scsi_debug and mutipath, was Re: [PATCH 1/1] block: rework flush sequencing for blk-mq

[Christoph Hellwig <hch@infradead.org>]

On Fri, Mar 14, 2014 at 10:13:01AM -0400, Mike Snitzer wrote:
> I was more reacting to the assertion you made like multipath regresses
> all the time.  I'm not faulting you at all for not having tested
> multipath.  Hell, I even forget to test multipath more than I should.
> /me says with shame

And I didn't assert that, I just asserted that it gets little testing.

> Yeah, not sure why single path scsi_debug "just works", maybe it is a
> "feature" of the older multipathd I have kicking around?, but for basic
> data path testing scsi_debug is a quick means to an end.  I can look
> closer at _why_ it gets multipathd in a bit.  But maybe Ben or Hannes
> will have quicker insight?
> 
> For me, if multipathd is running, if I issue the following a multipath
> device gets layered ontop of the associated scsi_debug created sd
> device: modprobe scsi_debug dev_size_mb=1024
> 
> I think it useful to have scsi_debug work with multipath.  I know people
> in Red Hat's QE organization have even simulated multiple paths with
> it.. but I don't recall if they had to hack scsi_debug to do that.  I'll
> try to find out.

Looks like it really shouldn't attach as-is.  But scsi_debug should be
easily extendable to export two LUNs for the same backing store to help
multipath testing.