Subject: RE: iSCSI: Use of the A bit

From: "John Hufferd" <[email-masked]>
Date: Wed, 13 Mar 2002 19:12:21 -0800

Actually it does not actually say that.  It says "This is limited to
sessions that support error recovery and is implemented through the A bit
..."

I know what I am about to say, is nit picking, but ....
ErrorRecoveryLevel=0 is an error recovery technique.

Having said that, we probably should just change that statement to "This is
implemented through the A bit ..."

.
.
.
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: [email-masked]

"Rod Harrison" <[email-masked]>@ece.cmu.edu on 03/13/2002
01:05:51 PM

Sent by:    [email-masked]

To:    "Eddy Quicksall" <[email-masked]>, "BURBRIDGE,MATTHEW
       (HP-UnitedKingdom,ex2)" <[email-masked]>, "ips@ece. cmu.
       edu (E-mail)" <[email-masked]>
cc:
Subject:    RE: iSCSI: Use of the A bit

Eddy,

 You were right first time, in draft 11 section 2.5.1.5 paragraph 5
and 9.7.2 state that the A-bit is only supported if ErrorRecoveryLevel
is 1 or higher.

 - Rod

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of
Eddy Quicksall
Sent: Wednesday, March 13, 2002 8:03 PM
To: BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2); ips@ece. cmu. edu
(E-mail)
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:[email-masked]]
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:[email-masked]]
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:[email-masked]

---

From: Santosh Rao <[email-masked]>
Date: Wed, 13 Mar 2002 19:25:21 -0800 (PST)

> implemented through the A bit ..."

John,

Initiators that use "ErrorRecoveryLevel=0" must not be required to respond to
an A bit setting with a DataACK. "ErrorRecoveryLevel=0" implies the initiator
need not support/use SNACK pdu.  

A target can recycle its data buffers once it has completed sending the data,
since it knows that the initiator will not issue Data SNACK while operating as
"ErrorRecoveryLevel=0".

- Santosh

-- 
##################################
Santosh Rao
Software Design Engineer,
HP-UX iSCSI Driver Team,
Hewlett Packard, Cupertino.
email : [email-masked]
Phone : 408-447-3751
##################################

---

From: Ajit Aryan <[email-masked]>
Date: Thu, 14 Mar 2002 10:18:36 +0530

Santosh, your argument seems fair.

Aryan

---

From: "Rod Harrison" <[email-masked]>
Date: Thu, 14 Mar 2002 09:26:48 -0000

John,

	Actually 9.7.2 says almost exactly that ...

	"For sessions with ErrorRecoveryLevel 1 or higher, the target sets
this bit to 1 to indicate that it requests a positive acknowledgement
from the initiator for the data received."

	We've interpreted that to mean our initiator should ignore the A bit
if ErrorRecoveryLevel is 0.

	Having said that it is very easy to support just the positive form of
SNACK, so I would have no objection to removing the A bits current
link to ErrorRecoveryLevel.

	- Rod

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of
John Hufferd
Sent: Thursday, March 14, 2002 3:12 AM
To: Rod Harrison
Cc: Eddy Quicksall; BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2); ips@ece.
cmu. edu (E-mail)
Subject: RE: iSCSI: Use of the A bit

Actually it does not actually say that.  It says "This is limited to
sessions that support error recovery and is implemented through the A
bit
..."

Having said that, we probably should just change that statement to
"This is
implemented through the A bit ..."

.
.
.
John L. Hufferd

To:    "Eddy Quicksall" <[email-masked]>,
"BURBRIDGE,MATTHEW
       (HP-UnitedKingdom,ex2)" <[email-masked]>, "ips@ece.
cmu.
       edu (E-mail)" <[email-masked]>
cc:
Subject:    RE: iSCSI: Use of the A bit

Eddy,

 - Rod

-----Original Message-----

Eddy
-----Original Message-----

Eddy,

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

Comments?

---

From: Santosh Rao <[email-masked]>
Date: Wed, 13 Mar 2002 19:25:21 -0800 (PST)

John,

- Santosh