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On Wed, Sep 24, 2003 at 09:08:54PM -0700, Ben Cahill wrote:
>
>
> > -----Original Message-----
> > From: opengfs-devel-admin@lists.sourceforge.net
> > [mailto:opengfs-devel-admin@lists.sourceforge.net]On Behalf Of Dominik
> > Vogt
> > Sent: Wednesday, September 24, 2003 4:25 AM
> > To: opengfs-devel@lists.sourceforge.net
> > Subject: Re: [ogfs-dev]thinking about "clusterizing" etx3
>
> >
> > > I think it would be easy to put glock into its own module
> > > (combine it with harness). It looks quite portable, believe it
> > > or not, and provides the glops hooks that we would need for
> > > interfacing with the ext3 filesystem.
> >
> > The is good news. However, it lacks at least one important
> > feature: downgrading locks from exclusive to shared (easy) and
> > upgrading locks from shared to exclusive (difficult because of
> > possible deadlocks).
>
> Tell us more about the possible deadlocks,
1. Node A holds a shared lock on resource X.
Node B holds a shared lock on resource X.
2. Both nodes decide to upgrade their lock at the same time and
sends a message to the other nodes requesting them to release
their locks on resource X.
==>
Node A is waiting for node B to yield the shared lock.
Node B is waiting for node A to yield the shared lock.
> and why it is important to upgrade/downgrade (convert) without
> releasing the lock.
I don't know if we will need it, but it would have made the block
allocation code more efficient. As it is now, we have to lock
resource groups exclusively because to upgrade a lock we have to
release it first - allowing other nodes to invalidate the resource
group statistics.
> Maybe we can add a little extra to the glock layer.
> IIRC/IIUC, OpenDLM handles "lock conversions".
Yes. It's a common feature in (decent) locking systems.
Ciao
Dominik ^_^ ^_^
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