[Pvfs2-developers] ncache causes shared creat problems

[Rob Ross]

Hi all,

While I understand that the ncache does add some disparity to the views 
of various processes, I think it would be worth tracking down exactly 
what is happening here. It seems like this particular error is one that 
we should be able to avoid.

I agree with RobL that the ncache isn't going to be terribly helpful for 
most MPI-IO users, and with Phil that default disabling through the 
system interface is probably good.

Rob

Phil Carns wrote:
> Pete Wyckoff wrote:
>> The simul code, test #14, does a shared create:  all processes
>> try to do "creat(file, 0644)" at the same time through the VFS.
>> There is no O_EXCL, so what should happen here is that they all
>> succeed, although under the hood, all but one will probably have
>> to unwind the SYS_CREATE when they notice that the dirent already
>> exists from another process.
>>
>> This used to work just fine.  With the addition of the ncache code
>> to pvfs2-client, I'm guessing, things break.  The test works again
>> if I add "-n 0" to the pvfs2-client command line.
>>
>> My setup is all x86_64.  Two IO servers, one of which does MD too.
>> Two other nodes as clients, running:
>>
>>     mpiexec -pernode -np 2 $simul/simul -d /pvfs-ib -i 14 -n 200 -N 1
>>
>> eventually one will fail, usually around the second iteration, with
>>
>>     14:46:54: Process 1(ib26): FAILED in simul_creat, creat failed:
>>     No such file or directory
>>
>> Does anybody know the ncache code well enough to figure this out?
>> I find the -EEXIST fixup code in client-core, but can't see what
>> kind of ncache invalidation should presumably happen around there.
>>
>>         -- Pete
> 
> What happens on each iteration?  Does the code at some point delete a 
> file with a particular name and then create a new one with the same name?
> 
> -Phil
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