Re: Re: NFS as a Cluster File System.

["David B. Ritch" <dritch@hpti.com>]

I agree that cache coherency is not a sensible goal for a cluster
filesystem.  However, cache coherency of metadata is rather important. 
For example, when one node creates a file of intermediate data, it is
important for the other nodes to be able to see that.  Using actime=0 is
the conventional mechanism for allowing file creation and deletion to be
propagated quickly.  Usually, one can tweak that a bit to reduce the
burden on the server.  However, it might be be nice if there were a
mechanism to propagate this sort of metadata change without dumping all
metadata over a second or two old.

dbr

On Mon, 2003-01-13 at 14:36, Neil Brown wrote:
> On Thursday January 9, alanr@unix.sh wrote:
> > 
> > NFS V3 and before have problems with "cache coherency".  That is, the 
> > different nodes in the cluster are not guaranteed to see the same contents.
> > 
> > I think this is supposed to be fixed in v4.
> > 
> 
> NFSv4 does not try to "fix" this.  It makes no attempts at "cache
> coherency" beyond what NFSv2/3 provide which is "close to open"
> cohenrence, meaning that if only one process has a file open at a
> time, then everythnig will appear coherent, and if multiple processes
> have the file open at the same time, they need to use record locking.
> 
> I really don't think total cache coherency is a sensible goal for a
> network filesystem, even a cluster filesystem.  It imposes lots of
> extra network traffic that most of the time will be of no value.
> If an application needs some degree of coherence, it should be
> explicit about it's needs (using open/close or locking) so that the
> protocol can provide it then, and only then.
> 
> NeilBrown
> 
> 
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David B. Ritch
High Performance Technologies, Inc.


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