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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: Framing DiscussionI agree with David. For the sake of clarity however I would like to add that even for those cases in which buffers are anonymous (like some appliance fileservers or disk controllers) the data that comes from the wire are not necessarily entire storage data blocks and have to be placed somewhere within a storage buffer. That particular placement can be made itself anonymous only by "placing" a "scather-gather" burden on the storage end of the wire and that is either expensive or outright impossible. Julo Black_David@emc.com on 20/12/2000 23:40:05 Please respond to Black_David@emc.com To: ips@ece.cmu.edu cc: Subject: RE: Framing Discussion > No. Why do you need a copy ? These buffers are consumed > by iscsi layer and may be returned later to the same > pool. These are just some details. > Why are they not interchangeable ? I have a hunch that Mohan has never worked with local filesystem or block device code :-). The reason is that SCSI code inside operating systems does not work that way. Operating systems perform reads from SCSI devices by allocating memory at a layer above SCSI and then telling the SCSI driver to read data into the memory it has allocated. This is the case even when the operating system (e.g., filesystem) is the originator of the I/O. The allocation operation cannot be moved into the SCSI driver without significant code changes to the OS (e.g., the device driver interface has to change, and then we can talk about the impact on the filesystem). To keep existing code happy, the act of iSCSI "consuming" the data in the above scenario will include copying (or otherwise moving) it to the location at which it was supposed to arrive. > This is an > assumption not stated anywhere. Is there > a list of other assumptions that is documented ? I suggest looking at the SCSI/block device driver interface inside an operating system like Solaris, which I'm sure someone at Sun can explain :-) . It's very different from the interface to network devices (e.g., no unsolicited inbound data). --David --------------------------------------------------- David L. Black, Senior Technologist EMC Corporation, 42 South St., Hopkinton, MA 01748 +1 (508) 435-1000 x75140 FAX: +1 (508) 497-8500 black_david@emc.com Mobile: +1 (978) 394-7754 ---------------------------------------------------
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