RE: RE: [Opendlm-devel] ODLM/OGFS Recovery

On Thu, 2004-04-29 at 09:18, Zickus II, Don wrote:
> Hello Stan,
> 
> We actually used this function on trucluster.  However, its main
> use was _after_ detection of a failed node and even then to revalidate
>  the lock value blocks and resources assuming you are using lock value blocks.
>  
> I have thinking about this whole problem and wondering what everyone's
>  definition of 'persistent' lock is?  Is it expected to have every resource
>  and its information replicated across the entire cluster or just have each
>  node keep track of its own resource locks?
> 
> -Don
> 


This is the same question I had many years ago when I first heard about
persistent locking.  As I have said before, a particular implementation
of persistent locking was used to implement a cluster file system.

>From what I remember (It has been more than 4 years since I worked
on this).  As it was explained to me, the whole point of persistent
locks was that the application did not want to keep a bunch of locks
open to be able to find out that some event has occurred which would
require an application recovery before using the lock.  Previously,
the application would keep the locks open and if it got a invalid
value block back, it would know that a recovery is required.  With
"persistent" locks, the application did not have to keep the lock
open, but would still get the invalid value block if process holding
the lock died or a node died.

Re-reading the trucluster man pages I pointed to before (like this one):
http://h30097.www3.hp.com/docs/cluster_doc/cluster_16/MAN/MAN3/0021____.HTM

This implementation seems much more complicated than what I remember.
This looks like this DLM does keep persistent locks around if they
are invalid after the process closes them (and they get marked invalid
if a node dies).  Thus, a new lock request will see the invalid lock
even when all previous users have gone away.  I'm guessing that is
what is meant by persistent.

The more simple implementation approach I remember (as best as I can --
at least for the node death case anyway -- and it really isn't that
simple).  If a node dies, then all new persistent locks would return
invalid until the dlm_rd_validate() is made.  Also, existing dlm locks
could get marked invalid during dlm recovery.

This implementation was then used to implement CFS.  Recovery went
something like:
	running nomally
	node dies
	DLM recovery runs - non-persistent locks recover
		persistent locks return invalid
	deadman locks from dead node(s) for each file system are granted
	start file system recovery:
		surviving nodes pick one to run log replay
			id = dlm_attach()
			replay file system log
			if validate(id) == MORE_RECOVERY
				re-start file system recovery
				 since there's been another death
			dlm_validate(id) - persistent locks valid now
		other surviving nodes wait for replay
	file system recovery complete
	
	if another node dies in the middle of file system recovery
		abort current recovery and start over 

		
This is simpler because persistent locks returned invalid based on
whether a node died until the validate call was made.  Every node
in the cluster did not have to replicated each lock information.

The use of persistent locks prevented surviving nodes from accessing
metadata BEFORE file system log replay had finished.  We used separate
lock domains for each file system, so file system recovery happened in
parallel.
	

Sorry for rambling, but I think I answered the question in there
somewhere.

Daniel



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