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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: iSCSI: Use of the A bitI also do not like the comment about having to wait until maxBurstsize byte to set the A-bit. I wanted to use it as Eddie suggested. To force the initiator to acknowledge the IO so that I can release resources on the target. This will be especially useful for hardware accelerated IOs where the chip cannot declare the IO complete until it has been Ack'd. Could we just change the wording to: ---- For sessions with ErrorRecoveryLevel 1 or higher the target sets this bit to 1 to indicate that it requests from the initiator a positive acknowledgement for the data received. The target should use the A bit moderately; the A-bit MAY only be set to 1 at the end of a data sequence. On receiving a Data-In PDU with the A bit set to 1 the initiator MUST issued a SNACK of type DataACK. If the initiator has detected holes in the input sequence, it MUST postpone issuing the SNACK of type ACKN until the holes are filled. --- This way I can always set the A bit at the end of an IO. Kevin Lemay -----Original Message----- From: Eddy Quicksall [mailto:Eddy_Quicksall@ivivity.com] Sent: Thursday, March 14, 2002 5:51 AM To: BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2); 'Rod Harrison'; John Hufferd Cc: ips@ece. cmu. edu (E-mail) Subject: RE: iSCSI: Use of the A bit I see a lot of responses talking about ErrorRecoveryLevel = 0. That was not my original point. ErrorRecoveryLevel = 0 already has this solved. I am talking about ErrorRecoveryLevel > 0 (I made an error in my first EMAIL and the point may have been lost). If the initiator has the option to ignore the A bit, the target will get stuck. What is wrong with requiring the initiator to respond? Can we just get that answered first? I suspect that there is a concern that this may cause too much traffic. My answer would be "don't buy a cheep target ... their lack of memory may cause degraded performance" (not intended for the spec, of course :-) ). I also do not see the rational in not allowing the A bit more often than MaxBurstSize. The ULP layers may not know anything about the transport layers. So their resource issues are not known by the transport(s). The MaxBurstSize is an iSCSI issue and may not apply to future protocols. So, why not just say something like: 1) The initiator MUST send a DataACK SNACK if it sees the A bit set (and if ErrorRecoveryLevel > 0) 2) The A bit may be set only when the F bit is set. And leave it go at that? Eddy -----Original Message----- From: BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2) [mailto:matthew_burbridge@hp.com] Sent: Thursday, March 14, 2002 7:22 AM To: 'Rod Harrison'; John Hufferd Cc: Eddy Quicksall; ips@ece. cmu. edu (E-mail) Subject: RE: iSCSI: Use of the A bit I think that what Santosh and others have stated is the correct think to do for initiators. If the revovery level is 0 and the A bit is set then the initiator is quite with in its rights to ignore the A bit (If the target is at fault then penalise the target and not the initiator). IMHO changing the spec to make it more specific is not the right thing and it could lead to inter operability problems. However, I do think something needs to be added so I propose adding the statement: "If ErrorRecoveryLevel=0 then the initiator MAY ignore the A bit (i.e. assume A=0)". One could argue for the MAY to be a MUST but if there is a kind initiator out there then let it do its stuff - it can only help the rogue target. Matthew -----Original Message----- From: Rod Harrison [mailto:rod.harrison@windriver.com] Sent: Thursday, March 14, 2002 9:27 AM To: John Hufferd Cc: Eddy Quicksall; BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2); ips@ece. cmu. edu (E-mail) Subject: RE: iSCSI: Use of the A bit 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: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]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 ..." 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: hufferd@us.ibm.com "Rod Harrison" <rod.harrison@windriver.com>@ece.cmu.edu on 03/13/2002 01:05:51 PM Sent by: owner-ips@ece.cmu.edu To: "Eddy Quicksall" <Eddy_Quicksall@ivivity.com>, "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 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: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]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: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
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