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[Cluster-devel] Re: [PATCH 1/2] NLM failover unlock commands

On Thu, Jan 17, 2008 at 03:23:42PM -0500, J. Bruce Fields wrote:
> To summarize a phone conversation from today:
> On Thu, Jan 17, 2008 at 01:07:02PM -0500, Wendy Cheng wrote:
> > J. Bruce Fields wrote:
> >> Would there be any advantage to enforcing that requirement in the
> >> server?  (For example, teaching nlm to reject any locking request for a
> >> certain filesystem that wasn't sent to a certain server IP.)
> >>
> >> --b.
> >>   
> > It is doable... could be added into the "resume" patch that is currently  
> > being tested (since the logic is so similar to the per-ip base grace  
> > period) that should be out for review no later than next Monday.
> >
> > However, as any new code added into the system, there are trade-off(s).  
> > I'm not sure we want to keep enhancing this too much though.
> Sure.  And I don't want to make this terribly complicated.  The patch
> looks good, and solves a clear problem.  That said, there are a few
> related problems we'd like to solve:
> 	- We want to be able to move an export to a node with an already
> 	  active nfs server.  Currently that requires restarting all of
> 	  nfsd on the target node.  This is what I understand your next
> 	  patch fixes.
> 	- In the case of a filesystem that may be mounted from multiple
> 	  nodes at once, we need to make sure we're not leaving a window
> 	  allowing other applications to claim locks that nfs clients
> 	  haven't recovered yet.
> 	- Ideally we'd like this to be possible without making the
> 	  filesystem block all lock requests during a 90-second grace
> 	  period; instead it should only have to block those requests
> 	  that conflict with to-be-recovered locks.
> 	- All this should work for nfsv4, where we want to eventually
> 	  also allow migration of individual clients, and
> 	  client-initiated failover.
> I absolutely don't want to delay solving this particular problem until
> all the above is figured out, but I would like to be reasonably
> confident that the new user-interface can be extended naturally to
> handle the above cases; or at least that it won't unnecessarily
> complicate their implementation.
> I'll try to sketch an implementation of most of the above in the next
> week.

Bah.  Apologies, this is taking me longer than it should to figure
out--I've only barely started writing patches.

The basic idea, though:

In practice, it seems that both the unlock_ip and unlock_pathname
methods that revoke locks are going to be called together.  The two
separate calls therefore seem a little redundant.  The reason we *need*
both is that it's possible that a misconfigured client could grab locks
for a (server ip, export) combination that it isn't supposed to.

So it makes sense to me to restrict locking from the beginning to
prevent that from happening.  Therefore I'd like to add a call at the
beginning like:

	echo " /exports/example" > /proc/fs/nfsd/start_grace

before any exports are set up, which both starts a grace period, and
tells nfs to allow locks on the filesystem /exports/example only if
they're addressed to the server ip  Then on shutdown,

	echo "" >/proc/fs/nfsd/unlock_ip

should be sufficient to guarantee that nfsd/lockd no longer holds locks
on /exports/example.

(I think Wendy's pretty close to that api already after adding the
second method to start grace?)

The other advantage to having the server-ip from the start is that at
the time we make lock requests to the cluster filesystem, we can tell it
that the locks associated with are special: they may migrate
as a group to another node, and on node failure they should (if
possible) be held to give a chance for another node to take them over.

Internally I'd like to have an object like

	struct lock_manager {
		char *lm_name;

for each server ip address.  A pointer to this structure would be passed
with each lock request, allowing the filesystem to associate locks to
lock_manager's.  The name would be a string derived from the server ip
address that the cluster can compare to match reclaim requests with the
locks that they're reclaiming from another node.

(And in the NFSv4 case we would eventually also allow lock_managers with
single nfsv4 client (as opposed to server-ip) granularity.)

Does that seem sane?

But it's taking me longer than I'd like to get patches that implement
this.  Hopefully by next week I can get working code together for people
to look at....


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