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

[Andreas Dilger]

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.