Re: [SSI-devel] Newbie question about CFS

[David B. Z. <dav...@hp...>]

Arcot Arumugam wrote:

>Hi
>
>>From the documentation I understand that the CFS layer handles the
>read/write tokens which coordinates access to the filesystem. My question is
>why do we need that ? Does not the filesystem code itself handle read/write
>locks? I guess the question could be posed as multiple threads/processes can
>access the same file and the filesystem has locking mechanisms to prevent
>data corruption. Why should CFS handle locking when it is already being done
>by the file system?
>
>Any answers on illuminating me would be greatly appreciated.
>
>Arcot
>
>  
>
The standard filesystem code only handles locking on a single machine 
even with multiple CPUs (SMP).  CFS handles locking across machines, so 
that filesystem coherency is maintained across nodes.  For example, I 
write a file on node 1 and immediately execute a process on node 2 which 
reads the file.  Will it see the newly written data?  In a cases like 
this, NFS client will NOT work, but CFS does.

-- 
David B. Zafman                | Hewlett-Packard Company
mailto:dav...@hp...     | http://www.hp.com
"Computer Science" is no more about computers than astronomy is about telescopes                                            - E. W. Dijkstra