RE: iSCSI: "Wedge" drivers

[Black_David@emc.com]

> While many of you keep telling me that wedge drivers won't go away nobody
> gave one good example to support that assertion.

With my co-chair hat off, here are four, two from the mail exchange:
(1) Sharing SCSI state across array interface processors involves some
	work.  Use of multiple SCSI connections and wedge drivers
	for fault tolerance and load balancing will persist in systems that
	choose not to follow that implementation path.
(2) If iSCSI is implemented in hardware, so that a system has multiple iSCSI
	HBAs rather than multiple NICs with a common iSCSI software
	stack, multiple SCSI connections and wedge drivers are again
	the path of least resistance to fault tolerance and load balancing.
And two new ones:
(3) Naive greedy load balancing is not optimal.  Significant improvements
	are possible based on an understanding of how the storage behaves.
	PowerPath contains Symmetrix-specific logic that does this,
	and I don't think that's appropriate for standardization.
(4) Some arrays need additional support for fail-over.  Clariion uses
	an active fail-over architecture in which the array must be
	instructed to fail-over, and the fail-over occurs across separate
	SCSI connections.  I don't think that the Clariion-specific
	logic that does this is appropriate for standardization.
As I wrote earlier, I have no problem with specifying a session abstraction
for iSCSI, but it won't eliminate wedge drivers.

With my co-chair hat on, the current consensus is to specify a session
abstraction and review the result.

--David

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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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