[dm-devel] [RFC] How to fix system stall on root volume multipath

Christophe Varoqui christophe.varoqui at free.fr
Tue Nov 13 01:01:24 UTC 2007


Le lundi 12 novembre 2007 à 11:24 -0500, Kiyoshi Ueda a écrit :
> Hi Alasdair,
> 
> On Sat, 10 Nov 2007 03:31:13 +0000, Alasdair G Kergon <agk at redhat.com> wrote:
> > On Fri, Nov 09, 2007 at 06:17:19PM -0500, Kiyoshi Ueda wrote:
> > > If we use multipath for "/", temporal all-paths failure could lead to
> > > system stall because multipathd depends on callout programs on "/".
> > > I would like to hear your comments about my idea to fix it.
> >  
> > I thought it avoided that problem by pre-loading everything into a ramdisk:
> > did that code get dropped for some reason?
> 
> Thank you for the information!  I had forgotten it!
> Yes, we used to have it, and it seems to be removed by the commit below:
>     http://git.kernel.org/gitweb.cgi?p=linux/storage/multipath-tools/.git;a=commitdiff;h=cad20ecf8919b2126ab6f4f793a1a738e9864f59
> 
> And probably the following thread is related to the commit.
>     https://www.redhat.com/archives/dm-devel/2005-July/msg00044.html
> 
> 
> Ben, Christophe,
> Is that code still problem for current multipathd?
> And what do you think about my proposal?
> 
I'm not found of yet-another user-visible ramfs for multipathd use,
that's why I started with the private namespace tricks. The problems are
still there, and will stay till we stop using pthreads.

That may happen someday, as one of the main reason for pthread was the
(blocking ioctl) libdevmapper event collection. And this is being
superseded by path status uevents.

But not soon enough, and the private namespace stuff suffers from lack
of friendliness anyway : we can't expect users to grasp easily that
changing their prioritizer in /sbin won't be seen by the multipath
daemon till restart, for example.

So I propose to start playing with your prioritizers-as-lib idea to see
if it's practical.

I prepared the following patch to that effect. It is not complete
(actually segfaults, no useful prioritizer ported) but can start fixing
bugs and go where ever your personnal interest leads.

Looking forward to your fixes,
Thanks,
cvaroqui
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