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Re: [libvirt] [PATCH v2 5/7] lock_driver_sanlock: Handle metadata flag gracefully



On Mon, Aug 20, 2018 at 03:59:23PM +0100, Daniel P. Berrangé wrote:
> On Fri, Aug 17, 2018 at 10:49:12AM -0400, John Ferlan wrote:
> > 
> > 
> > On 08/14/2018 07:19 AM, Michal Privoznik wrote:
> > > No real support implemented here. But hey, at least we will not
> > > fail.
> > > 
> > > Signed-off-by: Michal Privoznik <mprivozn redhat com>
> > > ---
> > >  src/locking/lock_driver_sanlock.c | 25 ++++++++++++++++++-------
> > >  1 file changed, 18 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/src/locking/lock_driver_sanlock.c b/src/locking/lock_driver_sanlock.c
> > > index 3e5f0e37b0..c1996fb937 100644
> > > --- a/src/locking/lock_driver_sanlock.c
> > > +++ b/src/locking/lock_driver_sanlock.c
> > > @@ -791,7 +791,8 @@ static int virLockManagerSanlockAddResource(virLockManagerPtr lock,
> > >      virLockManagerSanlockPrivatePtr priv = lock->privateData;
> > >  
> > >      virCheckFlags(VIR_LOCK_MANAGER_RESOURCE_READONLY |
> > > -                  VIR_LOCK_MANAGER_RESOURCE_SHARED, -1);
> > > +                  VIR_LOCK_MANAGER_RESOURCE_SHARED |
> > > +                  VIR_LOCK_MANAGER_RESOURCE_METADATA, -1);
> > >  
> > >      if (priv->res_count == SANLK_MAX_RESOURCES) {
> > >          virReportError(VIR_ERR_INTERNAL_ERROR,
> > > @@ -804,6 +805,11 @@ static int virLockManagerSanlockAddResource(virLockManagerPtr lock,
> > >      if (flags & VIR_LOCK_MANAGER_RESOURCE_READONLY)
> > >          return 0;
> > >  
> > > +    /* No metadata locking support for now.
> > > +     * TODO: implement it. */
> > > +    if (flags & VIR_LOCK_MANAGER_RESOURCE_METADATA)
> > > +        return 0;
> > > +
> > 
> > Doesn't this give someone the false impression that their resource is
> > locked if they choose METADATA?
> > 
> > Something doesn't feel right about that - giving the impression that
> > it's supported and the consumer is protected, but when push comes to
> > shove they aren't.
> > 
> > I'd be inclined to believe that we may want to do nothing with/for
> > sanlock allowing the virCheckFlags above take care of the consumer.
> 
> Yeah, this doesn't feel right to me. I think we need to treat the
> metadata locking as completely independant of the content locking.
> This implies we should have a separate configuration for metadata
> locking, where the only valid options are "lockd" or "nop".
> 
> eg our available matrix looks like
> 
>                         METADATA
>                  | nop   lockd  sanlock
>         ---------+--------------------	
>              nop |  Y      Y      N
> CONTENT    lockd |  Y      Y      N
>          sanlock |  Y      Y      N

Oh and even for virtlockd we need to consider the config separately
for content vs metdata.

For content we have the possibility to lock out of band files as a
proxy for the main file. This is primarily done for protecting
shared blocks devices as locking the device node doesn't apply
across hosts.

For metadata locking, we only ever care about the local device node
as changes to labelling/ownership don't propagate across hosts. IOW,
we should always directly lock the file in question for metadata
locking, and never use an out of band proxy for locking.

Regards,
Daniel
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