Hi Hannes, Now I see why you want this change in dm-multipath. I think I agree with these changes. But, it brings another question: what does dh_state provide ? Help to user to see which hardware handler a device is attached to ? I thought more about the scsi_dh_detach function (in the context of my earlier comment), adding it would require more housekeeping to associate one-to-one mapping between attach and detach. We can leave it the same way as the module will be detached when the device disappear eventually. Thanks, chandra On Tue, 2008-05-20 at 14:41 +0200, Hannes Reinecke wrote: > Hi Chandra, > > Chandra Seetharaman wrote: > > On Mon, 2008-05-19 at 12:21 +0200, Hannes Reinecke wrote: > >> Hi Chandra, > >> > [ .. ] > >> Yes, but this only verifies that the _handler_ exist, not that it is > >> attached to this sdev > > > > Agreed. And it works properly with the original design. > > > > Here is my thinking on how it would work with the new feature (ability > > to attach any device using dh_state) you added: > > - User adds a new device > > - User knows which scsi hardware handler their device can attach to, > > lets say, "mydev". > > - User attaches the device with scsi hardware handler (writing "mydev" > > to "dh_state"). > > Device is now attached to "mydev" > > - User updates his/her multipath.conf file with "mydev" for the > > specific device. > > - User invokes multipath, which checks for the existence of "mydev" > > and allows the table insertion. > > - When pg_init was required in the future, multipath calls "activate" > > for "mydev" correctly. > > > > Wouldn't this work ? > > > > Note: In the absence of this patch (7/7) only. > > > Yes, but only for the first time. After reboot the user would > have to do the reconfiguration again. > > >>> Hardware handler module is attached to the device itself (thru > >>> try_module_get() in attach), so, the module will exist as long as the > >>> device exists. > >>> > >> Not quite. It's only attached automatically if the device map has > >> an entry for this module. > >> However, given this patchset we can now easily manually attach any > >> device to the device handler. Or at least try to do so. > >> It's then up to the device handler to refuse binding if none of > >> the necessary pages/information etc. was found. > > > > Doesn't dh_state solve this issue ? > > > In theory, yes. > > >>> Hence, there is no need to attach it again from the multipath layer. > >>> > >> Yes, you do. The user might have it's own custom configuration file, > >> covering new/unknown/unhandled/testing devices, which out of necessity > >> are _not_ hardcoded in the device table of the device handler. > >> So multipath has to have a way of attaching device handler to those > >> devices, too. > > > > dh_state allows the user to do this, in which case why do we need to do > > this in multipath layer ? > > > Because then we (read: I) don't have to modify multipath-tools. > If we don't take this patch we'll have to modify multipath-tools > to check for the dh_state attribute explicitly and activate > the hardware handler directly via sysfs. > > But this also means that existing multipath-tools programs won't > be able to correctly activate the hardware handler in > _some_ cases (ie those cases, where the hardware table for the > device_handler doesn't contain the device in question). > And I can just imagine the bugs I'll be getting ... > So if we decide to take that road we should kill the hardware > handler feature from the device-mapper multipath interface > altogether to notify the user that the programs won't work > anymore. Otherwise it's just heading for trouble. > > So I'd rather keep the userland interface stable here and add > some code to the kernel. > > Cheers, > > Hannes -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
