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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: iSCSI: Keep aliveSteph, First I did mean connections, thank you. Though I disagree,somewhat ,with some of your comments, they all lead up to the last two paragraphs, which I do agree with. So, that is probably good enough. I was just getting anxious about all the Pinging stuff that was flying around, and wanted to limit it as much as possible. So again, I can not quarrel with your conclusions. . . . John L. Hufferd Senior Technical Staff Member (STSM) IBM/SSG San Jose Ca (408) 256-0403, Tie: 276-0403 Internet address: hufferd@us.ibm.com Stephen Bailey <steph@cs.uchicago.edu>@ece.cmu.edu on 11/08/2000 06:48:13 AM Sent by: owner-ips@ece.cmu.edu To: ips@ece.cmu.edu cc: Subject: Re: iSCSI: Keep alive John Hufferd, > It has been stated that there may be value in ensuring that a link has gone > down and re-establish it, without the SCSI application being aware. A link, or a connection? If a TCP connection has failed, it implies that the path has been interrupted in more than a momentary way. A retry strategy for a failed TCP connection should be non-aggressive. As such, being able to reestablish a failed TCP connection without having to notify the ULP is unlikely. So, I don't agree that this goal is feasible or desirable, but... > 1. If for what ever reason an iSCSI session thinks something has gone wrong > and is contemplating dropping the connection it should Ping before dropping > a session that maybe down, as a possible confirmation that the link is > down. What this amounts to is a decision by the iSCSI layer that a path which is `good enough' for TCP is not good enough for the iSCSI application. I believe that the iSCSI spec should not have anything to say about this. Any path which is good enough for a TCP connection should be good enough for iSCSI in general. Of course, specific implementations can make this choice, but how they determine that the connection is not good enough implementation dependent. Maybe it involves an iSCSI NOP-{In,Out}, or maybe it involves running a little SCSI command, or something else entirely. Therefore, I see no requirement to ping before `dropping' (actively closing) a connection. > 2. No point to pinging every 10 Seconds or so. A keep-alive like this should use an exponential backoff, rather than a constant interval. One might claim that the only justifiable keep-alive would be something of this form implemented by a target, comparable to (or equivalent to) the TCP keep-alive. The TCP keep-alive allows servers to free connection resources from clients to whom the path is lost and with whom no operations are currently outstanding. In iSCSI the target is the server and the initiator is the client. Since the client initiates all exchanges, including closing the connection in the nominal case, it does not need such a timer. In the case where the path has failed and the client attempts an activity which leads to it discovering the failed path, it will declare the connection closed. However, the server has no way of discovering that the client has declared this, and recover its connection state. Is a keep-alive the most efficient and appropriate way in iSCSI to recover the resources of nonviable idle connections on the server (target)? I think that a demand-driven approach is a better idea---when a target needs to recover connection resources it will initiate a close on an idle connection. It leads to much less network traffic than keep-alives, particularly given common quadratic connectivity scenarios. In this case the iSCSI spec only has to ensure that such a target-side close is not prohibited, and it might mention that target initiated closes are a way to handle the lost client problem. > Therefore, a ping at (and only at) the time of suspicion and to avoid a > inappropriate connection drop is a valid approach. Again, this seems to focusing on an initiator's desire to short-circuit the TCP connection management process, which I think is not an appropriate thing for iSCSI to specify. I am definitely not saying that this behavior should be prohibited, the mechanism to do this is implicitly present in NOP-{In,Out}, but it should not be specified or discussed in the iSCSI spec. One peculiar thing to note about pinging with NOP is that it actually involves 3 or 4 messages, since both NOP-In and NOP-Out will need TCP ACKs (the NOP-In ACK might be piggybacked). All the more reason to go crazy minimizing (or, I say eliminating) any specified pinging. Steph
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