[Linux-cluster] Can LOCK_NOLOCK be used in this situation?

[Srini Sankaran]

I don't have a GFS cluster running right now. I'd appreciate some 
guidance on using GFS for the following situation:

I need a cluster of nodes to have read and write access to a scalable 
and common pool of storage connected to an FC SAN. The entire pool of 
storage must appear as one single file system to the cluster nodes. So 
far so good. GFS fits. But...

The application running on each node is partitioned in such a way that 
at any given moment, only one node will need read / write access to a 
directory and its descendant file tree.

For example, let's say the file system is called "/big" and it has 
directories "a", "b", ... "z". Let's say that I have cluster node "1", 
"2", and "3". When node 1 needs access to "/big/a", the other nodes "2" 
and "3", won't need access to "/big/a". Those nodes will be reading and 
writing in to "/big/b" or "/big/c" or something else. In general, 
"/big/a" and other directories could have several million files.

A few minutes to hours later, node 2 might take over the read / write 
responsibilities for "/big/a", and node 1 might move over to "/big/b", 
etc.

 From reading the GFS documentation, it certainly appears that a 
standard GFS with locking (single or redundant servers) would work in 
this situation. But, I would like to avoid designating any single or 
multiple servers as lock servers. This is because the cluster is very 
dynamic. Nodes can constantly be added or removed, and the system 
administration environment isn't conducive for designating lock servers 
and protecting them. Besides, I am wondering why the lock servers 
should work so hard to maintain all the locks on the millions of files 
when I know 100% that no other node is going to access the files 
simultaneously.

So, my question is: Can I simply use LOCK_NOLOCK in this situation and 
avoid any lock server? Maybe the answer is no because the documentation 
warns "Do not allow multiple nodes to mount the same file system while 
LOCK_NOLOCK is used. Doing so causes one or more nodes to panic their 
kernels, and may cause file system corruption".

I am still asking the question because of this partitioned file system 
access characteristic of my application. Is this warning still valid if 
I can guarantee that no two files or directories will be accessed by 
two different nodes simultaneously? If I can't do LOCK_NOLOCK, is there 
any other idea I can use here?

Thanks for your time