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



On Thu, Jan 17, 2008 at 11:31:22AM -0500, Wendy Cheng wrote:
> J. Bruce Fields wrote:
>> On Thu, Jan 17, 2008 at 10:48:56AM -0500, Wendy Cheng wrote:
>>   
>>> 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 that last sentence:
>>
>> 	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.
>>
>> is still true in the cluster filesystem case, right?
>>
>> --b.
>>   
> Yes .... Wendy

In one situations (buggy client?  Weird network failure?) could that
fail to be the case?

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.


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