On Fri, Mar 14, 2014 at 12:21:11PM -0400, Mike Snitzer wrote: > DM multipath has a role in insuring the desired scsi_dh is attached and > that it holds a reference on the attached scsi_dh. > > I'm open to ideas of how dm-multipath could avoid having _any_ role here > but it isn't so simple to avoid, dm-multipath does 3 things in this > area (ranging from lightest to heaviest relative to scsi_dh interface use): > 1) get reference on scsi_dh that is already attached -- most widely used > now that the scsi_dh matching code has been improved to get correct > scsi_dh attached during scsi device scan) > 2) no scsi_dh was attached, but one should be -- really shouldn't happen > anymore > 3) switch from the scsi_dh that was auto-attached by scsi_dh matching to > some user-specified override -- shouldn't be needed now but a user may > have a custom scsi_dh they've developed. What we currently have surely is a bit of a layering violation. I don't think it's urgent or overly important to fix it now, but I see two ways out: a) move the handler registration to dm-multipath. This still leaves the problem on figuring out if a handler supports a device, but at least that problem is inside the handlers now. b) move the path selectors to the block layer, and have the methods provided indirect off the requeuest_queue b) seems like the cleanest layering, although in the case of SCSI (the only one that matters at the moment) that would give us a double indirection. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
