On 04/25/2013 05:31 PM, Mike Snitzer wrote: > On Thu, Apr 25 2013 at 10:50am -0400, > Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > >> >> >> On Thu, 25 Apr 2013, Mike Snitzer wrote: >> >>> On Thu, Apr 25 2013 at 9:48am -0400, >>> Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: >>> >>>> >>>> >>>> On Mon, 22 Apr 2013, Mike Snitzer wrote: >>>> >>>>> I spoke with Hannes at LSF, to address the potential crashes in the >>>>> endio path (e.g. stpg_endio) we'd have to bump the scsi_dh_data kref >>>>> where appropriate (e.g. for ALUA kref_get in submit_stpg and kref_put in >>>>> stpg_endio). >>>>> >>>>> But that is just the tip of the iceberg relative to scsi_dh lifetime. >>>>> Seems we've been playing it pretty fast and loose with scsi_dh issued >>>>> requests vs detach for quite some time. >>>>> >>>>> I'm now inclined to not care about this issue. Take away is: don't >>>>> switch the device handler (attach the correct one from the start). >>>> >>>> I did a patch that disables device handler switching and it was NACKed by >>>> Hannes. The problem that he pointed out was - when we load SCSI device >>>> handler modules, they attach automatically to SCSI devices they think they >>>> belong to. The user then can't set the desired device handler in multipath >>>> configuration because a different handler is already attached. >>> >>> The handler that is automatically attached _should_ be the correct >>> handler. We now have the .match() hook for scsi_dh and it has made for >>> reliable scsi_dh attachment of the correct handler. >> >> The EMC devices work with both ALUA and EMC handlers - so there is no one >> "correct" handler, the correct handler is the one that the user specified >> in multipath configuration. >> >>>> So we need a functionality to change device handlers. >>> >>> I really cannot think of a sequence where the scsi_dh .match() will >>> attach the incorrect handler. This is why I added the >>> "retain_attached_hw_handler" feature to mpath (commit a58a935d5). >> >> The automatic handler assigment can't change existing handler. >> >> But if one handler was automatically selected and the user selects a >> different handler in multipath configuration, the handler is changed. >> >>>> (or maybe stop the scsi device handlers from attaching automatically, but >>>> it would surely generate a lot of other regressions) >>> >>> The need to support changing device handlers (via multipath table load) >>> is overblown/historic. >> >> So - do you mean that we make "retain_attached_hw_handler" the default >> option and don't allow the user to change existing device handler in >> multipath configuration? >> >> That's what my patch did and it was NACKed by Hannes. The problem there is >> that behavior depends on module loading order - if you activate multipath >> with "EMC" option, it activates the EMC handler. If you load the ALUA >> module and activate multipath with "EMC" option, it stays with the ALUA >> handler. > > .match allows for correct scsi_dh selection in the decision of alua vs > emc (alua has the tpgs bit set) -- but both scsi_dh modules must be > loaded. > > If the incorrect handler is getting attached then it is either a bug in > the .match method (for the handler that should've been attached) or the > storage isn't configured how the user thought and they need to > adjust/reconfigure to have it be like they expected. > > Either way we really _could_ impose not allowing the scsi_dh handler to > be changed (by multipath) -- which is why I Acked your patch. There is > always the scsi_dh sysfs interface to allow the user to change the > scsi_dh (and possibly shoot themselves in the foot). > Always providing there _is_ a correct way. For eg RDAC might run in ALUA mode, but this is by no means exclusively; the original 'rdac' mode will work there, too. Plus some vendors / admins might prefer for whatever reasons to continue to use the original mode. So I don't think there is a 'correct' hardware handler. Only a preferred one. And the preference is set by the user, not the installation. Hence it would be a bad idea to disallow scsi_dh changes. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@xxxxxxx +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
