Lustre discuss - Mar 2010 - Question regarding caution statement in 1.8 manual for the consistent mode flock option

Hi Guys,

Thanks Andreas for pointing me to the flock options.  However, I see the
following caution statement for the consistent mode:

--
CAUTION: This mode has a noticeable performance impact and may affect
stability, depending on the Lustre version used. Consider using a newer
Lustre version which is more stable.
--

Is there an impact if the option is turned on, or only if it is turned on
and used?  Is the impact local to the file being locked, the machine on
which that file is locked, or the entire set of machines mounting that
lustre file system?

Thanks in advance,
-J
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On 2010-03-05, at 15:18, Jagga Soorma wrote:
> Thanks Andreas for pointing me to the flock options.  However, I see  
> the following caution statement for the consistent mode:
>
> --
> CAUTION: This mode has a noticeable performance impact and may  
> affect stability, depending on the Lustre version used. Consider  
> using a newer Lustre version which is more stable.
> --
>
> Is there an impact if the option is turned on, or only if it is  
> turned on and used?  Is the impact local to the file being locked,  
> the machine on which that file is locked, or the entire set of  
> machines mounting that lustre file system?

It only affects the performance of the file that is being flocked.  If  
it is enabled and no applications are using flock then it has no effect.

It used to be that we defaulted to "localflock" behaviour, which has  
minimal performance impact, but that was confusing to applications.   
The "noflock" default now reports an error as you saw and it is up to
the administrator to pick either "localflock" (fastest, low impact,  
not coherent between nodes) or "flock" (slower, performance impact for
use, coherent between nodes).

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

Hello!

On Mar 5, 2010, at 5:25 PM, Andreas Dilger wrote:
> On 2010-03-05, at 15:18, Jagga Soorma wrote:
>> Is there an impact if the option is turned on, or only if it is  
>> turned on and used?  Is the impact local to the file being locked,  
>> the machine on which that file is locked, or the entire set of  
>> machines mounting that lustre file system?
> It only affects the performance of the file that is being flocked.  If  
> it is enabled and no applications are using flock then it has no effect.
Actually another side effect is if you have a lot of flock activity going on
that might put a lot of (cpu) load on your MDS esp. if there are a lot of
conflicts.

Another gotcha is some application might try to use flock when they see
functionality as available and this is pretty slow on lustre, every
lock/unlock request directly translates to an RPC.

And lastly speaking of real flock (not posix locking through fcntl) there is
one additional limitation, you can''t actually pass on a file descriptor
to another
process and inherit the lock there (classic example you can find in any book
is that if you do flock, then fork, your child process can close/unlock the
file and parent process will lose the lock too. Does not happen with Lustre)

Bye,
    Oleg