Subject: RE: Persistent Reservation Proposal - Group Reservations Date: Tue, 18 Dec 2007 07:08:50 -0700 From: "Roger Cummings" <roger_cummings@symantec.com> To: "Kevin D Butt" <kdbutt@us.ibm.com> Cc: <t10@t10.org> X-Message-Number: 8313 Formatted message: HTML-formatted message Kevin, Thanks for your comments in turn. I think we actually agree about a lot more in relation to this subject than we disagree about, so we should be able to work through these issues and come up with a single proposal that works for both of us - I certainly don't want two different ways of doing this! But by the same token I don't want a disk-type group reservation and a tape-type group reservation either, and that means whatever we come up with has to be straightforwardly implementable by devices that support the RO & AR PR types today. And this is probably the major concern that I have with your proposal - I believe it requires a Target to remember which registrations should be included in the group for a Reservation, and to validate those registrations within the context of an atomic Reserve operation. Whereas my proposal prevents devices that are not in the group from registering in the first place, and thus Reservations work just as they do today - both the RO and AR types. That having been said, I do not work for a hardware Target supplier that supports PR today, so I might not be representing their concerns adequately. Would anyone else more qualified care to comment on this point? With regards to the other points you raised: a) I'm not sure we can rely on existing devices to check the type of an existing reservation before registering. Those existing devices may assume that they get access under ANY non-exclusive PR when they register. b) The point that you raise about registrations being denied is right, but resource issues do happen and I know we do see that and handle that in some of our code. On the other hand, a host that sees the Registration being rejected, assumes that PR therefore isn't supported, and falls back to RESERVE/RELEASE wouldn't be very good! I think we always now use PR IN to check for PR support, but I'll check that. c) Yes, I agree that the RO & AR types aren't very interesting or significant in the tape world, but they are interesting in other uses and some of those uses could really use the other features of your proposal. d) Re the corner cases, let me ask this question? If a device that has access under the reservation de-registers, and then registers again, does it have access? If we can hold the line of definitely not, then I agree that will be simpler, but I'm not sure that's feasible. e) I agree about the need for a new ASCQ, and yes there would be pain on the host side at the register point. But the host has to handle that action in the resource conflict case today, and handling it there saves the target a whole lot of work. f) I don't have an issue with reporting Reservation Conflict when one of the new Reservations are in place, but you'd only do that under the existing conflict rules, right? You'd still allow registrations to be accepted after the Reservation was in place, just that those new registrations would get access - that what I'm worried about. And BTW we still have another major issue to discuss. Do you want to change anything regarding Preempt? Regards, Roger ________________________________ From: Kevin D Butt [mailto:kdbutt@us.ibm.com] Sent: Monday, December 17, 2007 9:51 AM To: Roger Cummings Cc: t10@t10.org Subject: RE: Persistent Reservation Proposal - Group Reservations Roger, Thank you for your feedback. I am certainly willing to entertain other methods for accomplishing the end goal in an easier fashion. I am not sure I understand how your proposed method makes it more backward compatible. In my proposal PRin would show a different type of reservation and hence the application clients would not try to join the reservation because they don't know about the type. In your proposal, application clients would not be allowed to register. This is a deviation from what they can always do today - unless there is a resource issue. This seems more disruptive to me. I would assume that there would be a new additional sense code added for UNABLE TO REGISTER BECAUSE A GROUP RESERVATION IS IN PLACE (or analogous). This would be a new thing for failure to register and there would be pain at the register point. Perhaps that is better than at the reserve point - but I would think that it would be better handled as a reservation conflict since that is what it is instead of something the application client does not understand. As for "all registrants" type vs. "registrants only" I didn't see where the difference would be interesting, but I am not opposed to providing a way to switch between which of these two types is done. Whether it is additional types or some bit during registration etc. As for some of the corner cases mentioned below, if each I_T nexus that is supposed to be part of the group reservation is required to be registered before the reservation is made, and if the reservation is released when the last group reservation participant is unregistered, then I think we don't have an issue. I would prefer that we work together to shape a mutually beneficial proposal as opposed to have "competing" proposals. I am willing to modify my proposal where it can be made easier and such. I am not sold that my proposed method is the only way or even the best - it's just the way I thought of doing it. I admit that I have always been very confused about the usefulness of RA and AR types. They make absolutely no sense in the tape world. Thanks, Kevin D. Butt SCSI & Fibre Channel Architect, Tape Firmware MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744 Tel: 520-799-2869 / 520-799-5280 Fax: 520-799-2723 (T/L:321) Email address: kdbutt@us.ibm.com http://www-03.ibm.com/servers/storage/ "Roger Cummings" <roger_cummings@symantec.com> Sent by: owner-t10@t10.org 12/14/2007 12:10 PM To Kevin D Butt/Tucson/IBM@IBMUS cc <t10@t10.org> Subject RE: Persistent Reservation Proposal - Group Reservations Kevin, First of all, let me say that I completely support what you're trying to do here. I think that providing a method in persistent reservations (PRs) to support shared access between ONLY a specifically-designated set of systems is a worthy goal, and something we should do in SPC-4. Adding a set of Transport IDs to Reserve as per your document 08-024 & 08-025 is certainly possible, but it's a massive change to the way that PRs work today, and it throws up a bunch of nasty corner cases and backwards compatibility issues. The massive change comes from the fact that now the Target will have to remember which registrations are in the Reservation, and which are not. It will probably have to preserve all of the transport information for the life of the reservation. The corner cases are things like, what happens if there's no longer a registration that corresponds to the transport ID in the Reserve? Does the Reserve succeed? What happens if a registration comes in later, after the reservation has been established - does that device it get access? Backwards compatibility issues may arise like this: An existing device registers, and finds it has no access, so it does a PR In and finds out that a reservation is in place, retries its access and still it has no access. What does it do next, preempt the reservation because it assumes the Target is broken? Reserve also has to be an "atomic" command, and I've always thought that was why it's functionality is as compact as it is today. Most of the complex operations related to addresses and keys are done at registration time, and those operations don't have to be atomic. One more thing: you chose for your new "group" reservations to follow the "all registrants" approach is terms of the definition of the reservation holder. While that's fine by me (obviously), I suspect there are also situations where group reservations that follow the "registrants only" approach might be useful. The bottom line from my point of view is this: Your proposal is feasible and we can probably make it work. But I wonder if there's an easier way to achieve the same goal that is more compatible with existing practice and requires less of a change in functionality on the Target side. What if we didn't add any new reservations types, but instead added some new functionality to the registration process? What I'm thinking of a new Register feature that causes the Target to kill all existing registrations, create the registrations identified in the transport IDs in the Register command, and not accept any future registrations. That way, we don't need any changes to Reserve, and an Initiator with existing functionality would just not be able to register and therefore would not be confused. Does that make sense to you? Is there a chance this is an easier approach? If so, I'll write up a detailed proposal that's the equivalent of 08-025r0 and we can compare and contrast at the next CAP. Again, thanks for getting this started, I think it's a worthwhile endeavor and I'll be glad to put some cycles towards defining this sort of functionality for SPC-4. Regards, Roger ________________________________ From: owner-t10@t10.org [mailto:owner-t10@t10.org] On Behalf Of Kevin D Butt Sent: Monday, December 10, 2007 4:18 PM To: t10@t10.org Subject: Persistent Reservation Proposal - Group Reservations I have posted two documents related to an additional Persistent Reservation Type. The first document is a presentation on where persistent reservations are today and where they fall short in the scenarios covered by the proposal. It also covers the intent of the proposal and what will be proposed. The second is the actual proposal Your PDF file will be posted at: http://www.t10.org/ftp/t10/document.08/08-024r0.pdf http://www.t10.org/ftp/t10/document.08/08-025r0.pdf Normally, the posting/archiving process takes about 30 minutes. Kevin D. Butt SCSI & Fibre Channel Architect, Tape Firmware, IBM MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744 Tel: 520-799-2869 / 520-799-5280 Fax: 520-799-2723 (T/L:321) Email address: kdbutt@us.ibm.com http://www-03.ibm.com/servers/storage/