RE: iSCSI: Use of the A bit

[Eddy Quicksall <Eddy_Quicksall@ivivity.com>]

Why is it not of paramount importance? 

How would the target throttle its Data-In's if DataACK is optional when
ErrorRecoveryLevel != 0?

I don't know a lot about FC but parallel SCSI has this capability.

Eddy

-----Original Message-----
From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
Sent: Wednesday, March 13, 2002 8:17 PM
To: Eddy Quicksall
Cc: ips@ece. cmu. edu (E-mail); BURBRIDGE,MATTHEW
(HP-UnitedKingdom,ex2); owner-ips@ece.cmu.edu
Subject: RE: iSCSI: Use of the A bit



Eddy,

You need the target to support SNACK so that you need some recovery
capabilities.
And yes the consensus of the group was that the scenario you describe is
not of paramount importance.
So you may ask for ACK and keep the data for a while hoping to get it :-)

Julo


 

                      Eddy Quicksall

                      <Eddy_Quicksall@i        To:       "BURBRIDGE,MATTHEW

                      vivity.com>               (HP-UnitedKingdom,ex2)"

                      Sent by:                  <matthew_burbridge@hp.com>,
"ips@ece. cmu.   
                      owner-ips@ece.cmu         edu (E-mail)"
<ips@ece.cmu.edu>              
                      .edu                     cc:

                                               Subject:  RE: iSCSI: Use of
the A bit         
 

                      13-03-02 22:02

                      Please respond to

                      Eddy Quicksall

 

 




Then my only concern is that the initiator may ignore the A bit if it deems
that the bit is being set aggressively.

If it ignored it, then the target would be stalled waiting for he ACK.

Eddy
      -----Original Message-----
      From: BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2)
      [mailto:matthew_burbridge@hp.com]
      Sent: Wednesday, March 13, 2002 1:39 PM
      To: 'Eddy Quicksall'; ips@ece. cmu. edu (E-mail)
      Subject: RE: iSCSI: Use of the A bit
      Importance: High

      Eddy,

      The target is quite within its rights to use the A bit when at
      recovery level 0.  If the session is re-established due to recovery
      7.11.4 then the relevant command is aborted anyway and so there is no
      reason to keep hold of the data any way: With recovery level 0 there
      is no recovery mechanism that requires the target to keep the data.
      Therefore the A bit is redundant when the recovery level is 0.

      The spec says that the initiator MUST issue a SNACK if the A bit is
      set.  However, the MaxBurstSize restriction is there to prevent the
      initiator from having to send a SNACK on every PDU in the case where
      a target inadvertently sets the A bit in (for example) every data in
      PDU. The target may set the A bit more often than the MaxBurstSize
      but it should not expect a SNACK more often than this.

      Matthew Burbridge
            -----Original Message-----
            From: Eddy Quicksall [mailto:Eddy_Quicksall@ivivity.com]
            Sent: Wednesday, March 13, 2002 3:12 PM
            To: ips@ece. cmu. edu (E-mail)
            Subject: iSCSI: Use of the A bit

            Here is a case that I want to go over and if there is not
            already a solution, I think a refinement to the A bit could
            solve it.

            The problem is that a target (perhaps an iSCSI disk drive) does
            not have enough memory to transfer the full READ request so it
            must read from the medium as much as it can, transmit that,
            when that transmission is known to be good, read the next
            bunch, transmit that and so on.

            The problem we have is that the target must keep the buffer
            around until the transfer has been "ack'd" via ExpStatSN. But
            that status can't be sent because all of the requested data has
            not been sent. So the target would have to refuse to do the
            command.

            I was going to use the A bit for this thinking it would force
            the initiator to give an "ack" but our current wording does not
            make this a sure fire thing:

            1) The initiator may not want to run at ErrorRecoveryLevel 1.
            2) The initiator may ignore the A bit if it deems that the bit
            is being set aggressively.
            3) The target may set the A bit no more frequently than
            MaxBurstSize.

            Comments?

            mailto:Eddy_Quicksall@iVivity.com