RE: iSCSI: Use of the A bit

[Sukha Ghosh <sukha_ghosh@ivivity.com>]

Is it possible that A bit is defined to be valid when F bit is '1' in Data-In PDU so that the target sets the A bit (if it requires DataACK SNACK) only on the last Data-In PDU of a sequence. This may reduce some aggressiveness and still satisfies the DataACK requirement from Initiator.

-----Original Message-----
From: Eddy Quicksall [mailto:Eddy_Quicksall@ivivity.com]
Sent: Wednesday, March 13, 2002 3:03 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: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