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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: iSCSI AutosenseRalph, How are volume change events supposed to be treated with CA, ACA and autosense (or for that matter any Async. Event that intends to convey to an initiator state information that the initiator should be aware of before sending commands - i.e. how is state sync. achieved)? I was under the impression that this is one of vestigial uses of CA. It is obvious that autosense is fine but until I can't be convinced that it is completely orthogonal to CA use (or to some other mechanism that can assure state sync) I would be reluctant to force it. If state sync. is provided by some other means that I will be happy to oblige and assume autosense is always there. Thanks, Julo Ralph Weber <ralphoweber@compuserve.com> on 30/08/2000 06:12:53 Please respond to ENDL_TX@computer.org To: IPS Reflector <ips@ece.cmu.edu> cc: (bcc: Julian Satran/Haifa/IBM) Subject: Re: iSCSI Autosense This is going to be fun, trying to respond to all the information and misinformation that's been bandied about today. David Black's note is correct. The method for bridging a CA-only (Contingent Allegiance only) device to an Autosense representation of that device is well known and trivial. In addition to being documented in the requirements, the method is documented in the first paragraph of 5.3 in draft-satran-iscsi-01.txt. I have two minor complaints about the 5.3 wording. 1) The method will work all the time, so the suggestion that it 'may' work is incorrect. 2) I would prefer that the REQUEST SENSE command be explicitly mentioned not alluded to. As regards CA, let me restate my position more emphatically. CA is a historical artifact that dates to the single byte of status model employed by the parallel SCSI bus. To the best of my knowledge it is extremely difficult to implement the CA model in a packetized transfer model because the CA condition is cleared by the next command to arrive at the target regardless of what kind of command that is. Since most packetized protocols allow several commands to be in flight concurrently between initiator and target, the most probably case in a packetized protocol is that an in flight command will clear a CA condition long before the initiator finds out about it and has a chance to fire off the needed REQUEST SENSE in to get the sense data. The bottom line here is that Autosense works for packetized protocols and CA does not. Since it is trivial to translate a packetized Autosense front end to the CA behavior of an older backend SCSI device, the right thing for iSCSI to do is to make Autosense mandatory and not to provide any way whatsoever to disable it. The fact that SAM is written with a preference for CA is (like CA itself) a historical artifact. So long as the wording continues to be correct, it is unlikely that T10 will change the wording (if it ain't broke don't fix it). However, the ancient leanings of SAM should not under any circumstances be used as a justification for iSCSI support for a CA-like interface. As noted above, CA doesn't work for packetized protocols like iSCSI. ACA (Auto Contingent Allegiance) is absolutely positively NOT the super-duper, all fixed up for a packetized world brother of CA. ACA is orthogonal to Autosense and actually has no place in this discussion. IN ALL CASES, ACA is optional. Simply return 0 in the NormACA bit (bit 5 byte 3) of the Standard INQUIRY Data and you don't support ACA and you are expected to refuse to process any CDB with the NACA bit set to 1. ACA is more akin to the SCSI-2 ECA (Extended Contingent Allegiance) than it is to either CA or Autosense. The assumption in ACA and ECA is that the initiator needs to send several commands to the target in order to cleanup whatever error occurred. The most frequently cited excuse for this is the cleanup needed when a tape writes past the EOT (End Of Tape) reflective strip. Thanks. Ralph Weber ENDL Texas
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