Re: Re: NFS as a Cluster File System (Locking question)

[Tom McNeal <trmcneal@attbi.com>]

Hi -

 > best.  however, on Linux, the client purges the entire file
 > from its cache when a file is locked, rather than just the
 > areas that were byte-range locked.

The purge must occur at lock time (or lock attempt time), right?  Or
does the metadata also keep lock status, and therefore should occur
even when just a stat is done? (I don't think this is the case, but I
thought I'd make sure, not having looked at it recently)

Tom


Lever, Charles 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.
>>
> 
> well, coherency is partially addressed in NFSv4 with delegations.
> a server can delegate a file to a client, allowing the client
> to cache the file and trust that the server will notify it when
> another client wants to access the file (read or write).
> 
> for an aggressively shared file, this doesn't perform well, but
> NFS has always assumed that there is little concurrent sharing
> of files.
> 
> this paradigm probably doesn't fit well with typical file
> usage in clusters, where files are very very large, and many
> nodes may be working on independent pieces of the same file
> at the same time.  in that case, record locking might be
> best.  however, on Linux, the client purges the entire file
> from its cache when a file is locked, rather than just the
> areas that were byte-range locked.
> 
> 
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-- 
Tom McNeal
(650)906-0761(cell)
(650)964-8459(fax)
Email: trmcneal@attbi.com



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