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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: iSCSI: "Wedge" driversI essentially got the same feedback from a Fibre Channel HBA vendor who also indicated that they would be very unlikely to get into the "wedge driver" business for the reasons stated in the mail below. (Please correct me if I am wrong). I shall reserve my arguments about complexity till the proponents of the multiple-connections per session concept come up with their error handling mechanisms. Sincerely, Prasenjit Prasenjit Sarkar Research Staff Member IBM Almaden Research San Jose Black_David@emc.com@ece.cmu.edu on 08/25/2000 03:01:28 PM Sent by: owner-ips@ece.cmu.edu To: ips@ece.cmu.edu cc: Subject: iSCSI: "Wedge" drivers I guess I need to take my co-chair hat off and say a few things about EMC's and other "Wedge" drivers. Wedge drivers are motivated by fault tolerance and fail-over in addition to load balancing/spreading. EMC actually has two products, PowerPath for Symmetrix (does all three), and ATF for Clariion (fail-over only). Other products include HDS's SafePath, Veritas's DMP and HP's PV Links. Apologies to those I've left out. John's goal of eliminating of wedge drivers via iSCSI sessions may not be achievable in practice due to fault tolerance and fail-over concerns. The basic fault tolerance/failover concern arises from the requirement that failure of one interface processor in the disk array should not disable all access to the host's storage. Hence the host to storage connectivity usually encompasses more than one interface processor, facing array designers with the choice of using multiple SCSI connections and keeping the SCSI state in each interface processor vs. sharing the SCSI state across multiple interface processors, and making sure everything works right when one of them fails in an arbitrary fashion (ouch). The former is considerably easier to implement, and requires a wedge driver on the host side. The corresponding issue of how a host deals with possible failure of an iSCSI HBA has already been noted, and I agree that the most likely approach is a wedge driver. Since John works for IBM, I should note that the phrase "interface processor(s)" isn't directly applicable to the IBM Shark array, but nonetheless, this issue is still present -- for fault tolerance, storage access has to be spread across both RS/6000 systems running AIX in a Shark, and sharing SCSI state across AIX instances doesn't sound like an easy thing to do. The bottom line is that failure and fault tolerance concerns make it unlikely (IMHO) that an iSCSI session concept will lead to the extinction of wedge drivers. There have been some questions about difficulty of development of wedge drivers. EMC's experience is that fault tolerance and fail-over require much more effort than load balancing for a couple of reasons. The first is that more complex systems tend to have more complex ways of failing than of working correctly :-(. The second is that correct behavior of a wedge driver depends on the correct behavior of HBA drivers in error cases, and HBA vendors in general do not exhaustively test correct behavior of error cases before releasing drivers -- we find driver bugs on a regular basis. Given the intention to implement much/all of iSCSI in hardware, I suspect that this situation will continue relatively unchanged. I'm not sure what the latter implies for difficulty of development of error handling code for multi-connection iSCSI sessions when a single vendor is responsible for both the hardware and the driver. --David --------------------------------------------------- David L. Black, Senior Technologist EMC Corporation, 42 South St., Hopkinton, MA 01748 +1 (508) 435-1000 x75140, FAX: +1 (508) 497-6909 black_david@emc.com Cellular: +1 (978) 394-7754 ---------------------------------------------------
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