[dm-devel] Re: multipathing pending issues with rhel

[Christophe Varoqui]

Le Wed, 8 Oct 2008 17:11:48 -0500,
Benjamin Marzinski <bmarzins at redhat.com> a écrit :

> On Wed, Oct 08, 2008 at 11:07:25PM +0200, Christophe Varoqui wrote:
> > Ben,
> > 
> > I'd like to summarize all the issues I raised recently through my
> > employers support channel on the multipath subsystem.
> > And see if something can be done about it, at least in the upstream
> > concerned codebases.
> > 
> > 1/ multipathd private namespace pins lvm2 logical volumes maps
> > mounted at daemon startup, thus making "vgchange -ay" fail, even
> > after umounting the visible mount. In my context, it also means I
> > can't stop a clustered service build on this vg to start it on
> > another node. This problem does not affect upstream which does not
> > create a private namespace.
>
errata: "vgchange -an", though it's clear you read it as I meant :)
 
> This already has a fix queued for 5.3.  Multipathd now unounts all of
> the unnecessary mount points after creating the private namespace.
>
Good to know, thanks for this update.
 
> > 
> > 2/ can't map a rw multipath over read-only paths. Quick workaround
> > to create ro multipath, but ro->rw promotion is not automatic when
> > paths become writable. I keep thinking we should allow rw multipath
> > over ro paths. The ro->rw event might also work, but what will
> > trigger the kernel rw status change in the first place ? To my
> > knowledge, only a manual scsi device rescan can force this status
> > update ... which accounts for a less user-friendly solution than
> > the former.
> 
> The workaround is in place for 5.3, but I fully agree that a kernel
> patch to allow rw maps on top of ro devices is the way to go in the
> future.
>
Glad we share this point of view. Will you propose patches for inclusion
upstream ? Which update level do you estimate will include this fix ?
 
> > 3/ Can't use scsi-3 persistent reservations on clariion multipathed
> > luns : paths reserved on node A, writes submitted on node B should
> > be errored immediately to ensure data integrity. Instead, writes get
> > buffered in the "queue_if_no_path" logic, and finally corrupt the
> > data when reservation get cleared. In my context, reservation is the
> > prefered io fencing method for clusters.
> > The kernel knows the write io submitted on a path is refused due to
> > a reservation conflict, but this status is not propagated to
> > multipath for it to react by not queuing this io as it should.
> > 
This one is hard to understand, I empatize. Nonetheless it deserves
attention, as it is a data-corrupter for the clients using, or
eager to use, persistent reservations on these widespread Clariion
arrays. I know Mike Christie already tried to address the issue
years ago ... he might be willing to take over again. (hope)

> > Regards,
> > cvaroqui