[OpenAFS] file access

[Paul Blackburn]

Derrick J Brashear wrote:

>On 5 Dec 2002, amar deep kumar wrote:
>
>  
>
>>Hi,
>>    in AFS how file writes are synchronised, if suppose two 
>>processes are writing the same file, how the file will be updated. 
>>    
>>
>
>last writer wins
>  
>
Exactly. Just like with local files in a local filesystem.

>  
>
>>or is there any method to lock a file while writing to it, but 
>>again what would happen if that file is already cached on other 
>>machines, does the file server breaks the callback of all those 
>>machines.
>>    
>>
>
>it does, but if the file is open already that won't help.
>
There are various tricks.
One is to rename the file before trying to update it
and then return it to its original name when done.
(similar example: SCCS or RCS file checkout).
This is not perfect since a race condition can occur
if two try this at exactly the same time.

The general problem of mutual exclusion to access resources
one process at a time occurs in many places including local filesystems.

Mutual exclusion is a much studied problem
and a Dutch computer scientist, Edsger Dijkstra



http://www.thocp.net/biographies/dijkstra_edsger.htm

proposed a clever solution using "semaphores"
to the famous "five philosphers around a dining table" problem.
A semaphore operation is an atomic "test and set".
(test if free and set flag "used" in a single operation.)

The bottom line is: you have to have an agreed convention
(or protocol) between the consumer processes trying to
access the resource.

This problem is not any different in /afs than it is in other places.
--
cheers
paul                       http://acm.org/~mpb