[ovirt-devel] Sanlock fencing reservations

[Nir Soffer]

----- Original Message -----
> From: "Dan Kenigsberg" <danken at redhat.com>
> To: "Saggi Mizrahi" <smizrahi at redhat.com>, nsoffer at redhat.com
> Cc: devel at ovirt.org, "David Teigland" <teigland at redhat.com>
> Sent: Wednesday, April 16, 2014 10:33:39 AM
> Subject: Re: Sanlock fencing reservations
> 
> On Wed, Feb 26, 2014 at 11:14:33AM -0500, Saggi Mizrahi wrote:
> > I've recently been introduced to the this feature and I was wondering is
> > this really
> > the correct way to go for solving this particular problem.
> > 
> > My main issue is with making two unrelated flow dependent.
> > By pushing this into the existing sanlock data structures you limit
> > yourself
> > in the future from changing either to optimize or even solve problems for a
> > single use case.
> > 
> > Having an independent daemon to perform this task will give more room as to
> > how to implement the feature.
> 
> Saggi, are you thinking about something similar to fence_sanlockd
> http://linux.die.net/man/8/fence_sanlockd and its client fence_sanlock?
> 
> Using them, instead of reimplementing parts of them within Vdsm, seem
> reasonable in first glance.
> 
> However
> http://www.ovirt.org/Features/Sanlock_Fencing#Why_not_use_fence_sanlockd.3F
> claim that it wastes 2G of storage and requires explicit master domain
> upgrade.
> 
> Nir, could you explain why so much space is required by sanlockd? Can it
> be configured to use smaller area?

According to fence_sanlock manual, each host get a resource on the shared
storage. Each sanlock resource is 1MB (sector size * max hosts). So to serve
2000 hosts we need 2G.

We can use less space if we want to support smaller number of hosts, but 
we need a new domain format that include a new volume for the fencing.

Nir