[dm-devel] [PATCH 2/2] dm mpath: attach scsi_dh during table resume
Mike Snitzer
snitzer at redhat.com
Thu Apr 25 15:31:47 UTC 2013
On Thu, Apr 25 2013 at 10:50am -0400,
Mikulas Patocka <mpatocka at redhat.com> wrote:
>
>
> On Thu, 25 Apr 2013, Mike Snitzer wrote:
>
> > On Thu, Apr 25 2013 at 9:48am -0400,
> > Mikulas Patocka <mpatocka at redhat.com> 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).
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