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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: iSCSI - Change Proposal X bitJulian, Some comments on the below quoted scenarios : > session has 3 connections > on connection 1 I->T c1,c2,c3,C6 > on connection 2 I->T c4,c5,c7,c8 > Target receives 1,2,4,5,7,8 (miss 3 and 6) and acks 1 & 2 > Initiator closes 1 and resends c3, c4, c5,c7,c8 on connection 2 and 6 > on connection 3 > target receives all and starts executing and acks 8 on connection 3 but > connection 2 stalls after c3 for a LONG TIME > then (after 2 full sequence wraps) connection 2 is gets alive and > delivers c4,c5 etc (that are now valid) When the target acks CmdSN 8 on connection 3, it has, in effect, sent CmdSN ack's for CMdSNs 3,4,5,6,7,8. This implies that the commands with CmdSN 3, 4, 5, 7, & 8 were received by the target on connection 2 and their processing was commenced. Hence, the following does not make sense : > connection 2 stalls after c3 for a LONG TIME > then (after 2 full sequence wraps) connection 2 is gets alive and > delivers c4,c5 etc (that are now valid) c4, c5, etc were already delivered to the target and are not being re-delivered. There is no problem in this case. (??). Take the next scenario : > 2 connections: > > connection1 I->T c3,c4,c5 > status of 3 contains ack up to 6 and it and all other statuses are > lost > connection2 resend c3, c4 & c5 (no logout) and those are executed! Since the initiator got CmdSN ack's upto 6, the initiator should not be re-issuing these I/Os ?? I still don't see justification to require that initiators send a immediate NOP-OUT in the manner being advocated. On a more fundamental note, I see some issues with the initiator being allowed to re-issue the commands on a different connection without having first logged out the previous connection successfully. I see nothing in the draft that suggests such behaviour, while at the same time, it is not forbidden. By resorting to command retries on a different connection in an attempt to plug the hole, without first logging out the previous connection, the initiator is susceptible to encountering I/O failure of that I/O due to ULP timeout. Here's the scenario why such recovery should not be allowed : - Initiator sends CmdSN 3 on connection 1. - No CmdSN updates for a while and initiator re-sends CmdSn 3 on connection 2. - At the same time, target has sent CmdSN ack's for CmdSN 3 on connection 1. - Initiator has transferred the command allegiance on its side from connection 1 to connection 2 and is attempting the command on connection 2. However, the command does not go through, since the (ExpCmdSN, MaxCmdSN) window has advanced and the trget discards the command. - Target sends in data and/or R2T and/or status for CmdSN 3 on connection 1. Since the initiator is not expecting any traffic for that I/O on connection 1, it discards any PDUs received on that connection 1 for which no I/O state existed. In the above scenario, initiator will never get a CmdSN ack on connection 2 and will never be able to plug the hole despite repeated retries, finally, causing a ULP timeout, followed by session recovery. Given the above scenario, I suggest that the initiator must only re-issue commands on the same connection, and can re-issue them on another connection only following a successful logout. Comments ? Thanks, Santosh Julian Satran wrote: > > Santosh, > > The scenarios I am talking about are all derivatives of an initiator trying > to plug-in holes and switching connections. > As the initiator does know the "extent" of a hole it can send-out commands > that he did not have to. > I have sent the attached not to Mallikarjun a while ago. I think that > there might be many of this kind. I am also aware that X bit by itself > might have some bad scenarios but the new proposal fixes them all. > > Julo > > _____________________________ > > Mallikarjun, > > Take the following sequence scenario: > > session has 3 connections > on connection 1 I->T c1,c2,c3,C6 > on connection 2 I->T c4,c5,c7,c8 > Target receives 1,2,4,5,7,8 (miss 3 and 6) and acks 1 & 2 > Initiator closes 1 and resends c3, c4, c5,c7,c8 on connection 2 and 6 > on connection 3 > target receives all and starts executing and acks 8 on connection 3 but > connection 2 stalls after c3 for a LONG TIME > then (after 2 full sequence wraps) connection 2 is gets alive and > delivers c4,c5 etc (that are now valid) > > That is not a very likely scenario, I admit, but it is possible. > With X bit I could not find any such scenario since an X either follows a > good one on the same connection or can be safely discarded. > I suspect that there are some more scenarios that involve immediate > commands or commands that carry their own ack in the status and are acked > like: > > 2 connections: > > connection1 I->T c3,c4,c5 > status of 3 contains ack up to 6 and it and all other statuses are > lost > connection2 resend c3, c4 & c5 (no logout) and those are executed! > > I think we can avoid those be requiring a NOP exchange before reissuing a > command on a new connection or reissue the command with a task management > (that has an implied ordering) but why do it if X is an obvious and safe > solution. > > Julo > > Regards, > Julo > > > "Mallikarjun > C." To: Julian Satran/Haifa/IBM@IBMIL > <cbm@rose.hp.c cc: > om> Subject: Re: iscsi : X bit in SCSI Command PDU. > > 08-10-01 21:45 > Please respond > to cbm > > > > Julian, > > We currently have the following specified in section 2.2.2.1 - > > "The target MUST NOT transmit a MaxCmdSN that is more than > 2**31 - 1 above the last ExpCmdSN." > > It appears to me that the above is sufficient to ward off the > accidents of the sort you describe. Do you think otherwise? > -- > Mallikarjun > > Mallikarjun Chadalapaka > Networked Storage Architecture > Network Storage Solutions Organization > MS 5668 Hewlett-Packard, Roseville. > cbm@rose.hp.com > > Julian Satran wrote: > > > > Mallikarjun, > > > > There is at least one theoretical scenario in which an "old" command > > may appear in a "new window" and be reinstantiated. > > At 10Gbs and several connection that does not take months. With X the > > probability is far lower (not 0). I have no other strong arguments > > but I am still thinking. Matt Wakeley that insisted on it (against > > me) had some other argument that I am trying to find (I am note > > remembering). > > > > Julo > > > > "Mallikarjun C." > > <cbm@rose.hp.com> To: Julian > > Satran/Haifa/IBM@IBMIL > > 08-10-01 20:39 cc: > > Please respond to cbm Subject: Re: iscsi : X > > bit in SCSI Command PDU. > > > > > > > > Julian, > > > > Now that you put me on the spot, :-), my response - > > > > Santosh argued with me privately that X-bit no longer serves a > > useful purpose after the advent of task management commands to > > reassign. My response was that it never was a requirement per se, > > but always a "courtesy" extended by the initiator to help the > > target. I also suggested that X-bit may be considered for its > > usefulness in debugging. > > > > He still had some (very reasonable) comments for simplification > > - the most appealing of which (to me) was the opportunity to do > > away with the X-bit checking for *every* command PDU that the target > > has to endure now. > > > > If I missed a legitimate use of X-bit, please comment. Do you > > think it is a protocol requirement per se? I couldn't justify > > to myself so far (except the Login). > > > > Regards. > > -- > > Mallikarjun > > > > Mallikarjun Chadalapaka > > Networked Storage Architecture > > Network Storage Solutions Organization > > MS 5668 Hewlett-Packard, Roseville. > > cbm@rose.hp.com > > > > > > > > Julian Satran wrote: > > > > > > Santosh, > > > > > > I am not sure you went through all scenarios. A conversation with > > your > > > colleague - Mallikarjun - and getting through the state table may go > > a > > > long way to clarify the need for X. > > > > > > And I am sure that by now you found yourself several . > > > > > > Julo > > > > > > Santosh Rao > > > <santoshr@cup.hp.com> To: IPS Reflector > > > Sent by: owner-ips@ece.cmu.edu <ips@ece.cmu.edu> > > > cc: > > > 06-10-01 01:56 Subject: iscsi : X > > > Please respond to Santosh Rao bit in SCSI Command PDU. > > > > > > > > > > > > All, > > > > > > With the elimination of command relay from iscsi [in the interests > > of > > > simplification ?], I believe that the X bit in the SCSI Command PDU > > > can > > > also be removed. As it exists today, the X bit is only being used > > for > > > command restart, which is at attempt by the initiator to plug a > > > potential hole in the CmdSN sequence at the target. It does this on > > > failing to get an ExpCmdSN ack for a previously sent command within > > > some > > > timeout period. > > > > > > Given the above usage of command restart, no X bit is required to be > > > set > > > in the SCSI Command PDU when command re-start is done. > > > > > > Either : > > > (a) the target had dropped the command earlier due to a digest > > error, > > > in > > > which case, the command restart plugs the CmdSN hole in the target. > > > > > > [OR] > > > > > > (b) the target had received the command and was working on it, when > > > the > > > initiator timed out too soon and attempted a command restart to plug > > > [what it thought was] a possible hole in the CmdSN sequence. > > > > > > In case (a), no X bit was required, since the target knows nothing > > of > > > the original command. In case (b), no X bit is required again, since > > > the > > > (ExpCmdSN, MaxCmdSN) window would have advanced and the target can > > > silently discard the received retry and continue working on the > > > original > > > command received. > > > > > > Removal of the X bit in the SCSI Command PDU has the following > > > benefits > > > : > > > > > > a) The CmdSN rules at the target are simplified. No need to look at > > X > > > bit, only validate received CmdSN with (ExpCmdSN, MaxCmdSN) window. > > > > > > b) The reject reason code "command already in progress" can be > > > removed. > > > There's no need for this reject reason code anymore, since X bit > > > itself > > > is not required, and the targets can silently discard commands > > outside > > > the command window and continue to work on the original instance of > > > the > > > command already being processed at the target. > > > > > > c) Less work for the target and less resources consumed since it no > > > longer needs to generate a Reject PDU of type "command in progress". > > > It > > > can just silently discard any command PDU outside the (ExpCmdSN, > > > MaxCmdSN) window. > > > > > > d) Less code for the target, since it does not need : > > > - any Reject code paths when it receives X bit command PDUs that are > > > already in progress. > > > - No special casing of CmdSN checking rules. > > > - No overheads of verifying a received command based on its > > initiator > > > task tag, to check if the task is currently active, prior to sending > > a > > > Reject response with "command in progress". > > > > > > Comments ? > > > > > > Thanks, > > > Santosh > > > > > > -- > > > ################################## > > > Santosh Rao > > > Software Design Engineer, > > > HP-UX iSCSI Driver Team, > > > Hewlett Packard, Cupertino. > > > email : santoshr@cup.hp.com > > > Phone : 408-447-3751 > > > ################################## > > > Santosh Rao > <santoshr@cup. To: IPS Reflector <ips@ece.cmu.edu> > hp.com> cc: > Sent by: Subject: Re: iSCSI - Change Proposal X bit > owner-ips@ece. > cmu.edu > > > 23-10-01 22:50 > Please respond > to Santosh Rao > > > > Julian Satran wrote: > > > > However in order to drop "old" commands that might in the pipe on a > > sluggish connection - removing the X bit will require the initiator to > > issue an immediate NOP requiring a NOP response on every open connection > > whenever CmdSN wraps around (becomes equal to InitCmdSN). > > Julian, > > Can you please explain further the corner case you are describing above > ? Are you suggesting that special action should be taken every time > CmdSN wraps around, in case there were holes in the CmdSN sequence at > the wrap time ? Why is that ? > > Here's my understanding of how this plays out : > > Rule 1) > The CmdSN management rules at the target should be handling CmdSN wrap > case and the initiator cannot issue more than 2^32 -1 commands beyond > the last ExpCmdSN update it has received from the target, since the > target MUST NOT transmit a MaxCmdSN that is more than 2**31 - 1 above > the last ExpCmdSN. (per Section 2.2.2.1) > > Rule 2) > Any holes that occur in the CmdSN sequence are attempted to be plugged > by the initiator by re-issuing the original command. If the CmdSN never > got acknowledged and the I/O's ULP timeout expired, the initiator MUST > perform session recovery. (per Section 8.6) > > Thus, going by the above 2 rules, if the CmdSN sequence wraps upto > ExpCmdSN, the initiator will not be able to issue further commands, > since the target will keep the CmdSN window closed. The window can only > re-open when the CmdSN holes are plugged allowing ExpCmdSN and thereby, > MaxCmdSN to advance. (rule 1 above). > > Under the above circumstances, the initiator will possibly try to plug > the CmdSN hole by re-issuing the original command. It may do this 1 or > more times before its ULP timeout expires. Either the holes get plugged > and the windoe re-opens, or ULP timeout occurs without the corresponding > CmdSN for that I/O having been acknowledged, resulting in session > logout. (rule 2 above). > > What is required over and beyond the above ? Why does removal of X-bit > require an immediate NOP to be issued every time CmdSN wraps and a hole > exists in the CmdSN sequence (??). > > Regards, > Santosh > > -- > ################################## > Santosh Rao > Software Design Engineer, > HP-UX iSCSI Driver Team, > Hewlett Packard, Cupertino. > email : santoshr@cup.hp.com > Phone : 408-447-3751 > ################################## -- ################################## Santosh Rao Software Design Engineer, HP-UX iSCSI Driver Team, Hewlett Packard, Cupertino. email : santoshr@cup.hp.com Phone : 408-447-3751 ##################################
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