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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: iSCSI: Flow ControlPierre, I am not sure that I agree. I do not understand how a credit process to an initiator, solves the problem (if it is a problem) with 100s of initiators some of which can be dormant for long periods of time and very active at others. I believe that the current command window meets all the real problems. You did put your finger on the key solution, that is the management of the Target Buffers, by the target. . . . John L. Hufferd Pierre Labat <pierre_labat@hp.com>@ece.cmu.edu on 10/06/2000 09:00:08 AM Sent by: owner-ips@ece.cmu.edu To: IPS@ece.cmu.edu cc: Subject: RE: iSCSI: Flow Control Hi, From the mails on this thread it seems that most people agree on the following ? A) Unsolicited data will be allowed by iSCSI. Well, i think that the protocol must be able to handle large quantities of unsolicited data to be performant for the writes on large latency networks. B) We need of course a flow control on the data because even if we have a flow control on the commands, just a few commands with huge unsolicited data could overwhelm the target buffers. C) We need a flow control on the commands to avoid the TASK SET FULL condition. If the target hit this condition it has to drop command (return task set full) and drops data associated. Avoiding the task set full condition avoid too the "dead locks". D) As noted by several, one TCP connection doesn't "block" the commands behind the data. The iSCSI adapters (on the initiator side) must be build in a way to send first the commands/task management requests. As soon as a command/task mgt is posted to the adapter, the next iSCSI PDU sent by the adapter will be the one encapsulating the command/task mgt request. This has to be true for any command (read/write...) and task mgt request. About managing these flow controls: ================================== The TCP window can handle efficiently the flow control for the data. Now we need to flow control the commands. The solutions can be: 1) Use the window described in the draft Advantage: --------- o Flow control the commands Disavantages: ------------- o Some said that as the flow control is per session, the target will flow control the whole session (several LUs) when the task set of one LU will become [almost] full. Hence one LU blocks the other ones in the session when they could continue to work. This problem can be workaround by the target using a right command blocks allocation policy. (A command block is a buffer containing the command pending (in the task set) in the target). Instead of having a policy where the command blocks are allocated per LU, the target could use a policy where the command blocks are allocated per session. The target maintains a pool of blocks per session not per LU. In this case one LU cannot block another one. That means that inside a session the balance of the commands in the various task sets is driven by the initiator not by the target. It doesn't seems to be wrong doing that, hence this disavantage is NOT a disavantage. o If the initiator send a mix of fast and slow commands the slow commands can close the command window even if the target have slots free for new commands. 2) Replace the window in the iSCSI draft with a command credit. The target advertises (with each response) a credit to the initiator. The command reference number and status reference numbers can be kept for ordering/recovery purpose. Now, the MaxCMdRN becomes a credit unrelated to the CmdRn, that's the only change compared to the draft. It specifies the maximum number of commands in flight over the session. Advantage --------- o flow control commands o get rid of the blocking of fast commands by slow commands. Disavantage ----------- o None 3) Use a (iSCSI) credit per LUN (advertised with the command responses) Advantage --------- o flow control the commands (per LUN) Disavantage ----------- o the iSCSI layer on the initiator needs to maintain a state and a flow control per LUN 4) T10 specifies a command credit advertising mechanism (credit returned with the status + some asynchronous advertisements (see Mike mail/IBA)) Advantage --------- o flow control the commands (per LUN) o the state of the flow control would be maintained at the SCSI layer, it's ok because the SCSI layer already mantains some states/LUN o will benefit to FC too Disavantage ----------- o are we able to persuade T10 to specify that quickly? o need to change the SCSI layer Conclusion ========== It seems to me that with 2) and even with only one TCP connection, we have a solution very performant in term of throughput, simple, and with no dead locks. Do you agree? Regards, Pierre
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