an invitation to undesirable denial of service possibilities.
    > I will note that the reservation example that Ralph used in his
    > message is the sort of thing that can break clustering software,
    > although I don't know for certain whether there's any existing
    > clustering software that it does in fact break.
    > 
    > I think Rob and Jim are correct when they describe the current
    > 2.4.3 text as being at odds with SAM-2.  So, I think:
    > 
    > - The text in 2.4.3 that is at odds with SAM-2 should come out.
    >         The fact that it was caused by a desire to use ACA makes
    >         it doubly suspect.
    > - The current mandate for ACA in Section 8.2 exceeds the level
    >         stated in the iSCSI requirements draft and I don't believe
    >         it represents rough consensus of the WG.  Support for
    >         ACA should be a "SHOULD", not a "MUST".
    > - The entire effort to use ACA or something like it to serialize
    >         error conditions for multiple commands in flight has not been
    >         explained well to T10, and what has been explained has not
    >         received a good reception.
    > 
    > The last bullet has a couple of consequences.  First, we should
    > take out anything else that's crept into iSCSI in support of this
    > use of ACA.  Second, those who want to use ACA or something ACA-like
    > for this error coordination across multiple commands in flight
    > should write up a submission to T10 explaining what the problem
    > is and how ACA or something like it solves the problem.
    > 
    > I think Ralph is conveying a message from T10 that whatever the
    > problem iSCSI has, T10 is not inclined to believe that more use
    > of ACA is NOT the solution.  It may take a while to work with T10
    > to get the right solution designed, but I'd prefer doing nothing
    > over force-fitting ACA to a use that T10 doesn't think is appropriate.
    > When we figure out what the right thing is, we can put in the right
    > text to describe it.
    > 
    > Comments?
    > --David
    > 
    > ---------------------------------------------------
    > David L. Black, Senior Technologist
    > EMC Corporation, 42 South St., Hopkinton, MA  01748
    > +1 (508) 435-1000 x75140     FAX: +1 (508) 497-8500
    > black_david@emc.com       Mobile: +1 (978) 394-7754
    > ---------------------------------------------------
    > 
    > > -----Original Message-----
    > > From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
    > > Sent: Wednesday, August 01, 2001 8:01 PM
    > > To: ips@ece.cmu.edu
    > > Subject: Re: iSCSI: draft 7: iSCSI response and SCSI sense data
    > >
    > >
    > >
    > > Elliot,
    > >
    > > This issue has some history.  After many took the position you are
    > > advocating
    > > and I had to drop the check condition all list participants that had
    > > something to say adopted the current position in order the
    > > benefit from the
    > > "serializing" effect of ACA whenever a check condition is
    > > originated at the
    > > target and the delivery subsystem is still operational.
    > >
    > > Julo
    > >
    > >
    > > "Elliott, Robert" <Robert.Elliott@compaq.com> on 02-08-2001 02:26:57
    > >
    > > Please respond to "Elliott, Robert" <Robert.Elliott@compaq.com>
    > >
    > > To:   "'ips@ece.cmu.edu'" <ips@ece.cmu.edu>
    > > cc:
    > > Subject:  iSCSI: draft 7: iSCSI response and SCSI sense data
    > >
    > >
    > >
    > >
    > > iSCSI draft 7 section 2.4.3 includes a table relating the iSCSI
    > > response codes to various CHECK CONDITION status additional
    > > sense codes:
    > >
    > >   "0x01 Target Failure
    > >     ASC=0x44 ASCQ=0x00 INTERNAL TARGET FAILURE
    > >   0x02 Delivery Subsystem Failure
    > >     ASC=0x47 ASCQ=0x03 INFORMATION UNIT CRC ERROR DETECTED
    > >   0x03 Unsolicited data rejected
    > >     ASC=0x49 ASCQ=0x00 INVALID MESSAGE ERROR
    > >   0x04 Not enough unsolicited [data]
    > >     ASC=0x49 ASCQ=0x00 INVALID MESSAGE ERROR
    > >   0x05 SNACK rejected [Command in progress]
    > >     ASC=0x47 ASCQ=0x03 INFORMATION UNIT CRC ERROR DETECTED
    > >
    > >   As listed above, each defined response code MUST be used (under the
    > >   conditions described in the 'Reason' field), only when the
    > >   corresponding SCSI CHECK CONDITION is signaled, to convey additional
    > >   protocol service information.  A SCSI CHECK CONDITION with the ASC
    > >   and ASCQ values as tabulated MUST be signaled by iSCSI targets for
    > >   all the instances in this document referring to usage of one of the
    > >   above defined response codes."
    > >
    > > No other SCSI protocol does this. iSCSI seems to be granting the
    > > SCSI-level status a priority over the iSCSI-level response data.
    > > I think the other protocols treat the response data as more
    > > important; if the response data indicates a problem, the SCSI layer
    > > is considered broken and the SCSI status is undefined.
    > >
    > > Forcing the iSCSI layer to fill in those fields seems to break the
    > > layering model.
    > >
    > > I suggest removing the CHECK CONDITION status and additional sense
    > > code linkage and leave the SCSI status undefined if the iSCSI
    > > response is nonzero.
    > >
    > > This is compatible with SAM-2, which only requires status be returned
    > > when the service response is TASK COMPLETE (SAM-2 revision 18
    > > section 5.3.1):
    > >   "Status shall be sent from the logical unit to the application
    > >    client whenever a command ends with a service response of
    > >    TASK COMPLETE or LINKED COMMAND COMPLETE."
    > >
    > > Furthermore, SAM-2 requires that status be ignored when the service
    > > response indicates an error (SAM-2 revision 18 section 5.1):
    > >   "Status: A one-byte field containing command completion status
    > >   (see 5.3). If the command ends with a service response of
    > >   SERVICE DELIVERY OR TARGET FAILURE, the application client
    > >   shall consider this parameter to be undefined."
    > >
    > > I think iSCSI's current wording may violate this rule.
    > > ---
    > > Rob Elliott, Compaq Server Storage
    > > Robert.Elliott@compaq.com
    > >
    > >
    > >
    > >
    


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Last updated: Tue Sep 04 01:04:07 2001
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