[linux-lvm] A couple of questions on locking library

[Shawn Guo]

On Fri, May 31, 2019 at 10:38:18AM +0200, Zdenek Kabelac wrote:
> Dne 31. 05. 19 v 9:13 Shawn Guo napsal(a):
> >Hi David, Zdenek,
> >
> >Comparing to stable-2.02 branch, I noticed that there are significant
> >changes around locking infrastructure on master branch.  I have a
> >couple of questions regarding to these changes.
> >
> >1. I see External Locking support was removed as part of clvmd
> >removal. What's the reason for dropping External Locking support?  I'm
> >asking because we are investigating the possibility to use hardware
> >assisted locking for cluster, in form of External Locking extension.
> >
> >2. It seems there have never been real support for LV (Logic Volume)
> >locking.  On stable-2.02 branch, the LV locking interface is used as
> >activation path instead of real locking on LV.  And on master branch,
> >activation path gets separated off from locking infrastructure.  As
> >the result, the LV interface is dropped completely from locking
> >infrastructure.  My question is why there have never been LV locking
> >support.  Is the LV locking support an invalid requirement at all?  Or
> >it's just because no one cares about it enough to add the support?
> >
> >Thanks for your time, and appreciate any comment you would give here.
> 
> With stable-2.02 branch - there was always per-LV locking.
> The 'state' of the LV was matching state of lock.
> 
> http://people.redhat.com/agk/talks/CLVM-UKUUG2007/
> 
> See the 'DLM – Distributed Lock Manager'

That's only for lvmlockd but not clvmd, right?  IIUC, LV locking
semantic is used as activation for clvmd on stable-2.02.

> 
> The lock however was always took only for top-level LV - never for those
> component LVs.
> 
> --
> 
> There is however believe, that users no longer use volumes with shared
> activation (active at one time on multiple hosts) - so lvm2 is now being
> moved
> transformed to theoretically less complex locking scheme...

So back to my question 1, it looks that "less complex" means it's not
possible to use an external locking extension any more, right?  lvmlockd
becomes the only clustering locking solution.  While it supports sanlock
and dlm locking library, there is no extension support for plugging an
external locking library in.

Is my understanding above correct?  I'm just trying to understand the
feasibility of adding external locking library under new lvm2 locking
scheme.  Thanks much!

Regards,
Shawn