RE: iSCSI: Use of the A bit

["John Hufferd" <hufferd@us.ibm.com>]

If it hurts, don't do that.  Keep it within MaxBurstSize constraints.  And
set MaxBurstSize correctly for the device.

.
.
.
John L. Hufferd
Senior Technical Staff Member (STSM)
IBM/SSG San Jose Ca
Main Office (408) 256-0403, Tie: 276-0403,  eFax: (408) 904-4688
Home Office (408) 997-6136, Cell: (408) 499-9702
Internet address: hufferd@us.ibm.com


Eddy Quicksall <Eddy_Quicksall@ivivity.com>@ece.cmu.edu on 03/13/2002
12:02:38 PM

Sent by:    owner-ips@ece.cmu.edu


To:    "BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2)"
       <matthew_burbridge@hp.com>, "ips@ece. cmu. edu (E-mail)"
       <ips@ece.cmu.edu>
cc:
Subject:    RE: iSCSI: Use of the A bit




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