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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: iSCSI:Request/Response Ordering>Howevere i think that there should be some way in which write access to the >target can be reserved for some initiators. ( i dont know if that already >exists) or there should be some way of ordering of requests fos a target LU >in task router/ task manager I'm not certain what you're trying to get at. There's the various forms of reservation. Persistent reservations have been particularly well thought out for multi-initiator or cluster operation. There are some inherent difficulties with asking a target to coordinate or order writes. From an initiator's perspective, a command is outstanding while it is in flight to the target, while the target is queuing and processing the command, and while status is in flight back to the initiator. A target only has visibility to the middle part, it cannot do anything with commands that are in flight. For example, it does no good to require that the target perform writes in order if they can become re-ordered while in flight to the target. This pretty well means that any coordination has to be done using extra commands, not the write command itself. The extra commands are equivalent to a form of reserve and release. There are some rather complex approaches that would let targets do such things. But it's tantamount to making targets active participants in cluster lock managers. Noone's wanted to pursue approaches with that level of complexity (I think with good reason). The accepted way to do shared access is that the hosts / initiators coordinate everything using their lock manager. At the target level operations can be performed in whatever order is convenient. A number of vendors have built cluster software from such an approach. It appears to work well. Edward A. Gardner eag@ophidian.com Ophidian Designs 719 593-8866 voice 1262 Hofstead Terrace 719 593-8989 fax Colorado Springs, CO 80907 719 210-7200 cell
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