[PATCH] sanlock: host_message

David Teigland teigland at redhat.com
Mon Mar 3 15:55:05 UTC 2014


On Sun, Mar 02, 2014 at 05:32:07AM -0500, Nir Soffer wrote:
> ----- Original Message -----
> > From: "David Teigland" <teigland at redhat.com>
> > To: "Nir Soffer" <nsoffer at redhat.com>
> > Cc: "Allon Mureinik" <amureini at redhat.com>, "Ayal Baron" <abaron at redhat.com>, sanlock-devel at lists.fedorahosted.org,
> > fsimonce at redhat.com, smizrahi at redhat.com
> > Sent: Wednesday, February 26, 2014 11:58:25 PM
> > Subject: Re: [PATCH] sanlock: host_message
> > 
> > I think the following example illustrates the problem with the current
> > plan to use the vdsm lockspace for fencing:
> > 
> > hostA and hostB vdsm using a common lockspace
> 
> Do you mean the host id lockspace?

Sorry for being unclear here.  I'm not entirely sure what lockspace(s)
vdsm is using, so I was being vague.  I guess we'll need to sort out
exactly what lockspaces we're talking about.


> > hostB sends hostA a "reset yourself" message via the vdsm lockspace
> > hostA storage fails for vdsm lockspace around the same time
> 
> What do you mean by fails for vdsm lockspace?

I meant that sanlock fails to renew its host_id lease in this vaguely
defined lockspace.  We speak about this condition with some interchangable
terms:

- storage is lost / storage access is lost
- the storage is the storage on which the sanlock leases exist
- the effect is that sanlock can no longer renew its host_id lease
- we may say that the lockspace fails at this point or that lockspace
  enters recovery

That's all referencing the same situation.


> > Create a new lockspace dedicated to the fencing behavior.
> > (This is what the fence_sanlock daemon/agent do.)
> 
> Do you mean host id like lockspace (1MB for 2000 hosts)?

Yes, a sanlock_write_lockspace() / sanlock_add_lockspace() that is
dedicated to the fencing behavior and not used for anything else.


> > This new lockspace would *not* be killed or gracefully cleaned up
> > if storage is lost.  This way, either the lockspace message would cause
> > the host to be reset, or if lockspace storage is lost, (and no messages
> > are possible), the failure to renew the lockspace lease would cause the
> > host to be reset.  The same result is guaranteed in either case.
> 
> I don't think we like to fence a host if it lost access to some storage,
> or even to all storage.
> 
> If we can access the host through the network, we would like to migrate
> the vms on it to another host instead of killing the vms.

OK, I'll need to learn a little more about what behavior you want in each
circumstance.



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