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



J. Bruce Fields wrote:
Remind me: why do we need both per-ip and per-filesystem methods?  In
practice, I assume that we'll always do *both*?

Failover normally is done via virtual IP address - so per-ip base method should be the core routine. However, for non-cluster filesystem such as ext3/4, changing server also implies umount. If there are clients not following rule and obtaining locks via different ip interfaces, umount would fail that ends up aborting the failover process. That's the place we need the per-filesystem method.

ServerA:
1. Tear down the IP address
2. Unexport the path
3. Write IP to /proc/fs/nfsd/unlock_ip to unlock files
4. If unmount required,
write path name to /proc/fs/nfsd/unlock_filesystem, then unmount.
5. Signal peer to begin take-over.

Sometime ago we were looking at "export name" as the core method (so per-filesystem method is a subset of that). Unfortunately, the prototype efforts showed the code would be too intrusive (if filesystem sub-tree is exported).
We're migrating clients by moving a server ip address from one node to
another.  And I assume we're permitting at most one node to export each
filesystem at a time.  So it *should* be the case that the set of locks
held on the filesystem(s) that are moving are the same as the set of
locks held by the virtual ip that is moving.

This is true for non-cluster filesystem. But a cluster filesystem can be exported from multiple servers.
But presumably in some scenarios clients can get confused, and we need
to ensure that stale locks are not left behind?

Yes.

We've discussed this before, but we should get the answer into comments
in the code (or on the patches).

ok, working on it. or should we add something into linux/Documentation to describe the overall logic ?

-- Wendy


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